Dieter Suter

Publications

[1] Two-dimensional chemical exchange and cross-relaxation spectroscopy of coupled nuclear spins
S. Macura, Y. Huang, D. Suter and R. R. Ernst
J. Magn. Res. 43, 259-281 (1981)
AbstractThe features of two-dimensional cross-relaxation and chemical exchange spectroscopy of coupled spins are investigated theoretically and by experiment. It is shown that spin-spin couplings can lead to J cross-peaks in analogy to cross-peaks in two-dimensional autocorrelated spectroscopy. They reflect a coherent magnetization transfer in contrast to the incoherent processes responsible for cross-relaxation and for chemical exchange. Possibilities of selectively suppressing J cross-peaks are discussed.

[2] Spectral spin diffusion in the presence of an extraneous dipolar reservoir
D. Suter and R. R. Ernst
Phys. Rev. B 25, 6038-6041 (1982)
AbstractAn explanation is presented for the observed inverse quadratic dependence of the spectral spin-diffusion rate constant on the frequency difference of the two involved spin species in the presence of a large extraneous dipolar reservoir. The result applies to spin diffusion in the laboratory frame and to cross polarization in a rotating frame.

[3] Composite pulse excitation in three-level systems
M. H. Levitt, D. Suter and R. R. Ernst
J. Chem. Phys. 80, 3064-3068 (1984)
AbstractIt is demonstrated by theory and experiment that it is possible to excite coherence uniformly in three-level systems with a wide range of anharmonicities by using a composite pulse exctitation method. The theory is based on a formal analogy between the anharmonic three-level case and the simpler sytem with equally spaced energy levels where composite pulse techniques are already well known to compensate for the effects of mismatch between photon energies and level separations. Computer simulations and solid-state NMR experiments on the three-level system of deuterinum (spin 1) verify the uniformity of the excitation. The NMR powder spectrum of deuterated polymethylmethacrylate is faithfully reproduced even using a radio-frequency (rf) field strength weaker than the quadrupole interactions.

[4] Time-domain zero field magnetic resonance with field pulse excitation
R. Kreis, D. Suter and R. R. Ernst
Chem. Phys.Lett. 118, 120-124 (1985)

[5] Spin diffusion in resolved solid state NMR spectra
D. Suter and R. R. Ernst
Phys. Rev. B 32, 5608-5627 (1985)
AbstractSpectral spin diffusion in resolved solid-state NMR spectra is analyzed for various types of systems, including dipolar coupled spin-systems in the presence of extraneous spins, and systems of quadrupolar spins with and without an additional dipolar reservoir. Diffusion of Zeeman and quadrupolar order via single-quantum and double-quantum spin-diffusion mechanisms is considered. Special attention is paid to the frequency offset dependence of the spin-diffusion rate. The theoretical predictions are verified by spin-diffusion measurements using two-dimensional spectroscopy techniques for 13C, deuterium, and 14N resonance in single crystals.

[6] Radiofrequency-pulse excitation in time-domain zero-field magnetic resonance
R. Kreis, D. Suter and R. R. Ernst
Chem. Phys.Lett. 123, 154 (1986)

[7] Spin dynamics and thermodynamics in solid-state NMR cross polarization
M. H. Levitt, D. Suter and R. R. Ernst
J. Chem. Phys. 84, 4243-4255 (1986)
AbstractWe investigate spin thermodynamic processes taking place during Hartmann-Hahn cross-polarization experiments in solids, in which spin polarization is transferred between two nuclear spin species by the application of two strong rf fields. As is well known, optimum cross polarization is achieved for a particular ratio of the two rf field strengths called the Hartmann-Hahn condition; we find experimentally that this condition provides the optimum transfer not only on kinetic grounds, as is usually supposed, but also for thermodynamic reasons. By measurement of the evolution of the spin observables in a ferrocene single crystal we demonstrate the existence of a quasi-equilibrium state with nonuniform spin temperature and less than maximum entropy. A modified spin thermodynamic theory is developed whose main features are the important role of the dipolar energy for the quasi-equilibrium state and the existence of constants of the motion other than the total energy. A new cross-polarization experiment is suggested which is shown to provide efficient cross polarization even for mismatched Hartmann-Hahn conditions.

[8] Indirect phase detection of NMR spinor transitions
D. Suter, A. Pines and M. Mehring
Phys. Rev. Lett. 57, 242-244 (1986)
AbstractThe behavior of quantum mechanical state functions under selective rotations is discussed. If a two-level transition is phase shifted with composite z pulses, the component state function shared with a connected transition can be multiplied by i, thereby shifting the corresponding coherence into an orthogonal channel. Application to the sensitive indirect detection of NMR transitions is demonstrated.

[9] Multiple quantum NMR excitation with a one-quantum Hamiltonian
D. Suter, S. B. Liu, J. Baum and A. Pines
Chem. Phys. 114, 103 (1987)

[10] Theory of broadband heteronuclear decoupling in multispin systems
D. Suter, K. V. Schenker and A. Pines
J. Magn. Reson. 73, 90-98 (1987)
AbstractThe Waugh theory of broadband decoupling in NMR for I-S pairs is extended to arbitrary spin systems. It is shown that complete decoupling is achieved over a certain bandwidth if the irradiation scheme generates an average Hamiltonian for the I spins whose eigenvectors and eigenvalues are independent of resonance offset. If the observed S spins are only weakly coupled, it is possible to calculate the resulting spectrum directly from the offset dependence of the average Hamiltonian of the isolated I-spin system under the influence of the periodic decoupling sequence. The treatment applies to indirectly observed multiple-quantum transitions as well as to directly observable single-quantum resonance lines.

[11] Broadband heteronuclear decoupling in the presence of homonuclear dipolar and quadrupolar interactions
K. V. Schenker, D. Suter and A. Pines
J. Magn. Reson. 73, 99-113 (1987)
AbstractA new family of composite decoupling sequences designed for heteronuclear broadband decoupling of spin-1 and spin- in solids and liquid crystals is introduced. The pulse sequences are windowless and perform net rotations in spin space about magic-angle axes. Computer simulations and experimental results are presented to demonstrate their decoupling performance and to discuss their susceptibility toward pulse imperfections. We describe the evaluation of the new sequences, called COMARO (composite magic-angle rotation), and point out some possibilities for further improvements.

[12] Theory of multiple pulse NMR at low and zero fields
C. J. Lee, D. Suter and A. Pines
J. Magn. Reson. 75, 110 (1987)
AbstractCoherent averaging with time-dependent magnetic fields at low and zero static magnetic fields encounters several features which are unfamiliar in high-field magnetic resonance. The principal differences are that magnetic field pulses act generally on all spin species in the sample and that the Hamiltonian contains additional terms that are normally discarded in a high static magnetic field. We illustrate how the full Hamiltonian or different terms of the Hamiltonian may be averaged to zero by sequences of 90* rotations around the x, y, and z axes. The two limiting cases of ideal delta-function pulses and windowless sequences are treated. We also show that the duality between rotations of space coordinates and spin coordinates allows one to replace spatial reorientations of the sample, such as magic-angle spinning, by time-dependent magnetic fields. Sequences of delta-function pulses at zero field are analogous to recursive expansion schemes of multiple-pulse sequences at high field. The terms of the full Hamiltonian appear also in the average Hamiltonian of high-field pulse sequences and can be manipulated by the same sequence of configurations as in zero-field multiple-pulse NMR.

[13] Berry's phase in magnetic resonance
D. Suter, G. Chingas, R. A. Harris and A. Pines
Mol. Phys. 61, 1327-1340 (1987)
AbstractAccording to Berry, quantum states of a hamiltonian which varies adiabatically through a circuit C in parameter space may acquire geometrical phase factors exp (ig(C)) in addition to the normal dynamical phase factors exp ((-i/r?) f E(t) dt). We present N.M.R. experiments in the rotating frame which bear out these predictions for simple conical circuits, and point out that they are related to familiar behaviour based on the classical Bloch equations and on Haeberlen-Waugh coherent averaging theory. Extensions to coupled spins and electric quadrupolar effects are discussed.

[14] Observation of spin diffusion in zero field magnetic resonance
D. Suter, T. P. Jarvie, B. Sun and A. Pines
Phys. Rev. Lett. 59, 106 (1987)
AbstractWe report the measurement of spin diffusion at zero field, observed by two-dimensional deuterium magnetic resonance of a polycrystalline sample. This demonstrates for the first time an appealing feature of pulsed zero-field magnetic resonance, namely the potential for structure determination in solids without the need for single crystals or oriented samples.

[15] Recursive evaluation of interaction pictures
D. Suter and A. Pines
J. Magn. Reson. 75, 509-512 (1987)

[16] Broadband heteronuclear spin decoupling in solids
D. Suter, A. Pines, H. J. Lee and G. Drobny
Chem. Phys. Lett. 144, 324-327 (1988)

[17] Experimental classification of multi-spin coherence under the full rotation group
D. Suter and J. G. Pearson
Chem. Phys. Lett. 144, 328-332 (1988)

[18] Study of the Aharonov-Anandan phase by NMR interferometry
D. Suter, K. T. Mueller and A. Pines
Phys. Rev. Lett. 60, 1218-1220 (1988)
AbstractAharonov and Anandan have recently reformulated and generalized Berry's phase by showing that a quantum system which evolves through a circuit C in projective Hilbert space acquires a geometrical phase p(C) related to the topology of the space and the geometry of the circuit. We present NMR in-terferometry experiments in a three-level system which demonstrate the Aharonov-Anandan phase and its topological invariance for different circuits.

[19] Suppression of the zero frequency peak in zero field NMR
T. P. Jarvie, K. Takegoshi, D. Suter, A. Pines and D. B. Zax
Chem. Phys. Lett. 158, 325-328 (1989)

[20] Monochloro- and dichloroacetic acids as carbon and energy sources for a stable, methanogenic mixed culture
C. Egli, M. Thüer, D. Suter, A. M. Cook and T. Leisinger
Arch. Microbiol. 152, 218-223 (1989)
AbstractA stable methanogenic mixed culture was enriched from an industrial environment to utilize chloroacetate as sole carbon and energy source for growth. It immobilized spontaneously on activated charcoal and grew reproducibly on this carrier in a fluidized bed reactor when supplied with an anaerobic mineral salts medium. Substrate disappearance was complete. Methane, CO2 and chloride ions were conclusively identified as the metabolic products and quantified. The growth yield from chloroacetate was about 1 g of protein/mol of carbon. The calculated degradation rate in the fluidized bed reactor was 0.2 to 0.8 mmol/l-Sumh. The first metabolic intermediate from {$[$}2-13C{$]$}monochloroacetate in portions of biofilm-coated carrier was shown by 13C-NMR to be glycolate, from which 13CO2 and 13CH4 were formed. Glycolate was formed in an oxygen-insensitive hydrolysis, but its conversion to CO2 and CH4 was strictly anaerobic and sensitive to inhibition by bromoethanesulfonate. Degradation of {$[$}1-14C{$]$}-and {$[$}2-14C{$]$}-chloroacetate each yielded the same amount of {$[$}14C{$]$}-methane. We thus presume glycolate to be cleaved to CO2 and H2, which were the substrates for methanogenesis. Dehalogenation was limited to chlorobromo-, iodo- and dichloroacetate. These four compounds and glycolate were utilized as the sole carbon and energy sources by the methanogenic mixed culture.

[21] Optically Induced Coherent Spin Transients in an Atomic Ground State
J. Mlynek, M. Rosatzin and D. Suter
in: Editors: E. Wolf, L. Mandel and J. Eberly
Plenum,, New York, 763-767 (1990)

[22] Optically driven spin nutations in the ground state of atomic Sodium
D. Suter, M. Rosatzin and J. Mlynek
Phys. Rev. A 41, 1634-1644 (1990)
AbstractWe report detailed studies of the dynamic response of spin coherence to a step input of optically resonant narrow-band laser radiation in the presence of a static transverse magnetic field. The evolution of the spins is observed with a cw optical probe beam using polarization-selective detection of the transmitted light. In extension of previous optical-pumping experiments, the dynamics of the system are studied systematically as a function of optical-resonance detuning and intensity. Our ex-periments are performed on the 3s 2S 1/2 ground state of sodium, using the D1 line for optical excitation. The experimental-data are compared with theoretical predictions based on a Bloch-type equation of motion for an optically driven spin system in a J =i ground state.

[23] Phase and amplitude variations of optically induced spin transients
M. Rosatzin, D. Suter, W. Lange and J. Mlynek
J. Opt. Soc. Am. B 7, 1231-1238 (1990)

[24] Light-shift-induced spin echoes in a J=1/2 atomic ground state
M. Rosatzin, D. Suter and J. Mlynek
Phys. Rev. A 42, 1839-1841 (1990)
AbstractWe report on a spin-echo phenomenon that is induced between Zeeman sublevels by two laser pulses close to an optical resonance. The echo formation is due to a Zeeman light shift during the second off-resonant pulse; this light pulse creates a fictitious magnetic field that leads to the phase reversal of the spins. Our experimental results on the *S1/2 sodium ground state are in good qualitative agreement with theoretical predictions.

[25] Dynamics of atomic sublevel coherences during modulated optical pumping
D. Suter and J. Mlynek
Phys. Rev. A 43, 6124-6134 (1991)
AbstractThe dynamics of coherences between sublevels of an electronic ground state is investigated under conditions of modulated optical pumping. It is shown that the modulation corresponds to an effective reduction of the apparent sublevel splitting, thereby allowing the excitation of coherences between atomic sublevels whose energy splitting would require very high laser intensities for dc ex-citation. In addition, the frequency and phase of the modulation introduce additional degrees of freedom that can be utilized for the design of new experiments. The theoretical evaluations are compared with experiments on the ground state of atomic sodium.

[26] Optically induced coherence transfer echoes between Zeeman substates
D. Suter, M. Rosatzin and J. Mlynek
Phys. Rev. Lett. 67, 34-37 (1991)
AbstractWe report on optically induced spin-echo phenomena associated with Zeeman coherences in a sublevel manifold. The experiments on the ground state of sodium show multiple echoes occurring periodically after a double-pulse excitation sequence in an inhomogeneous magnetic field. The echo formation mechanism is attributed to a light-shift-induced transfer of sublevel coherence among the nondegenerate sub-states. This mechanism is verified experimentally, using a modulated excitation scheme with subsequent phase-sensitive detection.

[27] Laser excitation and detection of magnetic resonance
D. Suter and J. Mlynek
Editors: W. S. Warren
Academic Press, San Diego, 1-83 (1991)

[28] Evanescent Wave Spectroscopy of Sublevel Resonances near a Glass/Vapor Interface
D. Suter, J. Aebersold and J. Mlynek
Opt. Commun. 84, 269-274 (1991)

[29] Time-resolved twodimensional spectroscopy of optically driven atomic sublevel coherences
D. Suter, H. Klepel and J. Mlynek
Phys. Rev. Lett. 67, 2001-2004 (1991)
AbstractCoherences between atomic substates are driven by a resonant interaction with light and transferred between individual sublevel transitions. We present an experimental scheme, based on two-dimensional Fourier-transform spectroscopy, that allows direct observation of this coherence transfer.

[30] Propagation of light in a J=1/2J'=1/2 resonant medium
D. Suter
Opt. Commun. 86, 381-385 (1991)

[31] Evanescent Wave Spectroscopy of Atomic Sublevel Coherences

in: ''Laser Spectroscopy'', Eds.: M. Ducloy, E. Giacobino, and G. Camy (World Scientific, Singapore, 1992) 10, 143-144 (1992)

[32] Transverse optical pumping with polarization-modulated light
H. Klepel and D. Suter
Optics Commun. 90, 46-50 (1992)

[33] Optically Excited Zeeman Coherences in Atomic Groundstates: Nuclear Spin Effects
D. Suter
Phys. Rev. A 46, 344-350 (1992)
AbstractOptical experiments in alkali-metal atomic gases are usually interpreted in terms of the J=1/2 ground state, treating the electronic ground state as a degenerate two-level system. While this simplified level scheme has been quite successful in describing many experimental results, the nuclear spin can lead to significant modifications of the behavior. Apart from the obvious differences, such as the existence of hyperfine splitting, some more subtle effects are present that modify the dynamical as well as the equilibrium behavior of the system. An an example, the optical pumping process in the true atomic ground state is nonexponential and slower by at least an order of magnitude, compared to a hypothetical atom with nuclear spin zero. The limitations of the J = 1/2 model are analyzed theoretically and experimentally for atomic sodium, and experimental methods are demonstrated that can help disentangle the contributions from different hyperfine components.

[34] Indirect observation of 'forbidden' Raman transitions by laser-induced coherence transfer
D. Suter and H. Klepel
Europhys. Lett. 19, 469-474 (1992)

[35] Lateral displacement of a laser beam by a precessing magnetic dipole
T. Blasberg and D. Suter
Helv. Phys. Acta 65, 830-831 (1992)

[36] Two-dimensional spectroscopy of 'forbidden' Raman transitions by laser-induced coherence transfer
D. Suter and H. Klepel
Helv. Phys. Acta 65, 832-833 (1992)

[37] Sensitivity of Optically Excited and Detected Magnetic Resonance
D. Suter
J. Magn. Reson. 99, 495-506 (1992)
AbstractCompared to other forms of spectroscopy, the sensitivity of magnetic resonance experiments is relatively low, so that large numbers of spins are necessary for an experiment. This contrasts with related fields, most notably optical spectroscopy, where it has become possible, in recent years, to obtain spectroscopic information from individual atomic and molecular systems. This article investigates possible means for transferring some of this sensitivity advantage to the domain of magnetic resonance and gives numerical estimates of the expected signal-to-noise ratios, as well as experimental examples.

[38] Displacement of a laser beam by a precessing magnetic dipole
T. Blasberg and D. Suter
Phys. Rev. Lett. 69, 2507-2510 (1992)
AbstractExchange of angular momentum between photons and atoms cannot only cause optical pumping, but also a lateral displacement of a laser beam traveling through a homogeneous atomic vapor. We present experimental data from the ground state of atomic sodium demonstrating this effect.

[39] Coherent Effects in Laser Spectroscopy of Magnetic Multilevel Systems
D. Suter, T. Blasberg, H. Klepel and J. Mlynek
SPIE proceedings , (1992)

[40] Polarization Oscillations of Coupled Laser Beams in an Optically Pumped Atomic Vapour
D. Suter
Optics Commun. 95, 255-259 (1993)

[41] Quantum Time Translation Machine: An Experimental Realization
D. Suter, M. Ernst and R. R. Ernst
Mol. Phys 78, 95-102 (1993)

[42] Rotation properties of multipole moments in atomic sublevel spectroscopy
D. Suter, T. Marty and H. Klepel
Optics Letters 18, 531-533 (1993)

[43] Laser induced dynamics of atomic sublevel coherences
D. Suter and T. Marty
Optics Commun. 100, 443-450 (1993)

[44] Absolute Sign Determination of Nuclear Quadrupole Couplings by Laser - Radiofrequency Double Resonance
D. Suter and T. Blasberg
Helv. Phys. Acta 66, 427-428 (1993)

[45] Coherent Raman Beats: High-Order Interference Effects
D. Suter and T. Marty
Optics Letters 18, 1663-1665 (1993)

[46] Determination of the absolute sign of nuclear quadrupole interactions by laser - radio frequency double resonance experiments
T. Blasberg and D. Suter
Phys. Rev. B 48, 9524-9527 (1993)
AbstractThe interaction between the quadrupole moment of nuclear spins I > 1/2 with the electric-field-gradient (EFG) tensor leads to a splitting of the energy of the nuclear spin states. We show how the combination of laser and radio-frequency irradiation allows measurements of nuclear spin transitions in quadrupolar systems that are, in contrast to purely magnetic experiments, sensitive to the absolute sign of the quadrupole interaction. This determination of the sign is essential for comparison with calculated EFG tensors.

[47] Nuclear Spin Relaxation of Pr3+ in YAlO3: a Temperature-Dependent Optical-rf Double Resonance Study
T. Blasberg and D. Suter
Chem. Phys. Lett. 215, 668-673 (1993)

[48] Stabilization of transverse solitary waves by a nonlocal response of the nonlinear medium
D. Suter and T. Blasberg
Phys. Rev. A 48, 4583-4587 (1993)
AbstractOptical pumping of atomic vapors can lead to self-focusing of resonant laser beams. The saturation of the optical pumping process at low intensity allows the study of transverse solitons in the form of self-trapped laser beams, using low-power cw lasers. The long lifetime of the optically pumped atoms allows the atomic diffusion to transport the excitation away from the interaction region. Numerical simulations show that the resulting nonlocal response of the medium represents an important mechanism that stabilizes the self-trapped beam. The theoretical predictions are confirmed by experimental results from sodium vapor.

[49] Experimental Observation of the Rotation Properties of Atomic Multipoles
D. Suter and T. Marty
J. Opt. Soc. Am. B 11, 242-252 (1994)

[50] Determination of the absolute sign of nuclear quadrupole interaction by laser - radiofrequency double- resonance
T. Blasberg and D. Suter
Z. Naturf. 49a, 14-18 (1994)

[51] Excitation of coherent Raman beats in rare earth solids with a bichromatic laser field
T. Blasberg and D. Suter
Optics Commun. 109, 133-138 (1994)

[52] Large frequency shifts of absorption profiles due to the combination of optical pumping, light shift, and magnetic fields in sodium vapor
B. Roehricht, P. Eschle, C. Wigger, S. Dangel, R. Holzner and D. Suter
Phys. Rev. A 50, 2434-2437 (1994)

[53] Bilder von Atomen
D. Suter
Technische Rundschau Dezember 1994, 22-24 (1994)

[54] Weak measurements'' and the ''quantum time-translation machine'' in a classical system
D. Suter
Phys. Rev. A 51, 45-49 (1995)
AbstractA simple optical experiment is discussed that implements the ''quantum time-translation machine'' suggested by Aharonov et al . Phys. Rev. Lett. 64,2965 (1990)] for photons. The time-translation effect was observed experimentally as the shift of the fringe pattern in a modified Mach-Zehnder interferometer. The possibility of describing the effect classically, in terms of Maxwell's equations, suggests an alternative interpretation that does not invoke time-translation effects.

[55] Interference of Scattering Pathways in Raman Heterodyne Spectroscopy of Multilevel Atoms
T. Blasberg and D. Suter
Phys. Rev. B 51, 12439-12450 (1995)
AbstractRaman heterodyne detection of nuclear magnetic resonance is a sensitive technique for optical detection of an NMR transition, which is driven by a resonant radio-frequency field. The observed signal is linear in the transition matrix elements of the magnetic resonance transition and two optical transitions. This linearity is the reason that signal contributions from different sites or different rf transitions can interfere, in many cases destructively. In this article, we discuss such an interference effect, which appears to have been overlooked in the past. It occurs between signal contributions that originate from the same magnetic resonance transition of atoms with different positions within the inhomogeneously broadened optical resonance line. These atoms contribute to the coherent Raman scattering through different scattering paths, which involve optical transitions to different nuclear spin states of the same electronic state. We show that the interference between all possible scattering paths leads to complete signal cancellation, if the atoms that are involved in the different scattering paths are equally polarized. To study the effect and to eliminate the interference, we used a pump-and-probe technique with two laser beams. With two independent laser frequencies, it becomes possible to separate the individual scattering paths. We calculate the dependence of the signal on the frequency of both laser beams as well as on the radio frequency and compare the results to experimental data from Pr:YAl03. Our results show that the interference reduces the signal amplitude of the conventional Raman heterodyne experiment but can be eliminated. in the new experiment.

[56] Bichromatic Excitation of Coherent Raman Beats in Rare Earth Solids
T. Blasberg and D. Suter
Phys. Rev. B 51, 6309-6318 (1995)
AbstractIn coherent Raman beat experiments a laser field coherently scatters off a material excitation that evolves under the internal Hamiltonian of the system. The Raman signal, which is observed as a beat signal between the test laser and the scattered Raman field, provides information on energy-level splittings and dephasing rates. We propose a modification of this technique, which allows, in contrast to the conventional method, the excitation of coherent Raman beats also in systems with large sublevel splittings and small oscillator strengths. For this purpose, we use a bichromatic laser field to excite the coherent superposition in the medium. We calculate the evolution of the coherence during the laser excitation analytically and discuss its dependence on various experimental parameters. We compare the theoretical results with experimental data from Pr3+:YA103.

[57] Determination of Relative Oscillator Strengths by Coherent Raman Beats
T. Blasberg and D. Suter
Optics Commun. 120, 55-62 (1995)

[58] Wall Relaxation of Spin Polarized Sodium Measured by Reflection Spectroscopy
S. Grafstroem and D. Suter
Optics Letters 20, 2134-2136 (1995)
AbstractUsing reflection spectroscopy of optically pumped sodium vapor, we have performed the first direct measurement of the spatially inhomogeneous spin polarization near a glass surface. On the basis of a theoretical description of the reflection at an inhomogeneous, anisotropic medium, we deduce the magnitude of the small residual magnetization at the surface from an analysis of the optical line shape. This allows us to specify the depolarizing properties of the surface.

[59] Determination of Oscillator Strengths in Pr3+:YAlO3 by Raman Heterodyne and Hole Burning Spectroscopy
T. Blasberg and D. Suter
J. Luminesc. 65, 199-209 (1995)

[60] Application of absorption measurements to the temperature determination of a cloud of cold Cesium atoms
T. Marty and D. Suter
Helv. Phys. Acta 68, 504-505 (1995)

[61] Optically Enhanced Magnetic Resonance
D. Suter
Editors: D. M. Grand and R. K. Harris
John Wiley & Sons, Chichester, 3376-3385 (1996)

[62] Reflection Spectroscopy of Spin Polarized Atoms near a Dielectric Surface
S. Grafstroem, T. Blasberg and D. Suter
J. Opt. Soc. Am. B 13, 3-10 (1996)

[63] Interaction of Spin-Polarized Atoms with a Surface Studied by Optical Reflection Spectroscopy
S. Grafstroem and D. Suter
Phys. Rev. A 54, 2169-2179 (1996)

[64] Optically Enhanced Magnetic Resonance for the Study of Atom-Surface Interaction
S. Grafstroem and D. Suter
Zeitschr. Physik D 38, 119-132 (1996)
AbstractWe study surface-induced spin relaxation with a laser-assisted magnetic resonance experiment. Optical pumping with polarization-modulated light in a trans- verse magnetic field creates the spin polarization. For detection a probe laser beam is reflected at the surface and the change of its polarization is monitored. We present a comprehensive theoretical description, taking into account the spin relaxation at the surface, which leads to a spatially inhomogeneous magnetization near the sur- face as a result of the transient behavior of the atoms in this region. Analytical expressions are derived for the magnetic resonance signal, which show that the wall relaxation causes a clear modification of the line shape, characterized by pronounced wings. The experimental results obtained with bare and silicone-coated Pyrex-glass surfaces are well described by the theory. The bare glass surface causes strong relaxation, whereas the silicone- coated surface is only weakly depolarizing. The analysis of the magnetic-resonance line shape indicates that the de- polarization probability per wall collision is &0.01 in the latter case. The results are compared with corresponding results from the analysis of the optical resonance line measured with the same setup. Both types of measure- ments can be interpreted within the same theoretical framework and are fully consistent with one another.

[65] Nuclear magnetic resonance polarization and coherence echoes in static and rotating solids
M. Tomaselli, S. Hediger, D. Suter and R. R. Ernst
J. Chem. Phys. 105, 10672-10681 (1996)
AbstractThe mechanisms of defocusing and refocusing of spin order in extended dipolar coupled nuclear spin systems are investigated by experiments on static and on rotating solids. It is demonstrated that polarization or coherence echoes are possible also under magic-angle sample spinning. The dipolar interactions, averaged by the spinning, are recovered by rotor-synchronized multiple-pulse sequences. By a simple modification of the pulse sequences, it is possible to reverse the sign of the effective dipolar Hamiltonian and to induce the refocusing of polarization or coherence. The creation of multiple-spin order in the course of polarization evolution or free precession is monitored by a modified echo experiment. Experimental results for a polycrystalline sample of calcium formate are presented.

[66] Optical detection of transition metal ion electron paramagnetic resonance by coherent Raman spectroscopy
S. J. Bingham, D. Suter, A. Schweiger and A. J. Thomson
Chem. Phys. Lett. 266, 543-547 (1997)

[67] The Physics of Laser-Atom Interaction
D. Suter
Cambridge University Press, Cambridge (1997)
ISBN 0-525-46239-8

[68] The design and sensitivity of microwave frequency optical heterodyne receivers
S. J. Bingham, B. Börger, D. Suter and A. J. Thomson
Rev. Sci. Instrum. 69, 3403-3409 (1998)
AbstractRecent advances in high speed photodetector and microwave receiver technology make microwave frequency optical heterodyning an attractive approach for the detection of a number of coherent Raman and Brillouin scattering experiments. We have therefore analyzed the sensitivity of microwave frequency optical heterodyne receivers. Experimental tests on a visible wavelength receiver operating at 13.5 GHz confirm the expectation of shot noise limited sensitivity. The relative merits of microwave frequency optical heterodyne detection and the alternative Fabry-Perot interferometry approach are discussed.

[69] ac Stark effect in a doubly driven three-level atom
C. Wei, D. Suter, A. S. M. Windsor and N. B. Manson
Phys. Rev. A 58, 2310-2318 (1998)
AbstractIn this paper we present a detailed treatment of the ac Stark effect of a three-level atom driven by two strong laser fields in a cascade scheme. We consider two situations where there is a weak laser field probing a new transition starting from one of the three levels to a fourth level. In one case the initial level of the probed transition is the ground state and in the other case the initial level is the intermediate state. For both situations we derive an analytical expression for the absorptive and dispersive responses of the weak probe field and present the spectrum obtained from numerical calculation. The general feature of the spectrum has a three-peak structure. The positions and relative intensities of the three spectral components are affected strongly by the driving field intensities and detunings. An account of the spectrum is given in terms of the dressed-state formalism.

[70] Breaking the Stokes-anti-Stokes symmetry in Raman heterodyne detection of magnetic-resonance transitions
R. Neuhaus, M. J. Sellars, S. J. Bingham and D. Suter
Phys. Rev. A 58, 4961-4966 (1998)
AbstractCoherent Raman scattering can generate Stokes and anti-Stokes fields of comparable intensities. When the Raman shift is due to a magnetic resonance transition ~usually in the MHz to GHz range!, the Raman fields are generally detected by optical heterodyne detection, using the excitation laser as the local oscillator. In this case, the two sidebands generate beat signals at the same frequency and are therefore indistinguishable. Separation of the two contributions becomes possible, however, by superheterodyne detection with a frequency-shifted optical local oscillator. We compare the two scattering processes, and show how the symmetry between them can be broken in Pr:YAlO .

[71] Optically detected electron paramagnetic resonance by microwave modulated magnetic circular dichroism
B. Börger, S. J. Bingham, J. Gutschank, M. O. Schweika, D. Suter and A. J. Thomson
J. Chem. Phys. 111, 8565-8568 (1999)
AbstractElectron paramagnetic resonance (EPR) can be detected optically, with a laser beam propagating perpendicular to the static magnetic field. As in conventional EPR, excitation uses a resonant microwave field. The detection process can be interpreted as coherent Raman scattering or as a modulation of the laser beam by the circular dichroism of the sample oscillating at the microwave frequency. The latter model suggests that the signal should show the same dependence on the optical wavelength as the MCD signal. We check this for two different samples (cytochrome c-551, a metalloprotein, and ruby (Cr3+:Al2O3)). In both cases, the observed wavelength dependence is almost identical to that of the MCD signal. A quantitative estimate of the amplitude of the optically detected EPR signal from the MCD also shows good agreement with the experimental results.

[72] Depth profile of the implantation-enhanced intermixing of Ga+ focused ion beam in AlAs/GaAs quantum wells
S. Eshlaghi, C. Meier, D. Suter, D. Reuter and A. D. Wieck
J. Appl. Phys. 86, 6605-6607 (1999)
AbstractThe implantation-induced intermixing depth profile for 100 keV Ga 1 ions was determined by photoluminescence measurements on a series of samples containing quantum wells at variable depth from the surface but identical thickness. They were uniformly implanted and subsequently a rapid thermal annealing was applied. The measured maximum of the intermixing occurred at a depth of about 70 nm, significantly deeper than theoretical predictions. These results are important for achieving sufficient intermixing with a low implantation dose, thereby optimizing crystal quality and lateral resolution.

[73] Probing the electronic structure of transition metal ion centres in proteins by coherent Raman-detected electron paramagnetic resonance spectroscopy
S. J. Bingham, B. Börger, J. Gutschank, D. Suter and A. J. Thomson
JBIC 5, 30-35 (2000)
AbstractThe simultaneous excitation of a paramag- netic sample with optical (laser) and microwave radia- tion can cause an amplitude or phase modulation of the transmitted light at the microwave frequency. The detection of this modulation indicates the presence of coupled optical and electron paramagnetic resonance (EPR) transitions in the sample. Here we report the first application of this technique to a biomolecule: the blue copper centre of Pseudomonas aeruginosa azurin. Using optical excitation at 686 nm, in the thiol to copper(II) charge transfer band, we measure a coherent Raman-detected EPR spectrum of a frozen aqueous solution. Its lineshape is characteristic of the magnetic circular dichroism along each principal g-value axis. This information allows electronic and structural models of transition metal ion centres in proteins to be tested.

[74] Functional Magnetic Resonance Imaging in Real Time (FIRE): Sliding-Window Correlation Analysis and Reference-Vector Optimization
D. Gembris, J. G. Taylor, S. Schor, W. Frings, D. Suter and S. Posse
Mag. Reson. Med. 43, 259-268 (2000)
AbstractNew algorithms for correlation analysis are presented that al-low the mapping of brain activity from functional MRI (fMRI) data in real time during the ongoing scan. They combine the computation of the correlation coefficients between measured fMRI time-series data and a reference vector with ''detrending,'' a technique for the suppression of non-stimulus-related signal components, and the ''sliding-window technique.'' Using this technique, which limits the correlation computation to the last

[75] Magnetic circular dichroism anisotropy from coherent Raman detected electron paramagnetic resonance spectroscopy: Application to spin-1/2 transition metal ion centers in proteins
S. J. Bingham, J. Gutschank, B. Börger, D. Suter and A. J. Thomson
J. Chem. Phys. 113, 4331-4339 (2000)
AbstractMeasurement of magnetic circular dichroism (MCD) anisotropy has contributed greatly to the understanding of the electronic structure of transition metal ion centers in both biological and nonbiological materials. Compared to previous methods, optically detected electron paramagnetic resonance experiments can measure MCD anisotropy with dramatically improved orientational resolution. In this paper the relevant theory for systems with an isolated Kramers doublet ground level is derived and its application illustrated using a transition metal ion center in a protein: low spin ferric haem.

[76] Effect of devitrification on the ionic diffusion of Li-disilicate
O. Kanert, R. Kuechler, D. Suter, G. N. Shannon and H. Jain
J. Non-Cryst. Solids 274, 202-207 (2000)
AbstractA simple theoretical model has been developed for describing the variation of diffusion of ions in an initially ho- mogeneous glass as it transforms into a two-phase structure. The model is applicable to those microstructures in which the second phase grows as a discontinuous dispersion without affecting the diffusion properties of the glassy matrix. The validity of the model is demonstrated by the measurement of ionic conductivity and of diffusion-induced 7Li nuclear spin relaxation during the devitrification of Li-disilicate, containing 0 and 1 mol% P2O5, respectively. Evaluation of the data yields the basic parameters of devitrification.

[77] Deconvolution and Assignment of Different Optical Transitions of the Blue Copper Protein Azurin from Optically Detected Electron Paramagnetic Resonance Spectroscopy
B. Börger, J. Gutschank, D. Suter, A. J. Thomson and S. J. Bingham
J. Am. Chem. Soc. 123, 2334-2339 (2001)
AbstractMagnetic circular dichroism is a powerful spectroscopic tool for the assignment of optical resonance lines. An extension of this technique, microwave-modulated circular dichroism, provides additional details, in particular information about the orientation of optical transition moments. It arises from magnetization precessing around the static magnetic field, excited by a microwave field, in close analogy to electron paramagnetic resonance (EPR). In this paper we investigate the visible and near-infrared spectrum of the blue copper protein Pseudomonas aeruginosa azurin. Using a nonoriented sample (frozen solution), we apply this technique to measure the variation of the optical anisotropy with the wavelength. A comparison with the optical anisotropies of the possible ligand-field and charge-transfer transitions allows us to identify individual resonance lines in the strongly overlapping spectrum and assign them to specific electronic transitions. The technique is readily applicable to other proteins with transition metal centers.

[78] Magnetic resonance investigations of the dynamics of F centers in LiF
T. Klempt, S. Schweizer, K. Schwartz, O. Kanert, D. Suter, U. Rogulis and J. -M. Spaeth
Sol. State Comm. 119, 453-458 (2001)
AbstractDamage caused in lithium uoride crystals by -rays and electrons includes, besides other point defects, F centers and their agglomerates. We have stud- ied the paramagnetic F centers created by radiation doses that vary by several orders of magnitude. We measured the electronic spin relaxation time T1e of the F centers at low temperatures by the recovery of the magnetic circular dichroism of absorption (MCDA) as well as the temperature dependence of the 19 F and 7 Li nuclear spin relaxation (NSR) times, T1n. Our results indicate that the nuclear spin relaxation is dominated by spin-diusion limited para- magnetic relaxation. The electron spin correlation function is determined by the electronic spin-lattice relaxation. In the studied temperature range from 4 K to 300 K, the electron spin-lattice relaxation time T1e is long compared to the nuclear Larmor period, !LT1e , 1 (!L : nuclear Larmor frequency).

[79] Magnetic and optical anisotropy of Clostridium pasteurianumrubredoxin from optically detected electron paramagnetic resonance
B. Börger and D. Suter
J. Chem. Phys. 115, 9821-9826 (2001)
AbstractThe high-spin Fe~III!-center of oxidized rubredoxin from Clostridium pasteurianum shows a complicated, temperature-dependent EPR spectrum. We combine conventional EPR spectroscopy with optically detected EPR ~ODEPR! to elucidate the electronic structure of this protein metal center. The ODEPR experiment, which can be considered as coherent Raman scattering or modulated magnetic circular dichroism ~MCD!, yields spectra that depend on the relative orientation of optical and magnetic dipole moments. A detailed analysis of the spectra shows that they correspond to a zero-field splitting of D5146.3 GHz and a strong rhombic distortion with E/ D 50.25. In the frozen solution, conformational strain gives rise to variation of the rhombicity, which can be measured quantitatively from the EPR line shape. Analysis of the ODEPR line shapes yields the orientation of the optical anisotropy with respect to the magnetic g-tensor. We compare the results from this study to published results on EPR, optical spectroscopy, and MCD.

[80] Magnetic resonance investigation of F centers in LiF caused by ionizing radiation
T. Klempt, S. Schweizer, K. Schwartz, O. Kanert, D. Suter, U. Rogulis and J. -M. Spaeth
Radiation effects & Defects in Solids 155, 159-163 (2001)

[81] A first-principles approach for diffusion tensor based fiber tracking
D. Gembris, H. Schumacher, D. Suter and K. Zilles
NeuroImage 13, 123 (2001)

[82] Coupling mechanisms for optically induced NMR in GaAs quantum wells
M. Eickhoff, B. Lenzmann, G. Flinn and D. Suter
Phys. Rev. B 65, 125301, 125301 (2002)
AbstractOptical pumping can increase the polarization of nuclear spins in semiconductors such as GaAs by many orders of magnitude, improving the sensitivity in conventionally detected nuclear magnetic resonance ~NMR! experiments. Optical detection of these NMR transitions provides an additional increase in sensitivity, and furthermore, can distinguish signal contributions from different quantum wells in multiple quantum well samples. In this article we study the coupling mechanisms for all-optical NMR experiments, where modulation of the cw optical excitation at the nuclear Larmor frequency induces transitions between the nuclear spin states.

[83] Scalable architecture for spin-based quantum computers with a single type of gate
D. Suter and K. Lim
Phys. Rev. A 65, 052309 (2002)
AbstractWe propose a scalable design for a solid-state spin-based quantum computer. It uses endohedral fullerenes like N@C60 or P@C60 , which can be positioned on Si surfaces with a scanning tunneling microscope. Each logical qubit is stored in two physical qubits, corresponding to the nuclear and electronic spins. We discuss the addressing of individual qubits by a magnetic field gradient, and the implementation of one- and two-qubit quantum gates by sequences of radio-frequency and microwave pulses.

[84] Trends and random fluctuations in athletics
D. Gembris, J. G. Taylor and D. Suter
Nature 417, 506 (2002)

[85] Architectures for a Spin Quantum Computer Based on Endohedral Fullerenes
W. Harneit, C. Meyer, A. Weidinger, D. Suter and J. Twamley
Phys. Stat. Sol. (b) 233, 453-461 (2002)
AbstractPACS: 03.67.Lx; 76.30.-v; 81.07.Nb; 85.35.-p We present a discussion of recent concepts for the construction of a spin quantum computer using endohedral fullerenes. The fullerene molecule is a static, room-temperature trap for atoms with slowly relaxing electron and nuclear spins. The fullerene ''containers'' can be used to arrange the spins in complex structures such as a linear chain, to form a spin quantum register. We discuss the probable properties of such registers and different strategies to use them in a quantum computer design, including gating and read-out methods.

[86] Synthesis and Molecular Structure of a Tricyclic Stannasiloxane Containing a Novel SiSn3O3F2 Structural Motif
J. Beckmann, K. Jurkschat, M. Schuermann, D. Suter and R. Willem|
Organometallics 21, 3819-3822 (2002)
AbstractThe synthesis of the fluorine-substituted stannasiloxane complex cyclo-Ph2Si(OSnt-Bu2)2O,t-Bu2SnF2 (7) is reported, and its molecular structure has been determined by single-crystal X-ray diffraction analysis.

[87] Magneto-optical and EPR transitions in Raman heterodyne spectroscopy
M. O. Schweika-Kresimon, J. Gutschank and D. Suter
Phys. Rev. A 66, 043816 (2002)
AbstractLaser and microwave fields can be mixed in paramagnetic samples in a coherent Raman process. The radiation scattered in the forward direction contains frequency components at the sum and difference frequency. We consider the details of this scattering process in a static magnetic field, where two types of signals appear: magnetic-resonance transitions give rise to signals at fixed magnetic fields, independent of the laser frequency. Another set of resonances is determined by the optical transition frequencies. These resonances are particularly prominent in samples with narrow optical absorption lines. We derive a comprehensive theory that describes both contributions and compare it with experimental data from the R lines of ruby. DOI: 10.1103/PhysRevA.66.043816 PACS number~s!: 42.65.2k, 33.55.2b, 76.30.2v

[88] F centers in LiF: A nuclear magnetic resonance study
T. Klempt, O. Kanert and D. Suter
Phys. Stat. Sol. b 236, 151-165 (2003)
AbstractPACS 61.82.Ms, 76.30.Mi, 76.60.Es Ionizing radiation like g-rays, electrons, or swift heavy ions create a variety of point defects in dielectric materials. The largest fraction of the defects consists of the paramagnetic F centers. Here, we study those F centers in LiF by nuclear magnetic resonance. Nuclear spin relaxation (T1) measurements serve as a probe for the F centers offering the possibility to investigate their dynamics as a function of temperature and irradiation dose. Moreover, one is able to estimate the content of the paramagnetic defects from the T1-data over a wide range of concentration. We further observed and analyzed radiation annealing occurring at temperatures above 360 K. phys. stat. sol. (b) 236, No. 1, 151-165 (2003) / DOI 10.1002/pssb.200301416

[89] Mapping of strain and electric fields in GaAs/AlGaAs quantum-well samples by laser-assisted NMR
M. Eickhoff, B. Lenzmann, D. Suter, S. E. Hayes and A. D. Wieck
Phys. Rev. B 67, 085308 (2003)
AbstractThe usefulness of semiconductor heterostructures derives from the possibility to engineer their electronic and optical properties to match the requirements of many different applications. Optically detected nuclear magnetic resonance provides the possibility to map microscopic properties of such samples with a high spatial resolution through the splitting of resonance lines. In a multiple quantum-well sample, we measure the distortion of the crystal lattice and find variations of the order of 10^-5 over distances of a few mm. Internal electric fields also cause resonance line splittings. Comparing the electric field-induced resonance line splittings in different quantum wells, we mapped the vertical variation of the electric field from a Schottky contact with a spatial resolution of some 40 nm. DOI: 10.1103/PhysRevB.67.085308 PACS number~s!: 73.30.1y, 76.70.Hb

[90] Size scaling of decoherence rates
C. S. Maierle and D. Suter
Editors: G. Leuchs and T. Beth
Wiley-VCH, Weinheim, 121-130 (2003)

[91] Nuclear quadrupole resonance of an electronically excited state from high-resolution hole-burning spectroscopy
R. Klieber, A. Michalowski, R. Neuhaus and D. Suter
Phys. Rev. B 67, 184103 (2003)
AbstractHole-burning spectroscopy can eliminate inhomogeneous broadening and thereby resolve the fine structure of optical transitions. In the case of rare-earth ions at low temperatures, the homogeneous linewidth is often small compared to the splittings due to nuclear-spin interactions. Hole-burning spectra can then be used to measure, e.g., nuclear quadrupole couplings. We have used this technique to study the hyperfine interaction of Pr in Pr31:YAlO3 in the electronic ground state as well as in an electronically excited state. Using a stabilized ring dye laser system ~linewidth ,30 kHz), we obtained hole-burning spectra that clearly resolved the excitedstate interactions also. We show that the spectra depend sensitively on the relative orientation of the nuclearspin quantization axes of the two electronic states. This allows us to decide between different values for the tensor orientation that have been published before. DOI: 10.1103/PhysRevB.67.184103 PACS number~s!: 32.70.Cs, 32.10.Fn PHYSICAL REVIEW B 67, 184103 ~2003!

[92] K-dependent exchange interaction of quadrupole excitons in Cu2O
G. Dasbach, D. Fröhlich, H. Stolz, R. Klieber, D. Suter and M. Bayer
Phys. Stat. Sol. (b) 238, 541-547 (2003)
AbstractPACS 71.35.Cc, 78.20.
e., 78.40.Fy The long- and short-range exchange interaction for the quadrupole allowed transition to the yellow 1S orthoexciton in Cu2O is derived. While the three orthoexciton states still remain degenerate, when treating the exchange up to the order of K-linear terms, a fine structure arises when K-quadratic terms are included. This splitting is investigated experimentally by high resolution spectroscopy. Exchange contributions of few meV are resolved and compared to theory. The impact of strain on the exciton fine structure is discussed and evaluated. phys. stat. sol. (b) 238, No. 3, 541-547 (2003) / DOI 10.1002/pssb.200303186

[93] All-optical measurement of nuclear-spin relaxation
R. Klieber, A. Michalowski, R. Neuhaus and D. Suter
Phys. Rev. B 68, 054426 (2003)
AbstractNuclear-spin relaxation rates are usually measured by pulsed radio frequency excitation of nuclear-spin transitions. When the number of spins is too small for direct detection with conventional nuclear-magneticresonance spectrometers, pulsed excitation may be combined with optical detection of the nuclear spins for increased sensitivity. Here we demonstrate that such measurements can also be done purely optically, without radio frequency fields. This is achieved by preparing nonthermal populations by optical hole-burning and time-resolved probing of the populations as a function of the decay time. Measurements of the nuclear-spin relaxation of 141Pr in the electronic ground state of Pr:YAlO3 show that the relaxation rates for the three transitions differ by a factor of 3. DOI: 10.1103/PhysRevB.68.054426 PACS number~s!: 39.30.1w, 76.60.2k PHYSICAL REVIEW B 68, 054426 ~2003!

[94] Wave-Vector-Dependent Exciton Exchange Interaction
G. Dasbach, D. Froehlich, H. Stolz, R. Klieber, D. Suter and M. Bayer
Phys. Rev. Lett. 91, 107401 (2003)
AbstractThe exchange interaction for the yellow 1S orthoexciton in Cu2O is derived up to the order K2. The resulting exchange splittings are verified experimentally using high resolution spectroscopy. In agreement with theory the fine structure shows a characteristic dependence on the direction of the wave vector. DOI: 10.1103/PhysRevLett.91.107401 PACS numbers: 78.20.-e, 71.35.Cc, 78.40.Fy

[95] Pulsed optically detected NMR of single GaAs/AlGaAs Quantum Wells
M. Eickhoff and D. Suter
J. Magn. Reson. 166, 69-75 (2004)
AbstractWhile nuclear magnetic resonance (NMR) is one of the most important experimental tools for the analysis of bulk materials, the low sensitivity of conventional NMR makes it unsuitable for the investigation of small structures. We introduce an experimental scheme that makes NMR spectra of single, nanometer-sized quantum wells possible with excellent sensitivity and selectivity while avoiding the spectral broadening associated with some alternative techniques. The scheme combines optical pumping and pulsed radio frequency excitation of the nuclei with time-resolved detection of the free induction decay through the polarization of the photoluminescence. Key words: ODNMR; pulsed excitation; quantum well; GaAs PACS: 76.60.-k, 78.55.Cr, 78.67.De

[96] Orientational Information from Unoriented Metalloproteins by Optically Detected Electron Paramagnetic Resonance
J. Gutschank, D. Suter and B. Enkisch
J. Anal. At. Spectrom. 19, 34-40 (2004)
AbstractThe electronic structure of metalloproteins can be analysed with optically detected electron paramagnetic resonance (ODEPR). This relatively young technique combines electron paramagnetic resonance (EPR) and magnetic circular dichroism (MCD) with a coherent Raman scattering experiment. It can complement conventional EPR and MCD in the deconvolution and assignment of optical transitions. Information about the relative orientation of optical and magnetic transition dipoles in metalloproteins can be extracted with considerably higher resolution than by alternative techniques such as MCD. We discuss how these features provide information about the environment of metal ions in metalloproteins.

[97] Quantum Computing: A Short Course from Theory to Experiment
J. Stolze and D. Suter
Wiley-VCH, Berlin (2004)
ISBN 3-527-40438-4

[98] Wave-vector-dependent exchange interaction and its relevance for the effective exciton mass in Cu2O
G. Dasbach, D. Froehlich, R. Klieber, D. Suter, M. Bayer and H. Stolz
Phys. Rev. B 70, 045206 (2004)
AbstractThe wave-vector dependence of electron-hole exchange interaction is investigated. For the yellow 1S exciton in Cu2O the exchange is derived up to the order k2. The theoretical predictions are verified experimentally by high-resolution absorption experiments. In agreement with theory the fine structure shows a characteristic dependence on the direction of the wave vector. The exchange splitting of the orthoexciton triplet are distinguished from strain-induced perturbations. The exchange gives rise to an isotropic and an anisotropic correction of the effective exciton mass. This can explain the discrepancies in the measurements of the exciton mass in Cu2O. URL: http://link.aps.org/abstract/PRB/v70/e045206 doi:10.1103/PhysRevB.70.045206 PACS: 78.20.-e, 78.40.Fy, 71.35.Cc

[99] Scaling of Decoherence in Wide NMR Quantum Registers
H. G. Krojanski and D. Suter
Phys. Rev. Lett. 93, 090501 (2004)
AbstractAmong the most important parameters for the usefulness of quantum computers are the size of the quantum register and the decoherence time for the quantum information. The decoherence time is expected to get shorter with the number of correlated qubits, but experimental data are only available for small numbers of qubits. Solid-state nuclear magnetic resonance allows one to correlate large numbers of qubits (several hundred) and measure their decoherence rates.We use a modified magnetic dipole-dipole interaction to correlate the proton spins in a solid sample and observe the decay of the resulting highly correlated states. By systematically varying the number of correlated spins, we measure the increase of the decoherence rate with the size of the quantum register. DOI: 10.1103/PhysRevLett.93.090501 PACS numbers: 03.67.Pp, 03.65.Yz, 03.67.Lx, 82.56.Hg

[100] Hole-burning techniques for isolation and study of individual hyperfine transitions in inhomogeneously broadened solids demonstrated in Pr3+ :Y2 SiO5
M. Nilsson, L. Rippe, S. Kröll, R. Klieber and D. Suter
Phys. Rev. B 70, 214116 (2004)
AbstractDue to their narrow homogeneous linewidths, rare-earth ions in inorganic crystals at low temperatures have recently been given considerable attention as test materials for experiments in coherent quantum optics. Because these narrow linewidth transitions have been buried in a wide inhomogeneous line, the scope of experiments that could be carried out in these materials has been limited. However, here we present spectroscopic techniques, based on spectral hole burning and optical pumping, which allow hyperfine transitions that are initially buried within an inhomogeneously broadened absorption line to be studied with no background absorption from other transitions. A sequence of hole-burning pulses is used to isolate selected transitions between hyperfine levels, which makes it possible to directly study properties of the transitions, e.g., transition strengths, and gives access to information that is difficult to obtain in standard hole-burning spectroscopy, such as the ordering of hyperfine levels. The techniques introduced are applicable to absorbers in a solid with long-lived sublevels in the ground state and where the homogeneous linewidth and sublevel separations are smaller than the inhomogeneous broadening of the optical transition. In particular, this includes rare-earth ions doped into inorganic crystals and in the present work the techniques are demonstrated in spectroscopy of Pr3+ in Y2 SiO5 . Information on the hyperfine structure and relative transition strengths of the 3H 4 - 1D 2 hyperfine transitions in Pr3+ : Y2 SiO5 has been obtained from frequency-resolved absorption measurements, in combination with coherent and incoherent driving of the transitions. URL: http://link.aps.org/abstract/PRB/v70/e214116 DOI: 10.1103/PhysRevB.70.214116 PACS: 78.90.+t, 61.72.-y, 39.30.+w, 78.47.+p

[101] Quantum phase transition of ground-state entanglement in a Heisenberg spin chain simulated in an NMR quantum computer
X. Peng, J. Du and D. Suter
Phys. Rev. A 71, 012307 (2005)
AbstractUsing an NMR quantum computer, we experimentally simulate the quantum phase transition of a Heisenberg spin chain. The Hamiltonian is generated by a multiple-pulse sequence, the nuclear-spin system is prepared in its (pseudopure) ground state, and the effective Hamiltonian varied in such a way that the Heisenberg chain is taken from a product state to an entangled state, and finally to a different product state. URL: http://link.aps.org/abstract/PRA/v71/e012307 doi:10.1103/PhysRevA.71.012307 PACS: 03.67.Hk, 03.65.Ud, 05.70.Jk

[102] Anisotropic effective exciton mass in Cu2O
G. Dasbach, D. Froehlich, H. Stolz, R. Klieber, D. Suter and M. Bayer
Phys. Stat. Sol. C 2, 886-889 (2005)
AbstractThe degeneracy of the yellow 1S orthoexcitons in Cu2O is lifted by wave vector dependent electron-hole exchange. This interaction scales quadratically with k and strongly depends on the direction of the wave vector. Therefore it is interpreted as an anisotropic correction of the effective exciton mass. This can explain the discrepancies in the measurements of the exciton mass in Cu2O reported so far. PACS 71.35.Cc, 78.20.-c, 78.40.Fy

[103] High resolution spectroscopy of yellow 1S excitons in Cu2O
D. Froehlich, G. Dasbacha, G. B. H. v. Hoegersthal, M. Bayer, R. Klieber, D. Suter and H. Stolz
Sol. State Comm. 134, 139-142 (2005)
AbstractThe 1S ortho- and para-exciton of the yellow series in Cu2O are studied by high resolution spectroscopy. We observe that the 3-fold ortho-exciton (?5+-symmetry) is not degenerate. For a general k-direction there are three resonances with line width down to 1 ?eV. The splitting is quantitatively explained by taking into account the long range and short range exchange interaction. The group velocity of the quadrupole polariton is directly determined with use of 20 ns pulses with still high spectral resolution (?E=0.1 ?eV). Group velocities down to 4*104 m/s are quantitatively described by a one-oscillator quadrupole polariton model. The para-exciton (?2+-symmetry) is seen in absorption in magnetic fields down to 1 T. In selected crystals it shows a line width of 0.2 ?eV. Keywords: A. Cuprous oxide; D. Optical properties; D. Bose-Einstein condensation; D. Polaritons PACS: 78.20-e; 71.35.Cc; 71.36.+c; 78.40.Fy

[104] Quantum and classical parallelism in parity algorithms for ensemble quantum computers
R. Stadelhofer, D. Suter and W. Banzhaf
Phys. Rev. A 71, 032345 (2005)
AbstractThe determination of the parity of a string of N binary digits is a well-known problem in classical as well as quantum information processing, which can be formulated as an oracle problem. It has been established that quantum algorithms require at least N/2 oracle calls. We present an algorithm that reaches this lower bound and is also optimal in terms of additional gate operations required. We discuss its application to pure and mixed states. Since it can be applied directly to thermal states, it does not suffer from signal loss associated with pseudo-pure-state preparation. For ensemble quantum computers, the number of oracle calls can be further reduced by a factor 2k , with k epsilon 1,2,..., log2 (N/2) , provided the signal-to-noise ratio is sufficiently high. This additional speed-up is linked to (classical) parallelism of the ensemble quantum computer. Experimental realizations are demonstrated on a liquid-state NMR quantum computer. URL: http://link.aps.org/abstract/PRA/v71/e032345 DOI: 10.1103/PhysRevA.71.032345 PACS: 03.67.Lx

[105] Light-induced Knight shifts in GaAs/AlGaAs quantum wells
M. Eickhoff, S. Fustmann and D. Suter
Phys. Rev. B 71, 195332 (2005)
AbstractThe coupling between quantum-confined electron spins in semiconductor heterostructures and nuclear spins dominates the dephasing of spin qubits in III/V semiconductors. The interaction can be measured through the electron-spin dynamics or through its effect on the nuclear spin. Here, we discuss the resulting shift of the NMR frequency sthe Knight shiftd and measure its size as a function of the charge-carrier density for photoexcited charge carriers in a GaAs quantum well. DOI: 10.1103/PhysRevB.71.195332 PACS numberssd: 76.60.Cq, 76.70.Hb, 78.66.Fd

[106] Experimental demonstration of efficient and selective population transfer and qubit distillation in a rare-earth-metal-ion-doped crystal
L. Rippe, M. Nilsson, S. Kröll, R. Klieber and D. Suter
Phys. Rev. A 71, 062328 (2005)
AbstractIn optically controlled quantum computers it may be favorable to address different qubits using light with different frequencies, since the optical diffraction does not then limit the distance between qubits. Using qubits that are close to each other enables qubit-qubit interactions and gate operations that are strong and fast in comparison to qubit-environment interactions and decoherence rates. However, as qubits are addressed in frequency space, great care has to be taken when designing the laser pulses, so that they perform the desired operation on one qubit, without affecting other qubits. Complex hyperbolic secant pulses have theoretically been shown to be excellent for such frequency-addressed quantum computing [I. Roos and K. Molmer, Phys. Rev. A 69, 022321 (2004)]--e.g., for use in quantum computers based on optical interactions in rare-earth-metal-ion-doped crystals. The optical transition lines of the rare-earth-metal-ions are inhomogeneously broadened and therefore the frequency of the excitation pulses can be used to selectively address qubit ions that are spatially separated by a distance much less than a wavelength. Here, frequency-selective transfer of qubit ions between qubit states using complex hyperbolic secant pulses is experimentally demonstrated. Transfer efficiencies better than 90% were obtained. Using the complex hyperbolic secant pulses it was also possible to create two groups of ions, absorbing at specific frequencies, where 85% of the ions at one of the frequencies was shifted out of resonance with the field when ions in the other frequency group were excited. This procedure of selecting interacting ions, called qubit distillation, was carried out in preparation for two-qubit gate operations in the rare-earth-metal-ion-doped crystals. The techniques for frequency-selective state-to-state transfer developed here may be also useful also for other quantum optics and quantum information experiments in these long-coherence-time solid-state systems. URL: http://link.aps.org/abstract/PRA/v71/e062328 DOI: 10.1103/PhysRevA.71.062328 PACS: 03.67.Lx, 42.50.Md

[107] Correlating NQR transitions of ground and excited electronical states
R. Klieber and D. Suter
Phys. Rev. B 71, 224418 (2005)
AbstractNuclear spins can be used as probes of electronic charge distribution by measuring frequencies of NMR transitions. Spins with I> 1 / 2 are particularly sensitive to the charge distribution through the nuclear quadrupole coupling. While classical magnetic resonance experiments monitor only electronic ground states, it is possible to investigate also electronically excited states by using optical-radio frequency double resonance techniques. We use Raman heterodyne spectroscopy to directly correlate NMR transitions from an electronically excited state to the corresponding transitions in the electronic ground state of the same system in a time-resolved two-dimensional experiment. URL: http://link.aps.org/abstract/PRB/v71/e224418 DOI: 10.1103/PhysRevB.71.224418 PACS: 76.30.Kg, 76.70.-r, 76.60.-k

[108] Planar microresonators for EPR experiments
R. Narkowicz, D. Suter and R. Stonies
J. Magn. Reson. 175, 275-284 (2005)
AbstractEPR resonators on the basis of standing-wave cavities are optimised for large samples. For small samples it is possible to design different resonators that have much better power handling properties and higher sensitivity. Other parameters being equal, the sensitivity of the resonator can be increased by minimising its size and thus increasing the filling factor. Like in NMR, it is possible to use lumped elements; coils can confine the microwave field to volumes that are much smaller than the wavelength. We discuss the design and evaluation of EPR resonators on the basis of planar microcoils. Our test resonators, which operate at a frequency of 14 GHz, have excellent microwave efficiency factors, achieving 24 ns p/2 EPR pulses with an input power of 17 mW. The sensitivity tests with DPPH samples resulted in the sensitivity value 2.3 . 109 spins AE G 1Hz 1/2 at 300 K. Keywords: Small-volume EPR; Planar microresonators; Signal-to-noise ratio; Sensitivity; Microwave efficiency factor

[109] Efficient implementations of the quantum Fourier transform: an experimental perspective
K. Dorai and D. Suter
Int. J. Quant. Inf. 3, 413-424 (2005)

[110] Autler-Townes effect in a strongly driven electromagnetically induced transparency resonance
L. Yang, L. Zhang, X. Li, L. Han, G. Fu, N. B. Manson, D. Suter and C. Wei
Phys. Rev. A 72, 053801 (2005)
AbstractIn this paper we study the nonlinear behavior of an electromagnetically induced transparency EIT resonance subject to a coherent driving field. The EIT is associated with a three-level system where two hyperfine levels within an electronic ground state are coupled to a common excited state level by a coupling field and a probe field. In addition there is an radio-frequency rf field driving a hyperfine transition within the ground state. The paper contrasts two different situations. In one case the rf-driven transition shares a common level with the probed transition and in the second case it shares a common level with the coupled transition. In both cases the EIT resonance is split into a doublet and the characteristics of the EIT doublet are determined by the strength and frequency of the rf-driving field. The doublet splitting originates from the rf-field induced dynamic Stark effect and has close analogy with the Autler-Townes effect observed in three-level pump-probe spectroscopy study. The situation changes when the rf field is strong and the two cases are very different. One is analogous to two three-level systems with EIT resonance associated with each. The other corresponds to a doubly driven three-level system with rf-field-induced electromagnetically induced absorption resonance. The two situations are modeled using numerical solutions of the relevant equation of motion of density matrix. In addition a physical account of their behaviors is given in terms of a dressed state picture. DOI: 10.1103/PhysRevA.72.053801 PACS number s : 42.50.Gy, 42.62.Fi

[111] Quantification of complementarity in multiqubit systems
X. Peng, X. Zhu, D. Suter, J. Du, M. Liu and K. Gao
Phys. Rev. A 72, 052109 (2005)
AbstractComplementarity was originally introduced as a qualitative concept for the discussion of properties of quantum mechanical objects that are classically incompatible. More recently, complementarity has become a quantitative relation between classically incompatible properties, such as the visibility of interference fringes and ''which-way'' information, but also between purely quantum mechanical properties, such as measures of entanglement. We discuss different complementarity relations for systems of two-, three-, or n qubits. Using nuclear magnetic resonance techniques, we have experimentally verified some of these complementarity relations in a two-qubit system. URL: http://link.aps.org/abstract/PRA/v72/e052109 doi:10.1103/PhysRevA.72.052109 PACS: 03.65.Ta, 03.65.Ud, 76.60.-k

[112] Quantum information processing and communication
P. Zoller, Th. Beth, D. Binosi, R. Blatt, H. Briegel, D. Bruss, T. Calarco, J. I. Cirac, D. Deutsch, J. Eisert, A. Ekert, C. Fabre, N. Gisin, P. Grangiere, M. Grassl, S. Haroche, A. Imamoglu, A. Karlson, J. Kempe, L. Kouwenhoven, S. Kröll, G. Leuchs, M. Lewenstein, D. Loss, N. Lütkenhaus, S. Massar, J. E. Mooij, M. B. Plenio, E. Polzik, S. Popescu, G. Rempe, A. Sergienko, D. Suter, J. Twamley, G. Wendin, R. Werner, A. Winter, J. Wrachtrup and A. Zeilinger
Europ. Phys. J. D 36, 203-228 (2005)

[113] Time-resolved coherent double Raman detection of nuclear spin transitions
R. Klieber and D. Suter
Phys. Rev. B 73, 094408 (2006)
AbstractCoherent Raman scattering can be used for detection of nuclear spin transitions in solids and atomic vapors if both nuclear spin states of the spin transition to be detected are connected to a single nuclear spin state of a different electronic state by allowed optical transitions. This is not the case in crystals with high symmetry. Here, we introduce the coherent double Raman experiment, where the difference between nuclear spin transitions in two different electronic states is observed. In contrast to the conventional Raman scattering experiment, this scheme is applicable also to systems with high symmetry, where the nuclear spin does not change during an optical transition. URL: http://link.aps.org/abstract/PRB/v73/e094408 doi:10.1103/PhysRevB.73.094408 PACS: 76.70.Hb, 76.30.Kg, 32.10.Fn

[114] Adding salt to an aqueous solution of t-butanol: Is hydrophobic association enhanced or reduced?
D. Paschek, A. Geiger, M. J. Herve and D. Suter
J. Chem. Phys. 124, 154508 (2006)
AbstractRecent neutron scattering experiments on aqueous salt solutions of amphiphilic t-butanol by Bowron and Finney Phys. Rev. Lett. 89, 215508 2002 ; J. Chem. Phys. 118, 8357 2003 suggest the formation of t-butanol pairs, bridged by a chloride ion via O-H'Cl- hydrogen bonds, leading to a reduced number of intermolecular hydrophobic butanol-butanol contacts. Here we present a joint experimental/theoretical study on the same system, using a combination of molecular dynamics MD simulations and nuclear magnetic relaxation measurements. Both MD simulation and experiment clearly support the more classical scenario of an enhanced number of hydrophobic contacts in the presence of salt, as it would be expected for purely hydrophobic solutes. T. Ghosh et al., J. Phys. Chem. B 107, 612 2003 . Although our conclusions arrive at a structurally completely distinct scenario, the molecular dynamics simulation results are within the experimental error bars of the Bowron and Finney data. DOI: 10.1063/1.2188398

[115] Laser-assisted magnetic resonance: principles and applications
D. Suter and J. Gutschank
Lect. Notes Phys. 684, 115-141 (2006)

[116] Speedup of quantum-state transfer by three-qubit interactions: Implementation by nuclear magnetic resonance
J. Zhang, X. Peng and D. Suter
Phys. Rev. A 73, 062325 (2006)
AbstractUniversal quantum information processing requires single-qubit rotations and two-qubit interactions as minimal resources. A possible step beyond this minimal scheme is the use of three-qubit interactions. We consider such three-qubit interactions and show how they can reduce the time required for a quantum state transfer in an XY spin chain. For the experimental implementation, we use liquid-state nuclear magnetic resonance, where three-qubit interactions can be implemented by sequences of radio-frequency pulses. DOI: 10.1103/PhysRevA.73.062325 PACS number s : 03.67.Lx

[117] Reduced Decoherence in Large Quantum Registers
H. G. Krojanski and D. Suter
Phys. Rev. Lett. 97, 150503 (2006)
AbstractAmong the biggest obstacles for building larger (and thus more powerful) quantum-information processors is decoherence, the decay of quantum-information by the coupling between the quantum register and its environment. Procedures for reducing decoherence processes will be essential for successful operation of larger quantum processors. We study model quantum registers consisting of up to 4900 qubits and measure their decay as a function of the register size. We demonstrate that appropriate sequences of qubit rotations reduce the coupling between system and environment for all sizes of the quantum register, thus preserving the quantum-information 50 times longer than without decoupling.

[118] Quantum-information processing using strongly dipolar coupled nuclear spins
T. S. Mahesh and D. Suter
Phys. Rev. A 74, 062312 (2006)
AbstractDipolar coupled homonuclear spins present challenging, yet useful systems for quantum-information processing. In such systems, the eigenbasis of the system Hamiltonian is the appropriate computational basis and coherent control can be achieved by specially designed strongly modulating pulses. In this paper we describe the first experimental implementation of the quantum algorithm for numerical gradient estimation by nuclear magnetic resonance, using the eigenbasis of a four spin system.

[119] Decoherence in large NMR quantum registers
H. G. Krojanski and D. Suter
Phys. Rev. A 74, 062319 (2006)
AbstractDecoherence causes the decay of the quantum information that is stored in highly correlated states during quantum computation. It is thus a limiting factor for all implementations of a quantum computer. Because a scalable quantum computer with hundreds or thousands of qubits is not available yet, experimental data about decoherence rates was restricted to small quantum registers. With solid state nuclear magnetic resonance we create highly correlated multiqubit states that serve as a model quantum register and measure their decay. By measuring the decay as a function of the system size, we determined the scaling of the decoherence rate with the number of qubits. Using the same system, we also used decoupling techniques to reduce the coupling between system and environment and thereby the decoherence rate by more than an order of magnitude, independent of the system size. For the free decay as well as for the decoupled case, we found a relatively weak scaling with system size, which could be fitted to a power law p with an exponent p1/2. This raises the prospect for large-scale quantum computation.

[120] Experimental realization of 1->2 asymmetric phase-covariant quantum cloning
H. Chen, X. Zhou, D. Suter and J. Du
Phys. Rev. A 75, 012317 (2007)
AbstractWhile exact cloning of an unknown quantum state is prohibited by the linearity of quantum mechanics, approximate cloning is possible and has been used, e.g., to derive limits on the security of quantum communication protocols. In the case of asymmetric cloning, the information from the input state is distributed asymmetrically between the different output states. Here, we consider asymmetric phase-covariant cloning, where the goal is to optimally transfer the phase information from a single input qubit to different output qubits. We construct an optimal quantum cloning machine for two qubits that does not require ancilla qubits and implement it on an NMR quantum information processor. DOI: 10.1103/PhysRevA.75.012317 PACS number s : 03.67.Dd, 76.60. k

[121] Two-qubit gates between noninteracting qubits in endohedral-fullerene-based quantum computation
C. Ju, D. Suter and J. Du
Phys. Rev. A 75, 012318 (2007)
AbstractWe propose a scheme to implement the two-qubit gates between the nuclear spins of the encapsulated atoms in endohedral fullerenes 15N@C60 or 31P@C60, within today's magnetic resonance techniques. Since there is no interaction between the nuclear spins, this scheme employs the electronic spins as medium and two swap operations are proposed to transfer the information between the nuclear spin 1 2 and the electronic spin 3 2 , and between two electronic spins 3 2 . These two-qubit gates, along with the single-qubit rotation gates, compose a universal set of quantum gates in fullerene-based quantum computation.

[122] Spins as Probes of Different Electronic States
D. Suter and R. Klieber
Concepts Magn. Reson. A 30A, 116-126 (2007)
AbstractNuclear spins are efficient probes of electronic states. Because most NMR experiments are performed in thermal equilibrium, they probe the electronic ground state---the only state that is significantly populated under ambient conditions. Probing electronically excited states becomes possible, if magnetic resonance techniques are combined with optical (laser) excitation. Depending on the nature of the electronic state, drastic changes of the magnetic resonance parameters may be observed. We discuss the basic principles of this type of investigation. Depending on the lifetime of the electronically excited state, it is possible to measure separate spectra of ground and excited state if the lifetime is long on the NMR timescale, or an averaged spectrum if the lifetime is short. We present examples for both limiting cases using rare earth ions and semiconductor heterostructures.

[123] Decoherence in large quantum registers under variable interaction with the environment
M. Lovric, H. G. Krojanski and D. Suter
Phys. Rev. A 75, 042305 (2007)
AbstractEffective quantum-information processing requires coherent control of large numbers of qubits on a time scale that is short compared to the decoherence time of the system. It is therefore important to extrapolate and measure decoherence times for large quantum registers and to determine the effect of different couplings between system and environment on the decoherence rate. For this purpose, we have experimentally realized a system that allows one to generate model quantum registers with more than 100 qubits and measure the decay of the information in these states while adjusting the strength of the interaction between the quantum register and the environment. Our results indicate a power-law dependence of the decoherence rate on the number of qubits in the system, with an exponent of the order of 0.5. This behavior remains qualitatively unchanged when the coupling strength to the environment is reduced by about an order of magnitude.

[124] Effect of different sodium halides on the self-association of tertiary butanol in water
H. M. Jeufack and D. Suter
J. Chem. Phys. 126, 144501 (2007)
AbstractThe molecular self-association of hydrophobic substances is an important process for many biological systems. Here, the authors study the effect of salt on the molecular self-association of t-butanol in water solution, using NMR techniques. They compare the effects of different sodium halides (NaCl, NaBr, and NaI) as a function of their concentration.

[125] Effect of system level structure and spectral distribution of the environment on the decoherence rate
J. Zhang, X. Peng, N. Rajendran and D. Suter
Phys. Rev. A 75, 042314 (2007)
AbstractMinimizing the effect of decoherence on a quantum register must be a central part of any strategy to realize scalable quantum-information processing. Apart from the strength of the coupling to the environment, the decoherence rate is determined by the system level structure and by the spectral composition of the noise trace that the environment generates. Here, we discuss a relatively simple model that allows us to study these different effects quantitatively in detail. We evaluate the effect that the perturbation has on a nuclear magnetic resonance system while it performs a Grover search algorithm.

[126] Factorizing numbers with the Gauss sum technique: NMR implementations
T. S. Mahesh, N. Rajendran, X. Peng and D. Suter
Phys. Rev. A 75, 062303 (2007)
AbstractSeveral physics-based algorithms for factorizing large numbers were recently presented. A notable recent algorthm by Schleich et al. uses Gauss sums for distinguishing between factors and nonfactors. We demonstrate two NMR techniques that evaluate Gauss sums and thus implement their algorithm. The first one is based on differential excitation of a single spin magnetization by a cascade of rf pulses. The second method is based on spatial averaging and selective refocusing of magnetization for Gauss sums corresponding to factors. All factors of 16 637 and 52 882 363 are successfully obtained.

[127] Iterative quantum-state transfer along a chain of nuclear spin qubits
J. Zhang, N. Rajendran, X. Peng and D. Suter
Phys. Rev. A 76, 012317 (2007)
AbstractTransferring quantum information between two qubits is a basic requirement for many applications in quantum communication and quantum-information processing. In the iterative quantum-state transfer proposed by Burgarth et al. [Phys. Rev. A 75, 062327 (2007)], this is achieved by a static spin chain and a sequence of gate operations applied only to the receiving end of the chain. The only requirement on the spin chain is that it transfers a finite part of the input amplitude to the end of the chain, where the gate operations accumulate the information. For an appropriate sequence of evolutions and gate operations, the fidelity of the transfer can asymptotically approach unity. We demonstrate the principle of operation of this transfer scheme by implementing it in a nuclear magnetic resonance quantum-information processor.

[128] Evolution of Athletic Records: Statistical Effects versus Real Improvements
D. Gembris, J. G. Taylor and D. Suter
J. Appl. Stat. 34, 529-545 (2007)
AbstractAthletic records represent the best results in a given discipline, thus improving monotonically with time. As has already been shown, this should not be taken as an indication that the athletes' capabilities keep improving. In other words, a new record is not noteworthy just because it is a new record, instead it is necessary to assess by how much the record has improved. In this paper we derive formulae that can be used to show that athletic records continue to improve with time, even if athletic performance remains constant. We are considering two specific examples, the German championships and the world records in several athletic disciplines. The analysis shows that, for the latter, true improvements occur in 20--50% of the disciplines. The analysis is supplemented by an application of our record estimation approach to the prediction of the maximum body length of humans for a specified size of a population respectively population group from a representative sample.

[129] Measuring complete quantum states with a single observable
X. Peng, J. Du and D. Suter
Phys. Rev. A 76, 042117 (2007)
AbstractExperimental determination of an unknown quantum state usually requires several incompatible measurements. However, it is also possible to determine the full quantum state from a single, repeated measurement. For this purpose, the quantum system whose state is to be determined is first coupled to a second quantum system (the ``assistant'') in such a way that part of the information in the quantum state is transferred to the assistant. The actual measurement is then performed on the enlarged system including the original system and the assistant. We discuss in detail the requirements of this procedure and experimentally implement it on a simple quantum system consisting of nuclear spins.

[130] Experimental observation of a topological phase in the maximally entangled state of a pair of qubits
J. Du, J. Zhu, M. Shi, X. Peng and D. Suter
Phys. Rev. A 76, 042121 (2007)
AbstractQuantum mechanical phase factors can be related to dynamical effects or to the geometrical properties of a trajectory in a given space---either parameter space or Hilbert space. Here, we experimentally investigate a quantum mechanical phase factor that reflects the topology of the SO(3) group: since rotations by around antiparallel axes are identical, this space is doubly connected. Using a pair of nuclear spins in a maximally entangled state, we subject one of the spins to a cyclic evolution. If the corresponding trajectory in SO(3) can be smoothly deformed to a point, the quantum state at the end of the trajectory is identical to the initial state. For all other trajectories the quantum state changes sign.

[131] Effect of cholesterol on diffusion in surfactant bilayers
T. Pieper, S. Markova, M. Kinjo and D. Suter
J. Chem. Phys. 127, 165102 (2007)
AbstractBiological membranes consist of lipid bilayers with liquid-ordered and liquid-disordered phases. It is believed that cholesterol controls the size of the microdomains in the liquid-ordered phase and thereby affects the mobility as well as the permeability of the membrane. We study this process in a model system consisting of the nonionic surfactant C12E5 and water in the lamellar phase. We measure the diffusion of fluorescent probe molecules (rhodamine B) by fluorescence correlation spectroscopy. For different surfactant to water ratios, we measure how the molecular mobility varies with the amount of cholesterol added. We find that a reduction of the diffusion coefficient is already detectable at a molar ratio of 8 mol % cholesterol.

[132] Quantum Computing: A Short Course from Theory to Experiment
J. Stolze and D. Suter
Wiley-VCH, Berlin (2008)
ISBN 978-3-527-40787-3

[133] Spins as qubits: Quantum information processing by nuclear magnetic resonance
D. Suter and T. S. Mahesh
J. Chem. Phys. 128, 052206 (2008)
AbstractStoring information in quantum mechanical degrees of freedom and processing it by unitary transformation promises a new class of computers that can efficiently solve problems for which no efficient classical algorithms are known. The most straightforward implementation of this type of information processing uses nuclear spins to store the information and nuclear magnetic resonance for processing it. We discuss the basics of quantum information processing by NMR, with an emphasis on two fields of research: the design and implementation of robust logical gate operations and the loss of quantum information, which is known as decoherence.

[134] Detection of Quantum Critical Points by a Probe Qubit
J. Zhang, X. Peng, N. Rajendran and D. Suter
Phys. Rev. Lett. 100, 100501 (2008)

[135] Quantitative complementarity between local and nonlocal character of quantum states in a three-qubit system
X. Peng, J. Zhang, J. Du and D. Suter
Phys. Rev. A 77, 052107 (2008)
AbstractLocal or nonlocal character of quantum states can be quantified and is subject to various bounds that can be formulated as complementarity relations. Here, we investigate the local vs nonlocal character of pure three-qubit states by a four-way interferometer. The complete entanglement in the system can be measured as the entanglement of a specific qubit with the subsystem consisting of the other two qubits. The quantitative complementarity relations are verified experimentally in an NMR quantum information processor.

[136] Luminescence upconversion in GaAs quantum wells
S. Eshlaghi, W. Worthoff, A. D. Wieck and D. Suter
Phys. Rev. B 77, 245317 (2008)
AbstractPhotoluminescence upconversion removes energy from the medium. It has been applied extensively for laser cooling of atomic vapors and more recently also to condensed matter, such as dye solutions and glasses. The application to semiconductors has also been proposed, but so far, clear evidence is missing. We present a detailed experimental study of photoluminescence upconversion in GaAs quantum wells. We study the conversion efficiency over a wide temperature range as a function of the laser detuning. The best results are obtained when the laser detuning is comparable to the thermal energy, 2kBT.

[137] Evolving blackbox quantum algorithms using genetic programming
R. Stadelhofer, W. Banzhaf and D. Suter
AI EDAM 22, 285-297 (2008)
AbstractAlthough it is known that quantum computers can solve certain computational problems exponentially faster than classical computers, only a small number of quantum algorithms have been developed so far. Designing such algorithms is complicated by the rather nonintuitive character of quantum physics. In this paper we present a genetic programming system that uses some new techniques to develop and improve quantum algorithms. We have used this system to develop two formerly unknown quantum algorithms. We also address a potential deficiency of the quantum decision tree model used to prove lower bounds on the query complexity of the parity problem.

[138] Scaling of sensitivity and efficiency in planar microresonators for electron spin resonance
R. Narkowicz, D. Suter and I. Niemeyer
Rev. Sci. Instrum. 79, 084702 (2008)
AbstractElectron spin resonance (ESR) of volume-limited samples or nanostructured materials can be made significantly more efficient by using microresonators whose size matches that of the structures under investigation. We describe a series of planar microresonators that show large improvements over conventional ESR resonators in terms of microwave conversion efficiency (microwave field strength for a given input power) and sensitivity (minimum number of detectable spins). We explore the dependence of these parameters on the size of the resonator and find that both scale almost linearly with the inverse of the resonator size. Scaling down the loops of the planar microresonators from 500 down to 20 um improves the microwave efficiency and the sensitivity of these structures by more than an order of magnitude and reduces the microwave power requirements by more than two orders of magnitude.

[139] Experimental Study of the Validity of Quantitative Conditions in the Quantum Adiabatic Theorem
J. Du, L. Hu, Y. Wang, J. Wu, M. Zhao and D. Suter
Phys. Rev. Lett. 101, 060403 (2008)
AbstractThe quantum adiabatic theorem plays an important role in quantum mechanics. However, counterexamples were produced recently, indicating that their transition probabilities do not converge as predicted by the adiabatic theorem [K. P. Marzlin et al., Phys. Rev. Lett. 93, 160408 (2004); D. M. Tong et al., Phys. Rev. Lett. 95, 110407 (2005)]. For a special class of Hamiltonians, we examine the standard criterion for adiabatic evolution experimentally and theoretically, as well as three newly suggested adiabatic conditions. We show that the standard criterion is neither sufficient nor necessary.

[140] NMR implementation of factoring large numbers with Gauss sums: Suppression of ghost factors
X. Peng and D. Suter
Europhys. Lett. 84, 40006 (5pp) (2008)
AbstractFinding the factors of an integer can be achieved by various experimental techniques, based on an algorithm developed by Schleich et al. (Fortschr. Phys.,~54 (2006) 856), which uses specific properties of Gau\ss{}~sums. Experimental limitations usually require truncation of these series, but if the truncation parameter is too small, it is no longer possible to distinguish between factors and so-called "ghost" factors. Here, we discuss two techniques for distinguishing between true factors and ghost factors while keeping the number of terms in the sum constant or only slowly increasing. We experimentally test these modified algorithms in a nuclear spin system, using NMR.

[141] Quantum Adiabatic Algorithm for Factorization and Its Experimental Implementation
X. Peng, Z. Liao, N. Xu, G. Qin, X. Zhou, D. Suter and J. Du
Phys. Rev. Lett. 101, 220405 (2008)
AbstractWe propose an adiabatic quantum algorithm capable of factorizing numbers, using fewer qubits than Shor's algorithm. We implement the algorithm in a NMR quantum information processor and experimentally factorize the number 21. In the range that our classical computer could simulate, the quantum adiabatic algorithm works well, providing evidence that the running time of this algorithm scales polynomially with the problem size.

[142] Microslot NMR Probe for Metabolomics Studies
H. G. Krojanski, J. Lambert, Y. Gerikalan, D. Suter and R. Hergenröder
Analytical Chemistry 80, 8668-8672 (2008)
AbstractA NMR microprobe based on microstrip technology suitable for investigations of volume-limited samples in the low nanoliter range was designed. NMR spectra of sample quantities in the 100 pmol range can be obtained with this probe in a few seconds. The planar geometry of the probe is easily adaptable to the size and geometry requirements of the samples.

[143] Photoluminescence up-conversion in GaAs quantum wells
S. Eshlaghi, W. Worthoff, A. D. Wieck and D. Suter
Proc. SPIE 7228, 72280B (2009)

[144] Nuclear spin qubits in a trapped-ion quantum computer
M. Feng, Y. Y. Xu, F. Zhou and D. Suter
Phys. Rev. A 79, 052331 (2009)
AbstractPhysical systems must fulfill a number of conditions to qualify as useful quantum bits (qubits) for quantum-information processing, including ease of manipulation, long decoherence times, and high fidelity readout operations. Since these conditions are hard to satisfy with a single system, it may be necessary to combine different degrees of freedom. Here we discuss a possible system based on electronic and nuclear spin degrees of freedom in trapped ions. The nuclear spin yields long decoherence times, while the electronic spin, in a magnetic field gradient, provides efficient manipulation, and the optical transitions of the ions assure a selective and efficient initialization and readout.

[145] Probing Liquid-Liquid Interfaces with Spatially Resolved NMR Spectroscopy
J. Lambert, R. Hergenröder, D. Suter and V. Deckert
Angewandte Chemie International Edition 48, 6343-6345 (2009)
AbstractEn route to nanometer resolution: Spatially resolved NMR spectroscopy is used to directly probe liquid-liquid interfaces with exceptionally high spatial resolution in one selected dimension. The novel technique is a first step towards NMR spatial resolution in the nanometer range.

[146] Quantum Simulation of a System with Competing Two- and Three-Body Interactions
X. Peng, J. Zhang, J. Du and D. Suter
Phys. Rev. Lett. 103, 140501 (2009)
AbstractQuantum phase transitions occur at zero temperature, when the ground state of a Hamiltonian undergoes a qualitative change as a function of a control parameter. We consider a particularly interesting system with competing one-, two-, and three-body interactions. Depending on the relative strength of these interactions, the ground state of the system can be a product state, or it can exhibit genuine tripartite entanglement. We experimentally simulate such a system in a NMR quantum simulator and observe the different ground states. By adiabatically changing the strength of one coupling constant, we push the system from one ground state to a qualitatively different ground state. We show that these ground states can be distinguished and the transitions between them observed by measuring correlations between the spins or the expectation values of suitable entanglement witnesses.

[147] Spin qubits for quantum simulations
X. Peng and D. Suter
Frontiers of Physics in China 5, 1-25 (2010)
AbstractThe investigation of quantum mechanical systems mostly concentrates on single elementary particles. If we combine such particles into a composite quantum system, the number of degrees of freedom of the combined system grows exponentially with the number of particles. This is a major difficulty when we try to describe the dynamics of such a system, since the computational resources required for this task also grow exponentially. In the context of quantum information processing, this difficulty becomes the main source of power: in some situations, information processors based in quantum mechanics can process information exponentially faster than classical systems. From the perspective of a physicist, one of the most interesting applications of this type of information processing is the simulation of quantum systems. We call a quantum information processor that simulates other quantum systems a quantum simulator. This review discusses a specific type of quantum simulator, based on nuclear spin qubits, and using nuclear magnetic resonance for processing. We review the basics of quantum information processing by nuclear magnetic resonance (NMR) as well as the fundamentals of quantum simulation and describe some simple applications that can readily be realized by today's quantum computers. In particular, we discuss the simulation of quantum phase transitions: the qualitative changes that the ground states of some quantum mechanical systems exhibit when some parameters in their Hamiltonians change through some critical points. As specific examples, we consider quantum phase transitions where the relevant ground states are entangled. Chains of spins coupled by Heisenberg interactions represent an ideal system for studying these effects: depending on the type and strength of interactions, the ground states can be product states or they can be maximally entangled states representing different types of entanglement.

[148] Quantum-information-processing architecture with endohedral fullerenes in a carbon nanotube
W. L. Yang, Z. Y. Xu, H. Wei, M. Feng and D. Suter
Phys. Rev. A 81, 032303 (2010)
AbstractA potential quantum-information processor is proposed using an array of the endohedral fullerenes 15N@C60 or 31P@C60 contained in a single walled carbon nanotube (SWCNT). The qubits are encoded in the nuclear spins of the doped atoms, while the electronic spins are used for initialization and readout, as well as for two-qubit operations.

[149] Deformation of liquid-filled calcium alginate capsules in a spinning drop apparatus
S. Leick, S. Henning, P. Degen, D. Suter and H. Rehage
Physical Chemistry Chemical Physics 12, 2950--2958 (2010)
AbstractThis paper describes the mechanical properties of thin-walled, liquid-filled calcium alginate capsules by measuring the deformation of these particles in a spinning drop apparatus. By variation of the guluronic acid content of the alginate, the polymerization time and the calcium and alginate concentration we systematically studied the elastic properties of these capsules. In a series of experiments we observed for the first time new types of irreversibly deformed capsules, which can be described by plastic deformation. For comparison purposes, we also investigated liquid-filled calcium alginate particles in squeezing capsule experiments. The qualitative and quantitative results of both experiments in terms of the deformation properties and the surface Young moduli were in good agreement. Furthermore we also investigated liquid-filled calcium alginate particles by NMR microscopy to characterize the capsules in view of their membrane thickness. These results, in combination with the spinning capsule experiments allowed us to measure the kinetics of surface gelation and the mechanism of membrane growing.

[150] Experimental generation of pseudo-bound-entanglement
H. Kampermann, D. Bruß, X. Peng and D. Suter
Phys. Rev. A 81, 040304 (2010)
AbstractWe use nuclear magnetic resonance to experimentally generate a bound-entangled (more precisely: pseudo- bound-entangled) state, i.e., a quantum state which is nondistillable but nevertheless entangled. Our quantum system consists of three qubits. We characterize the produced state via state tomography to show that the created state has a positive partial transposition with respect to any bipartite splitting, and we use a witness operator to prove its pseudoentanglement.

[151] Ground-state entanglement in a system with many-body interactions
X. Peng, J. Zhang, J. Du and D. Suter
Phys. Rev. A 81, 042327 (2010)
AbstractEntanglement refers to the inability of many-body quantum mechanical systems to be separated into independent subsystems. This has been well investigated in systems consisting of two entangled subsystems. In larger systems, more complex types of entanglement exist. In a system consisting of three subsystems (e.g., qubits), it is possible that all three subsystems are entangled with each other in a way that cannot be reduced to bipartite entanglement, and it is known that two different, inequivalent forms of tripartite entanglement exist such as the GHZ and W states (GHZ denotes ``Greenberger-Horne-Zeilinger''). Here, we investigate a particularly interesting system with competing one-, two-, and three-body interactions. Its ground state can be a product state, a GHZ state, or a W state, depending on the type and strength of the spin-spin couplings. By varying an external control parameter, the system can be made to undergo quantum transitions between the various ground-state-entanglement phases. We implement the system in an NMR quantum simulator and use adiabatic evolution of the effective Hamiltonian to drive the system through the quantum transitions. In the experimental and numerical simulations, we check the suitability of different observables for making the quantum transitions visible and for characterizing the different phases.

[152] NMR Quantum Simulation of Localization Effects Induced by Decoherence
G. A. 'Alvarez and D. Suter
Phys. Rev. Lett. 104, 230403 (2010)
AbstractThe loss of coherence in quantum mechanical superposition states limits the time for which quantum information remains useful. Similarly, it limits the distance over which quantum information can be transmitted. Here, we investigate in a nuclear spin-based quantum simulator, the localization of the size of spin clusters that are generated by a Hamiltonian driving the transmission of information, while a variable-strength perturbation counteracts the spreading. We find that the system reaches a dynamic equilibrium size, which decreases with the square of the perturbation strength.

[153] Liquid-State NMR Quantum Computing
L. M. K. Vandersypen, I. L. Chuang and D. Suter
Editors: D. M. Grant and R. K. Harris
John Wiley & Sons, Chichester, 1-2 (2010)

[154] Performance comparison of dynamical decoupling sequences for a qubit in a rapidly fluctuating spin bath
G. A. 'Alvarez, A. Ajoy, X. Peng and D. Suter
Phys. Rev. A 82, 042306 (2010)
AbstractAvoiding the loss of coherence of quantum mechanical states is an important prerequisite for quantum information processing. Dynamical decoupling (DD) is one of the most effective experimental methods for maintaining coherence, especially when one can access only the qubit system and not its environment (bath). It involves the application of pulses to the system whose net effect is a reversal of the system-environment interaction. In any real system, however, the environment is not static, and therefore the reversal of the system-environment interaction becomes imperfect if the spacing between refocusing pulses becomes comparable to or longer than the correlation time of the environment. The efficiency of the refocusing improves therefore if the spacing between the pulses is reduced. Here, we quantify the efficiency of different DD sequences in preserving different quantum states. We use 13C nuclear spins as qubits and an environment of 1H nuclear spins as the environment, which couples to the qubit via magnetic dipole-dipole couplings. Strong dipole-dipole couplings between the proton spins result in a rapidly fluctuating environment with a correlation time of the order of 100 us. Our experimental results show that short delays between the pulses yield better performance if they are compared with the bath correlation time. However, as the pulse spacing becomes shorter than the bath correlation time, an optimum is reached. For even shorter delays, the pulse imperfections dominate over the decoherence losses and cause the quantum state to decay.

[155] Observation of the Ground-State Geometric Phase in a Heisenberg XY Model
X. Peng, S. Wu, J. Li, D. Suter and J. Du
Phys. Rev. Lett. 105, 240405 (2010)
AbstractGeometric phases play a central role in a variety of quantum phenomena, especially in condensed matter physics. Recently, it was shown that this fundamental concept exhibits a connection to quantum phase transitions where the system undergoes a qualitative change in the ground state when a control parameter in its Hamiltonian is varied. Here we report the first experimental study using the geometric phase as a topological test of quantum transitions of the ground state in a Heisenberg XY spin model. Using NMR interferometry, we measure the geometric phase for different adiabatic circuits that do not pass through points of degeneracy.

[156] Mechanical properties of liquid-filled shellac composite capsules
S. Leick, M. Kott, P. Degen, S. Henning, T. Pasler, D. Suter and H. Rehage
Phys. Chem. Chem. Phys. 13, 2765-2773 (2011)
AbstractThis paper describes the mechanical properties of thin-walled{,} liquid-filled composite capsules consisting of calcium pectinate and shellac. In a series of experiments we measured the deformation of these particles in a spinning drop apparatus. For different pH-values we studied the elastic properties of these particles and compared the obtained results with the mechanical response measured by squeezing capsule experiments. In analogy to these experiments{,} we also investigated liquid-filled unloaded calcium pectinate capsules without the addition of shellac. The deformation properties of these experiments and the surface Young moduli were in good agreement. Furthermore we investigated the liquid-filled calcium pectinate and the composite capsules by NMR microscopy. These experiments allowed investigations of the membrane thickness and the kinetics of membrane growing. Additional characterizations by stress controlled small amplitude surface shear experiments of similar composed gel layers provided coherent results for the surface Young modulus.

[157] An endohedral fullerene-based nuclear spin quantum computer
C. Ju, D. Suter and J. Du
Physics Letters A 375, 1441 - 1444 (2011)
AbstractWe propose a new scalable quantum computer architecture based on endohedral fullerene molecules. Qubits are encoded in the nuclear spins of the endohedral atoms, which posses even longer coherence times than the electron spins which are used as the qubits in previous proposals. To address the individual qubits, we use the hyperfine interaction, which distinguishes two modes (active and passive) of the nuclear spin. Two-qubit quantum gates are effectively implemented by employing the electronic dipolar interaction between adjacent molecules. The electron spins also assist in the qubit initialization and readout. Our architecture should be significantly easier to implement than earlier proposals for spin-based quantum computers, such as the concept of Kane [B.E. Kane, Nature 393 (1998) 133].

[158] Optimal pulse spacing for dynamical decoupling in the presence of a purely dephasing spin bath
A. Ajoy, G. A. 'Alvarez and D. Suter
Phys. Rev. A 83, 032303 (2011)
AbstractMaintaining quantum coherence is a crucial requirement for quantum computation; hence protecting quantum systems against their irreversible corruption due to environmental noise is an important open problem. Dynamical decoupling (DD) is an effective method for reducing decoherence with a low control overhead. It also plays an important role in quantum metrology, where, for instance, it is employed in multiparameter estimation. While a sequence of equidistant control pulses [the Carr-Purcell-Meiboom-Gill (CPMG) sequence] has been ubiquitously used for decoupling, Uhrig recently proposed that a nonequidistant pulse sequence [the Uhrig dynamic decoupling (UDD) sequence] may enhance DD performance, especially for systems where the spectral density of the environment has a sharp frequency cutoff. On the other hand, equidistant sequences outperform UDD for soft cutoffs. The relative advantage provided by UDD for intermediate regimes is not clear. In this paper, we analyze the relative DD performance in this regime experimentally, using solid-state nuclear magnetic resonance. Our system qubits are 13C nuclear spins and the environment consists of a 1H nuclear spin bath whose spectral density is close to a normal (Gaussian) distribution. We find that in the presence of such a bath, the CPMG sequence outperforms the UDD sequence. An analogy between dynamical decoupling and interference effects in optics provides an intuitive explanation as to why the CPMG sequence performs better than any nonequidistant DD sequence in the presence of this kind of environmental noise.

[159] Experimental demonstration of a unified framework for mixed-state geometric phases
J. Zhu, M. Shi, V. Vedral, X. Peng, D. Suter and J. Du
EuroPhys. Lett. 94, 20007 (2011)
AbstractGeometric phases have been found in every major branch of physics and play an important role in mathematics and quantum computation. Here, we unify two proposed definitions of the geometric phase in mixed states --Uhlmann's phase and Sjoeqvist's phase-- in a new formalism based on interferometry and further provide an experimental demonstration in NMR. This is also the first experimental measurement of Uhlmann's geometric phase.

[160] Robust Dynamical Decoupling for Quantum Computing and Quantum Memory
A. M. Souza, G. A. 'Alvarez and D. Suter
Phys. Rev. Lett. 106, 240501 (2011)
AbstractDynamical decoupling (DD) is a popular technique for protecting qubits from the environment. However, unless special care is taken, experimental errors in the control pulses used in this technique can destroy the quantum information instead of preserving it. Here, we investigate techniques for making DD sequences robust against different types of experimental errors while retaining good decoupling efficiency in a fluctuating environment. We present experimental data from solid-state nuclear spin qubits and introduce a new DD sequence that is suitable for quantum computing and quantum memory.

[161] Visualization of spin dynamics in single nanosized magnetic elements
A. Banholzer, R. Narkowicz, C. Hassel, R. Meckenstock, S. Stienen, O. Posth, D. Suter, M. Farle and J. Lindner
Nanotechnology 22, 295713 (2011)
AbstractThe design of future spintronic devices requires a quantitative understanding of the microscopic linear and nonlinear spin relaxation processes governing the magnetization reversal in nanometer-scale ferromagnetic systems. Ferromagnetic resonance is the method of choice for a quantitative analysis of relaxation rates, magnetic anisotropy and susceptibility in a single experiment. The approach offers the possibility of coherent control and manipulation of nanoscaled structures by microwave irradiation. Here, we analyze the different excitation modes in a single nanometer-sized ferromagnetic stripe. Measurements are performed using a microresonator set-up which offers a sensitivity to quantitatively analyze the dynamic and static magnetic properties of single nanomagnets with volumes of (100-*nm) 3 . Uniform as well as non-uniform volume modes of the spin wave excitation spectrum are identified and found to be in excellent agreement with the results of micromagnetic simulations which allow the visualization of the spatial distribution of these modes in the nanostructures.

[162] Localization effects induced by decoherence in superpositions of many-spin quantum states
G. A. 'Alvarez and D. Suter
Phys. Rev. A 84, 012320 (2011)
AbstractThe spurious interaction of quantum systems with their environment known as decoherence leads, as a function of time, to a decay of coherence of superposition states. Since the interactions between system and environment are local, they can also cause a loss of spatial coherence: correlations between spatially distant parts of the system are lost and the equilibrium states can become localized. This effect limits the distance over which quantum information can be transmitted, e.g., along a spin chain. We investigate this issue in a nuclear magnetic resonance quantum simulator, where it is possible to monitor the spreading of quantum information in a three-dimensional network: states that are initially localized on individual spins (qubits) spread under the influence of a suitable Hamiltonian apparently without limits. If we add a perturbation to this Hamiltonian, the spreading stops and the system reaches a limiting size, which becomes smaller as the strength of the perturbation increases. This limiting size appears to represent a dynamical equilibrium. We present a phenomenological model to describe these results.

[163] High fidelity quantum memory via dynamical decoupling: theory and experiment
X. Peng, D. Suter and D. A. Lidar
J. Phys. B 44, 154003 (2011)
AbstractQuantum information processing requires overcoming decoherence - the loss of 'quantumness' due to the inevitable interaction between the quantum system and its environment. One approach towards a solution is quantum dynamical decoupling - a method employing strong and frequent pulses applied to the qubits. Here we report on the first experimental test of the concatenated dynamical decoupling (CDD) scheme, which invokes recursively constructed pulse sequences. Using nuclear magnetic resonance, we demonstrate a near order of magnitude improvement in the decay time of stored quantum states. In conjunction with recent results on high fidelity quantum gates using CDD, our results suggest that quantum dynamical decoupling should be used as a first layer of defense against decoherence in quantum information processing implementations, and can be a stand-alone solution in the right parameter regime.

[164] Hyperfine characterization and spin coherence lifetime extension in Pr3+:La2(WO4)3
M. Lovric, P. Glasenapp, D. Suter, B. Tumino, A. Ferrier, P. Goldner, M. Sabooni, L. Rippe and S. Kröll
Phys. Rev. B 84, 104417 (2011)
AbstractRare-earth ions in dielectric crystals are interesting candidates for storing quantum states of photons. A limiting factor on the optical density and thus the conversion efficiency is the distortion introduced in the crystal by doping elements of one type into a crystal matrix of another type. Here we investigate the system Pr3+:La2(WO4)3, where the similarity of the ionic radii of Pr and La minimizes distortions due to doping. We characterize the praseodymium hyperfine interaction of the ground-state (3H4) and one excited state (1D2) and determine the spin Hamiltonian parameters by numerical analysis of Raman-heterodyne spectra, which were collected for a range of static external magnetic-field strengths and orientations. On the basis of a crystal-field analysis, we discuss the physical origin of the experimentally determined quadrupole and Zeeman tensor characteristics. We show the potential for quantum memory applications by measuring the spin coherence lifetime in a magnetic field that is chosen such that additional magnetic fields do not shift the transition frequency in first order. Experimental results demonstrate a spin coherence lifetime of 158 ms -- almost 3 orders of magnitude longer than in zero field.

[165] Measuring the Spectrum of Colored Noise by Dynamical Decoupling
G. A. 'Alvarez and D. Suter
Phys. Rev. Lett. 107, 230501 (2011)
AbstractDecoherence is one of the most important obstacles that must be overcome in quantum information processing. It depends on the qubit-environment coupling strength, but also on the spectral composition of the noise generated by the environment. If the spectral density is known, fighting the effect of decoherence can be made more effective. Applying sequences of inversion pulses to the qubit system, we developed a method for dynamical decoupling noise spectroscopy. We generate effective filter functions that probe the environmental spectral density without requiring assumptions about its shape. Comparing different pulse sequences, we recover the complete spectral density function and distinguish different contributions to the overall decoherence.

[166] Electron paramagnetic resonance investigation of the electron-doped manganite La1-xTexMnO3 (0.1 < x < 0.2)
J. Yang, X. Rong, D. Suter and Y. P. Sun
Phys. Chem. Chem. Phys. 13, 16343-16348 (2011)
AbstractThe electron paramagnetic resonance (EPR) properties of the electron-doped manganite La1-xTexMnO3 (0.1 [less-than-or-equal] x [less-than-or-equal] 0.2) are investigated based on the data of EPR spectra{,} resistivity{,} and magnetic susceptibility. With decreasing temperature from 400 K{,} the EPR linewidth [capital Delta]HPP decreases and passes through a minimum at Tmin{,} then substantially increases with further decreasing temperature. The broadening of the EPR linewidth above Tmin can be understood in terms of the increase in the relaxation rate of spin of eg polarons to the lattice with increasing temperature due to the similarity between the temperature dependence of the linewidth [capital Delta]Hpp(T) and the conductivity [sigma](T). For the samples with x = 0.1 and 0.15{,} the conductivity activation energy E[sigma] is comparable with the activation energy Ea deduced from the linewidth. Whereas for the x = 0.2 sample{,} there is a large difference between E[sigma] (0.2206 eV) and Ea (0.0874 eV).

[167] Spin Hamiltonian characterization and refinement for Pr3+:YAlO3 and Pr3+:Y2SiO5
M. Lovric, P. Glasenapp and D. Suter
Phys. Rev. B 85, 014429 (2012)
AbstractRare-earth ions in dielectric crystals play an important role in high-resolution laser spectroscopy and are interesting candidates for storing quantum states of photons. We characterize the praseodymium hyperfine interaction of the ground-state (3H4) and one excited-state (1D2) for two important compounds: praseodymium doped in yttrium aluminum perovskite and praseodymium doped in yttrium orthosilicate. The spin Hamiltonian parameters are determined by numerical analysis of Raman-heterodyne spectra, which were collected for a range of static external magnetic field strengths and orientations. For Pr3+:YAlO3, we present a full analysis without restrictions for the Zeeman and quadrupole tensors, and our new characterization for Pr3+:Y2SiO5 resolves a controversy in the literature.

[168] An Efficient Exact Quantum Algorithm for the Integer Square-free Decomposition Problem
J. Li, X. Peng, J. Du and D. Suter
Sci. Rep. 2, 260 (2012)
AbstractQuantum computers are known to be qualitatively more powerful than classical computers, but so far only a small number of different algorithms have been discovered that actually use this potential. It would therefore be highly desirable to develop other types of quantum algorithms that widen the range of possible applications. Here we propose an efficient and exact quantum algorithm for finding the square-free part of a large integer - a problem for which no efficient classical algorithm exists. The algorithm relies on properties of Gauss sums and uses the quantum Fourier transform. We give an explicit quantum network for the algorithm. Our algorithm introduces new concepts and methods that have not been used in quantum information processing so far and may be applicable to a wider class of problems.

[169] Absorption of Impralit-Kds by some Tropical Woods: An Electron Paramagnetic Resonnance Study
S. S. Zekeng and D. Suter
Annales de la Faculte des Sciences, Universite de Yaounde I. Ser. Physique, 39, 1-8 (2012)
AbstractWe studied the retention of Impralit-KDS (a 12.5% copper-based waterborne wood preservative) by three tropical species : AYOUS (Triplochoton Sclerylonk), FRAKE (Terminalia Superra) and MOABI (Baillonnella Toxisperma). The measurements were done with an Electron Paramagnetic Resonance (EPR) spectrometer on untreated and treated samples of heartwood. The treatment was a total immersion for a duration varying from 10 minutes to 24 hours

[170] Sealing liquid-filled pectinate capsules with a shellac coating
S. Henning, S. Leick, M. Kott, H. Rehage and D. Suter
J. Microencapsulation 29, 147-155 (2012)
AbstractLiquid-filled pectinate capsules have a large potential for the controlled and site-specific delivery of liquid drugs. Earlier studies have shown that pure pectinate capsules can store drugs only for a few minutes. Here, we show that the retention time can be extended to several hours by coating the capsules with the natural resin shellac. A bilberry extract containing anthocyanins with promising therapeutic properties was used as model drug to characterize the permeability of the capsules by in vitro drug release measurements. Characterizing the structure of the double-layered capsule membranes by NMR microscopy, we optimized the capsule production by adjusting the pH-value in the coating process and the gelation time of the pectinate hydrogel layer. A comparison of the layer thicknesses with drug release measurements reveals that capsules with the thinnest shellac layers provide the best entrapment. Additional squeezing experiments show that the shellac layer makes the capsules also mechanically more stable.

[171] Effects of time-reversal symmetry in dynamical decoupling
A. M. Souza, G. A. 'Alvarez and D. Suter
Phys. Rev. A 85, 032306 (2012)
AbstractDynamical decoupling is a technique for preserving the coherence of quantum-mechanical states in the presence of a noisy environment. It uses sequences of inversion pulses to suppress the environmental perturbations by periodically refocusing them. It has been shown that different sequences of inversion pulses show vastly different performance, in particular also concerning the correction of experimental pulse imperfections. Here, we investigate specifically the role of time-reversal symmetry in the building blocks of the pulse sequence. We show that using time-symmetric building blocks often improves the performance of the sequence compared to sequences formed by time-asymmetric building blocks. Using quantum state tomography of the echoes generated by the sequences, we analyze the mechanisms that lead to loss of fidelity and show how they can be compensated by suitable concatenation of symmetry-related blocks of decoupling pulses.

[172] Optically Enhanced Magnetic Resonance
D. Suter
Editors: R. K. Harris and R. Wasylishen
John Wiley & Sons, Chichester (2012)

[173] Medizinphysik in Übungen und Beispielen
W. A. Worthoff, H. G. Krojanski and D. Suter
De Gruyter (2012)
ISBN 978-3-11-026619-1

[174] Iterative rotation scheme for robust dynamical decoupling
G. A. 'Alvarez, A. M. Souza and D. Suter
Phys. Rev. A 85, 052324 (2012)
AbstractThe loss of quantum information due to interactions with external degrees of freedom, which is known as decoherence, remains one of the main obstacles for large-scale implementations of quantum computing. Accordingly, different measures are being explored for reducing its effect. One of them is dynamical decoupling (DD) which offers a practical solution because it only requires the application of control pulses to the system qubits. Starting from basic DD sequences, more sophisticated schemes were developed that eliminate higher-order terms of the system-environment interaction and are also more robust against experimental imperfections. A particularly successful scheme, called concatenated DD (CDD), gives a recipe for generating higher-order sequences by inserting lower-order sequences into the delays of a generating sequence. Here, we show how this scheme can be improved further by converting some of the pulses to virtual (and thus ideal) pulses. The resulting scheme, called (XY4)n, results in lower power deposition and is more robust against pulse imperfections than the original CDD scheme.

[175] Characterizing permeability and stability of microcapsules for controlled drug delivery by dynamic NMR microscopy
S. Henning, D. Edelhoff, B. Ernst, S. Leick, H. Rehage and D. Suter
J. Magn. Reson. 221, 11 - 18 (2012)
AbstractMicroscopic capsules made from polysaccharides are used as carriers for drugs and food additives. Here, we use NMR microscopy to assess the permeability of capsule membranes and their stability under different environmental conditions. The results allow us to determine the suitability of different capsules for controlled drug delivery. As a measure of the membrane permeability, we monitor the diffusion of paramagnetic molecules into the microcapsules by dynamic NMR microimaging. We obtained the diffusion coefficients of the probe molecules in the membranes and in the capsule core by comparing the measured time dependent concentration maps with numerical solutions of the diffusion equation. The results reveal that external coatings strongly decrease the permeability of the capsules. In addition, we also visualized that the capsules are stable under gastric conditions but dissolve under simulated colonic conditions, as required for targeted drug delivery. Depending on the capsule, the timescales for these processes range from 1 to 28 h.

[176] Robust dynamical decoupling for arbitrary quantum states of a single NV center in diamond
J. H. Shim, I. Niemeyer, J. Zhang and D. Suter
EPL (Europhysics Letters) 99, 40004 (2012)
AbstractDynamical decoupling is a powerful technique for extending the coherence time ( T 2 ) of qubits. We apply this technique to the electron spin qubit of a single nitrogen-vacancy center in type-IIa diamond. In a crystal with natural abundance of 13 C nuclear spins, we extend the decoherence time up to 2.2 ms. This is close to the T 1 value of this NV center (4 ms). Since dynamical decoupling must perform well for arbitrary initial conditions, we measured the dependence on the initial state and compared the performance of different sequences with respect to initial state dependence and robustness to experimental imperfections.

[177] Robust dynamical decoupling
A. M. Souza, G. A. 'Alvarez and D. Suter
Phil. Trans. Royal Soc. A 370, 4748-4769 (2012)
AbstractQuantum computers, which process information encoded in quantum mechanical systems, hold the potential to solve some of the hardest computational problems. A substantial obstacle for the further development of quantum computers is the fact that the lifetime of quantum information is usually too short to allow practical computation. A promising method for increasing the lifetime, known as dynamical decoupling (DD), consists of applying a periodic series of inversion pulses to the quantum bits. In the present review, we give an overview of this technique and compare different pulse sequences proposed earlier. We show that pulse imperfections, which are always present in experimental implementations, limit the performance of DD. The loss of coherence due to the accumulation of pulse errors can even exceed the perturbation from the environment. This effect can be largely eliminated by a judicious design of pulses and sequences. The corresponding sequences are largely immune to pulse imperfections and provide an increase of the coherence time of the system by several orders of magnitude.

[178] Experimental Implementation of Encoded Logical Qubit Operations in a Perfect Quantum Error Correcting Code
J. Zhang, R. Laflamme and D. Suter
Phys. Rev. Lett. 109, 100503 (2012)
AbstractLarge-scale universal quantum computing requires the implementation of quantum error correction (QEC). While the implementation of QEC has already been demonstrated for quantum memories, reliable quantum computing requires also the application of nontrivial logical gate operations to the encoded qubits. Here, we present examples of such operations by implementing, in addition to the identity operation, the NOT and the Hadamard gate to a logical qubit encoded in a five qubit system that allows correction of arbitrary single-qubit errors. We perform quantum process tomography of the encoded gate operations, demonstrate the successful correction of all possible single-qubit errors, and measure the fidelity of the encoded logical gate operations.

[179] Experimental protection of quantum gates against decoherence and control errors
A. M. Souza, G. A. 'Alvarez and D. Suter
Phys. Rev. A 86, 050301 (2012)
AbstractOne of the biggest challenges for implementing quantum devices is the requirement to perform accurate quantum gates. The destructive effects of interactions with the environment present some of the most difficult obstacles that must be overcome for precise quantum control. In this work we implement a proof of principle experiment of quantum gates protected against a fluctuating environment and control pulse errors using dynamical decoupling techniques. We show that decoherence can be reduced during the application of quantum gates. High-fidelity quantum gates can be achieved even if the gate time exceeds the free evolution decoherence time by one order of magnitude and for protected operations consisting of up to 330 individual control pulses.

[180] Room-temperature high-speed nuclear-spin quantum memory in diamond
J. H. Shim, I. Niemeyer, J. Zhang and D. Suter
Phys. Rev. A 87, 012301 (2013)
AbstractQuantum memories provide intermediate storage of quantum information until it is needed for the next step of a quantum algorithm or a quantum communication process. Relevant figures of merit are therefore the fidelity with which the information can be written and retrieved, the storage time, and also the speed of the read-write process. Here, we present experimental data on a quantum memory consisting of a single 13C nuclear spin that is strongly coupled to the electron spin of a nitrogen-vacancy (NV) center in diamond. The strong hyperfine interaction of the nearest-neighbor carbon results in transfer times of 300 ns between the register qubit and the memory qubit, with an overall fidelity of 88% for the write-storage-read cycle. The observed storage times of 3.3 ms appear to be limited by the T1 relaxation of the electron spin. We discuss a possible scheme that may extend the storage time beyond this limit.

[181] Robustness of dynamical decoupling sequences
M. A. Ali Ahmed, G. A. 'Alvarez and D. Suter
Phys. Rev. A 87, 042309 (2013)
AbstractActive protection of quantum states is an essential prerequisite for the implementation of quantum computing. Dynamical decoupling (DD) is a promising approach that applies sequences of control pulses to the system in order to reduce the adverse effect of system-environment interactions. Since every hardware device has finite precision, the errors of the DD control pulses can themselves destroy the stored information rather than protect it. We experimentally compare the performance of different DD sequences in the presence of an environment that was chosen such that all relevant DD sequences can equally suppress its effect on the system. Under these conditions, the remaining decay of the qubits under DD allows us to compare very precisely the robustness of the different DD sequences with respect to imperfections of the control pulses.

[182] Room temperature strong coupling between a microwave oscillator and an ensemble of electron spins
G. Boero, G. Gualco, R. Lisowski, J. Anders, D. Suter and J. Brugger
J. Magn. Reson. 231, 133 - 140 (2013)
AbstractWe demonstrate theoretically and experimentally the possibility to achieve the strong coupling regime at room temperature with a microwave electronic oscillator coupled with an ensemble of electron spins. The coupled system shows bistable behaviour, with a broad hysteresis and sharp transitions. The coupling strength and the hysteresis width can be adjusted through the number of spins in the ensemble, the temperature, and the microwave field strength.

[183] Broadband excitation by chirped pulses: application to single electron spins in diamond
I. Niemeyer, J. H. Shim, J. Zhang, D. Suter, T. Taniguchi, T. Teraji, H. Abe, S. Onoda, T. Yamamoto, T. Ohshima, J. Isoya and F. Jelezko
New J. Phys. 15, 033027 (2013)
AbstractPulsed excitation of broad spectra requires very high field strengths if monochromatic pulses are used. If the corresponding high power is not available or not desirable, the pulses can be replaced by suitable low-power pulses that distribute the power over a wider bandwidth. As a simple case, we use microwave pulses with a linear frequency chirp. We use these pulses to excite spectra of single nitrogenvacancy centres in a Ramsey experiment. Compared to the conventional Ramsey experiment, our approach increases the bandwidth by at least an order of magnitude. Compared to the conventional continuous wave-ODMR experiment, the chirped Ramsey experiment does not suffer from power broadening and increases the resolution by at least an order of magnitude. As an additional benefit, the chirped Ramsey spectrum contains not only allowed single quantum transitions, but also forbidden zero- and double quantum transitions, which can be distinguished from the single quantum transitions by phase-shifting the readout pulse with respect to the excitation pulse or by variation of the external magnetic field strength.

[184] Experimental Implementation of Assisted Quantum Adiabatic Passage in a Single Spin
J. Zhang, J. H. Shim, I. Niemeyer, T. Taniguchi, T. Teraji, H. Abe, S. Onoda, T. Yamamoto, T. Ohshima, J. Isoya and D. Suter
Phys. Rev. Lett. 110, 240501 (2013)
AbstractQuantum adiabatic passages can be greatly accelerated by a suitable control field, called a counter-diabatic field, which varies during the scan through resonance. Here, we implement this technique on the electron spin of a single nitrogen-vacancy center in diamond. We demonstrate two versions of this scheme. The first follows closely the procedure originally proposed by Demirplak and Rice [ J. Phys. Chem. A 107 9937 (2003)]. In the second scheme, we use a control field whose amplitude is constant but whose phase varies with time. This version, which we call the rapid-scan approach, allows an even faster passage through resonance and therefore makes it applicable also for systems with shorter decoherence times.

[185] High-Precision Nanoscale Temperature Sensing Using Single Defects in Diamond
P. Neumann, I. Jakobi, F. Dolde, C. Burk, R. Reuter, G. Waldherr, J. Honert, T. Wolf, A. Brunner, J. H. Shim, D. Suter, H. Sumiya, J. Isoya and J. Wrachtrup
Nano Letters 13, 2738-2742 (2013)
AbstractMeasuring local temperature with a spatial resolution on the order of a few nanometers has a wide range of applications in the semiconductor industry and in material and life sciences. For example, probing temperature on the nanoscale with high precision can potentially be used to detect small, local temperature changes like those caused by chemical reactions or biochemical processes. However, precise nanoscale temperature measurements have not been realized so far owing to the lack of adequate probes. Here we experimentally demonstrate a novel nanoscale temperature sensing technique based on optically detected electron spin resonance in single atomic defects in diamonds. These diamond sensor sizes range from a micrometer down to a few tens of nanometers. We achieve a temperature noise floor of 5 mK/Hz1/2 for single defects in bulk sensors. Using doped nanodiamonds as sensors the temperature noise floor is 130 mK/Hz1/2 and accuracies down to 1 mK for nanocrystal sizes and therefore length scales of a few tens of nanometers. This combination of precision and position resolution, combined with the outstanding sensor photostability, should allow the measurement of the heat produced by chemical interactions involving a few or single molecules even in heterogeneous environments like cells.

Page Not Found[186] Resonance Effects of Excitons and Electrons: Basics and Applications
I. Geru and D. Suter
Springer London (2013)
ISBN 9783642358067

[187] Faithful Solid State Optical Memory with Dynamically Decoupled Spin Wave Storage
M. Lovric, D. Suter, A. Ferrier and P. Goldner
Phys. Rev. Lett. 111, 020503 (2013)
AbstractWe report a high fidelity optical memory in which dynamical decoupling is used to extend the storage time. This is demonstrated in a rare-earth doped crystal in which optical coherences were transferred to nuclear spin coherences and then protected against environmental noise by dynamical decoupling, leading to storage times of up to 4.2 ms. An interference experiment shows that relative phases of input pulses are preserved through the whole storage and retrieval process with a visibility 1, demonstrating the usefulness of dynamical decoupling for extending the storage time of quantum memories. We also show that dynamical decoupling sequences insensitive to initial spin coherence increase retrieval efficiency.

[188] High-resolution MRI velocimetry compared with numerical simulations
D. Edelhoff, L. Walczak, S. Henning, F. Weichert and D. Suter
J. Magn. Reson. 235, 42--49 (2013)
AbstractAlterations of the blood flow are associated with various cardiovascular diseases. Precise knowledge of the velocity distribution is therefore important for understanding these diseases and predicting the effect of different medical intervention schemes. The goal of this work is to estimate the precision with which the velocity field can be measured and predicted by studying two simple model geometries with NMR micro imaging and computational fluid dynamics. For these initial experiments, we use water as an ideal test medium. The phantoms consist of tubes simulating a straight blood vessel and a step between two tubes of different diameters, which can be seen as a minimal model of the situation behind a stenosis. For both models, we compare the experimental data with the numerical prediction, using the experimental boundary conditions. For the simpler model, we also compare the data to the analytical solution. As an additional validation, we determine the divergence of the velocity field and verify that it vanishes within the experimental uncertainties. We discuss the resulting precision of the simulation and the outlook for extending this approach to the analysis of specific cases of arteriovascular problems.

[189] Quantum simulations of localization effects with dipolar interactions
G. A. 'Alvarez, R. Kaiser and D. Suter
Annalen der Physik 525, 833--844 (2013)
AbstractQuantum information processing often uses systems with dipolar interactions. Here a nuclear spin-based quantum simulator is used to study the spreading of information in such a dipolar-coupled system. While the information spreads with no apparent limits in the case of ideal dipolar couplings, additional perturbations limit the spreading, leading to localization. In previous work [Phys. Rev. Lett. 104, 230403 (2010)], it was found that the system size reaches a dynamic equilibrium that decreases with the square of the perturbation strength. This work examines the impact of a disordered Hamiltonian with dipolar 1/r3 interactions. It shows that the expansion of the cluster of spins freezes in the presence of large disorder, reminiscent of Anderson localization of non-interacting waves in a disordered potential.

[190] Medical Physics, Exercises and Examples
W. A. Worthoff, H. G. Krojanski and D. Suter
(2013)
ISBN paperback 978-3-11-030675-0; ebook: ISBN 978-3-11-030676-7

[191] A cryogenic receiver for EPR
R. Narkowicz, H. Ogata, E. Reijerse and D. Suter
J. Magn. Reson. 237, 79--84 (2013)
AbstractCryogenic probes have significantly increased the sensitivity of NMR. Here, we present a compact EPR receiver design capable of cryogenic operation. Compared to room temperature operation, it reduces the noise by a factor of 2.5. We discuss in detail the design and analyze the resulting noise performance. At low microwave power, the input noise density closely follows the emission of a cooled 50 resistor over the whole measurement range from 20 K up to room temperature. To minimize the influence of the microwave source noise, we use high microwave efficiency (1.11.7 mT W1/2) planar microresonators. Their efficient conversion of microwave power to magnetic field permits EPR measurements with very low power levels, typically ranging from a few W down to fractions of nW.

[192] Protected Quantum Computing: Interleaving Gate Operations with Dynamical Decoupling Sequences
J. Zhang, A. M. Souza, F. D. Brandao and D. Suter
Phys. Rev. Lett. 112, 050502 (2014)
AbstractImplementing precise operations on quantum systems is one of the biggest challenges for building quantum devices in a noisy environment. Dynamical decoupling attenuates the destructive effect of the environmental noise, but so far, it has been used primarily in the context of quantum memories. Here, we experimentally demonstrate a general scheme for combining dynamical decoupling with quantum logical gate operations using the example of an electron-spin qubit of a single nitrogen-vacancy center in diamond. We achieve process fidelities >98% for gate times that are 2 orders of magnitude longer than the unprotected dephasing time T2.

[193] Optimized multiple-quantum filter for robust selective excitation of metabolite signals
M. Holbach, J. Lambert and D. Suter
J. Magn. Reson. 243, 8--16 (2014)
AbstractThe selective excitation of metabolite signals in vivo requires the use of specially adapted pulse techniques, in particular when the signals are weak and the resonances overlap with those of unwanted molecules. Several pulse sequences have been proposed for this spectral editing task. However, their performance is strongly degraded by unavoidable experimental imperfections. Here, we show that optimal control theory can be used to generate pulses and sequences that perform almost ideally over a range of rf field strengths and frequency offsets that can be chosen according to the specifics of the spectrometer or scanner being used. We demonstrate this scheme by applying it to lactate editing. In addition to the robust excitation, we also have designed the pulses to minimize the signal of unwanted molecular species.

[194] Angular dependent ferromagnetic resonance analysis in a single micron sized cobalt stripe
C. Schoeppner, K. Wagner, S. Stienen, R. Meckenstock, M. Farle, R. Narkowicz, D. Suter and J. Lindner
J. Appl. Phys. 116, 033913 (2014)
AbstractWe demonstrate how planar microresonators (PMRs) can be utilized to investigate the angular dependent magnetic resonance response of single magnetic nanostructures. In contrast to alternative detection schemes like electrical or optical detection, the PMR approach provides a classical means of investigating the high frequency dynamics of single magnetic entities, enabling the use of well-established analysis methods of ferromagnetic resonance (FMR) spectroscopy. To demonstrate the performance of the PMR-based FMR setup for angular dependent measurements, we investigate the microwave excited magnons in a single Co stripe of 5 1 0.02 lm3 and compare the results to micromagnetic simulations. The evolution of excited magnons under rotation of one individual stripe with respect to a static magnetic field is investigated. Besides quasi uniform excitations, we observe magneto-static as well as localized excitations. We find a strong influence of inhomogeneousdynamicandstaticdemagnetizingfieldsforallmodes.

[195] Evidence for the light hole in GaAs/AlGaAs quantum wells from optically-pumped NMR and Hanle curve measurements
E. L. Sesti, W. A. Worthoff, D. D. Wheeler, D. Suter and S. E. Hayes
J. Magn. Reson. 246, 130--135 (2014)
AbstractAbstract Optically-pumped 69Ga NMR (OPNMR) and optically-detected measurements of polarized photoluminescence (Hanle curves) show a characteristic feature at the light hole-to-conduction band transition in a GaAs/AlxGa1xAs multiple quantum well sample. OPNMR data are often depicted as a profile of the OPNMR integrated signal intensity plotted versus optical pumping photon energy. What is notable is the inversion of the sign of the measured 69Ga OPNMR signals when optically pumping this light hole-to-conduction band energy in OPNMR profiles at multiple external magnetic fields (B0 = 4.7 T and 3 T) for both + and irradiation. Measurements of Hanle curves at B0 = 0.5 T of the same sample exhibit similar phase inversion behavior of the Hanle curves at the photon energy for light hole excitation. The zero-field value of the light-hole state in the quantum well can be predicted for the quantum well structure using the positions of each of these signal-inversion features, and the spin splitting term in the equation for the transition energy yields consistent values at 3 magnetic fields for the excitonic g-factor (gex). This study demonstrates the application of OPNMR and optical measurements of the photoluminescence to detect the light hole transition in semiconductors.

[196] Experimental Implementation of Adiabatic Passage between Different Topological Orders
X. Peng, Z. Luo, W. Zheng, S. Kou, D. Suter and J. Du
Phys. Rev. Lett. 113, 080404 (2014)
AbstractTopological orders are exotic phases of matter existing in strongly correlated quantum systems, which are beyond the usual symmetry description and cannot be distinguished by local order parameters. Here we report an experimental quantum simulation of the Wen-plaquette spin model with different topological orders in a nuclear magnetic resonance system, and observe the adiabatic transition between two Z2 topological orders through a spin-polarized phase by measuring the nonlocal closed-string (Wilson loop) operator. Moreover, we also measure the entanglement properties of the topological orders. This work confirms the adiabatic method for preparing topologically ordered states and provides an experimental tool for further studies of complex quantum systems.

[197] All-optical NMR in semiconductors provided by resonant cooling of nuclear spins interacting with electrons in the resonant spin amplification regime
E. A. Zhukov, A. Greilich, D. R. Yakovlev, K. V. Kavokin, I. A. Yugova, O. A. Yugov, D. Suter, G. Karczewski, T. Wojtowicz, J. Kossut, V. V. Petrov, Yu. K. Dolgikh, A. Pawlis and M. Bayer
Phys. Rev. B 90, 085311 (2014)
AbstractResonant cooling of different nuclear isotopes manifested in optically induced nuclear magnetic resonances (NMR) is observed in n-doped CdTe/(Cd,Mg)Te and ZnSe/(Zn,Mg)Se quantum wells and for donor-bound electrons in ZnSe:F and GaAs epilayers. By time-resolved Kerr rotation used in the regime of resonant spin amplification, we can expand the range of magnetic fields where the effect can be observed up to nuclear Larmor frequencies of 170 kHz. The mechanism of the resonant cooling of the nuclear spin system is analyzed theoretically. The developed approach allows us to model the resonant spin amplification signals with NMR features.

[198] Construction of arbitrary robust one-qubit operations using planar geometry
T. Ichikawa, J. G. Filgueiras, M. Bando, Y. Kondo, M. Nakahara and D. Suter
Phys. Rev. A 90, 052330 (2014)
AbstractWe show how to construct an arbitrary robust one-qubit unitary operation with a control Hamiltonian of Ax (t )x + Ay (t )y , where i is a Pauli matrix and Ai (t ) is piecewise constant. Our method, based on planar geometry, admits a simple and intuitive interpretation. Furthermore, the total execution time and the number of elementary gates of the obtained sequence are comparable to those of the shortest known concatenated composite pulses.

[199] Experimental implementation of quantum gates through actuator qubits
J. Zhang, D. Burgarth, R. Laflamme and D. Suter
Phys. Rev. A 91, 012330 (2015)
AbstractUniversal quantum computation requires the implementation of arbitrary control operations on the quantum register. In most cases, this is achieved by external control fields acting selectively on each qubit to drive single-qubit operations. In combination with a drift Hamiltonian containing interactions between the qubits, this allows the implementation of arbitrary gate operations. Here, we demonstrate an alternative scheme that does not require local control for all qubits: we implement one- and two-qubit gate operations on a set of target qubits indirectly, through a combination of gates on directly controlled actuator qubits with a drift Hamiltonian that couples actuator and target qubits. Experiments are performed on nuclear spins, using radio-frequency pulses as gate operations and magnetic-dipole couplings for the drift Hamiltonian.

[200] Tuner and radiation shield for planar electron paramagnetic resonance microresonators
R. Narkowicz and D. Suter
Review of Scientific Instruments 86, 024701 (2015)
AbstractPlanar microresonators provide a large boost of sensitivity for small samples. They can be manufac- tured lithographically to a wide range of target parameters. The coupler between the resonator and the microwave feedline can be integrated into this design. To optimize the coupling and to compensate manufacturing tolerances, it is sometimes desirable to have a tuning element available that can be adjusted when the resonator is connected to the spectrometer. This paper presents a simple design that allows one to bring undercoupled resonators into the condition for critical coupling. In addition, it also reduces radiation losses and thereby increases the quality factor and the sensitivity of the resonator.

[201] Hybrid magic state distillation for universal fault-tolerant quantum computation
W. Zheng, Y. Yu, J. Pan, J. Zhang, J. Li, Z. Li, D. Suter, X. Zhou, X. Peng and J. Du
Phys. Rev. A 91, 022314 (2015)
AbstractA set of stabilizer operations augmented by some special initial states known as magic states gives the possibility of universal fault-tolerant quantum computation. However, magic state preparation inevitably involves nonideal operations that introduce noise. The most common method to eliminate the noise is magic state distillation (MSD) by stabilizer operations. Here we propose a hybrid MSD protocol by connecting a four-qubit H-type MSD with a five-qubit T-type MSD in order to overcome the shortcomings of the previous MSD protocols. The hybrid MSD protocol further integrates distillable ranges of different existing MSD protocols and provides considerable improvement in qubit cost. Moreover, we experimentally demonstrate the four-qubit H -type MSD protocol using nuclear magnetic resonance technology, together with the previous five-qubit MSD experiment, to show the feasibility of the hybrid MSD protocol.

[202] Optimized selective lactate excitation with a refocused multiple-quantum filter
M. Holbach, J. Lambert, S. Johst, M. E. Ladd and D. Suter
J. Magn. Reson. 255, 34--38 (2015)
AbstractSelective detection of lactate signals in in vivo MR spectroscopy with spectral editing techniques is necessary in situations where strong lipid or signals from other molecules overlap the desired lactate resonance in the spectrum. Several pulse sequences have been proposed for this task. The double-quantum filter SSel-MQC provides very good lipid and water signal suppression in a single scan. As a major drawback, it suffers from significant signal loss due to incomplete refocussing in situations where long evolution periods are required. Here we present a refocused version of the SSel-MQC technique that uses only one additional refocussing pulse and regains the full refocused lactate signal at the end of the sequence.

[203] Localization-delocalization transition in the dynamics of dipolar-coupled nuclear spins
G. A. 'Alvarez, D. Suter and R. Kaiser
Science 349, 846-848 (2015)
AbstractNonequilibrium dynamics of many-body systems are important in many scientific fields. Here, we report the experimental observation of a phase transition of the quantum coherent dynamics of a three-dimensional many-spin system with dipolar interactions. Using nuclear magnetic resonance (NMR) on a solid-state system of spins at room-temperature, we quench the interaction Hamiltonian to drive the evolution of the system. Depending on the quench strength, we then observe either localized or extended dynamics of the system coherence. We extract the critical exponents for the localized cluster size of correlated spins and diffusion coefficient around the phase transition separating the localized from the delocalized dynamical regime. These results show that NMR techniques are well suited to studying the nonequilibrium dynamics of complex many-body systems.

[204] Stability and Permeability of Microcapsules for Controlled Drug Delivery from Magnetic Resonance Microscopy
chapter 9 of 'Microencapsulation and Microspheres for Food Applications'
D. Suter, P. Degen, D. Edelhoff and S. Henning
Editors: L. Sagis
Academic Press, 173-194 (2015)

[205] Experimental Protection of Two-Qubit Quantum Gates against Environmental Noise by Dynamical Decoupling
J. Zhang and D. Suter
Phys. Rev. Lett. 115, 110502 (2015)
AbstractHybrid systems consisting of different types of qubits are promising for building quantum computers if they combine useful properties of their constituent qubits. However, they also pose additional challenges if one type of qubits is more susceptible to environmental noise than the others. Dynamical decoupling can help to protect such systems by reducing the decoherence due to the environmental noise, but the protection must be designed such that it does not interfere with the control fields driving the logical operations. Here, we test such a protection scheme on a quantum register consisting of the electronic and nuclear spins of a nitrogen-vacancy center in diamond. The results show that processing is compatible with protection: The dephasing time was extended almost to the limit given by the longitudinal relaxation time of the electron spin.

[206] Measurement with microscopic MRI and simulation of flow in different aneurysm models
D. Edelhoff, L. Walczak, F. Frank, M. Heil, I. Schmitz, F. Weichert and D. Suter
Medical Physics 42, 5661-5670 (2015)
AbstractPurpose: The impact and the development of aneurysms depend to a significant degree on the exchange of liquid between the regular vessel and the pathological extension. A better understanding of this process will lead to improved prediction capabilities. The aim of the current study was to investigate fluid-exchange in aneurysm models of different complexities by combining microscopic magnetic resonance measurements with numerical simulations. In order to evaluate the accuracy and applicability of these methods, the fluid-exchange process between the unaltered vessel lumen and the aneurysm phantoms was analyzed quantitatively using high spatial resolution. Methods: Magnetic resonance flow imaging was used to visualize fluid-exchange in two different models produced with a 3D printer. One model of an aneurysm was based on histological findings. The flow distribution in the different models was measured on a microscopic scale using time of flight magnetic resonance imaging. The whole experiment was simulated using fast graphics processing unit-based numerical simulations. The obtained simulation results were compared qualitatively and quantitatively with the magnetic resonance imaging measurements, taking into account flow and spinlattice relaxation. Results: The results of both presented methods compared well for the used aneurysm models and the chosen flow distributions. The results from the fluid-exchange analysis showed comparable characteristics concerning measurement and simulation. Similar symmetry behavior was observed. Based on these results, the amount of fluid-exchange was calculated. Depending on the geometry of the models, 7% to 45% of the liquid was exchanged per second. Conclusions: The result of the numerical simulations coincides well with the experimentally deter- mined velocity field. The rate of fluid-exchange between vessel and aneurysm was well-predicted. Hence, the results obtained by simulation could be validated by the experiment. The observed deviations can be caused by the noise in the measurement and by the limited resolution of the simulation. The resulting differences are small enough to allow reliable predictions of the flow distribution in vessels with stents and for pulsed blood flow.

[207] High-fidelity gate operations for quantum computing beyond dephasing time limits
A. M. Souza, R. S. Sarthour, I. S. Oliveira and D. Suter
Phys. Rev. A 92, 062332 (2015)
AbstractThe implementation of quantum gates with fidelities that exceed the threshold for reliable quantum computing requires robust gates whose performance is not limited by the precision of the available control fields. The performance of these gates also should not be affected by the noisy environment of the quantum register. Here we use randomized benchmarking of quantum gate operations to compare the performance of different families of gates that compensate errors in the control field amplitudes and decouple the system from the environmental noise. We obtain average fidelities of up to 99.8%, which exceeds the threshold value for some quantum error correction schemes as well as the expected limit from the dephasing induced by the environment.

[208] Characterization of hyperfine interaction between an NV electron spin and a first-shell 13C nuclear spin in diamond
K. R. K. Rao and D. Suter
Phys. Rev. B 94, 060101 (2016)
AbstractThe nitrogen-vacancy (NV) center in diamond has attractive properties for a number of quantum technologies that rely on the spin angular momentum of the electron and the nuclei adjacent to the center. The nucleus with the strongest interaction is the C13 nuclear spin of the first shell. Using this degree of freedom effectively hinges on precise data on the hyperfine interaction between the electronic and the nuclear spin. Here, we present detailed experimental data on this interaction, together with an analysis that yields all parameters of the hyperfine tensor, as well as its orientation with respect to the atomic structure of the center.

[209] Colloquium : Protecting quantum information against environmental noise
D. Suter and G. A. Alvarez
Rev. Mod. Phys. 88, 041001 (2016)
AbstractQuantum technologies represent a rapidly evolving field in which the specific properties of quantum mechanical systems are exploited to enhance the performance of various applications such as sensing, transmission, and processing of information. Such devices can be useful only if the quantum systems also interact with their environment. However, the interactions with the environment can degrade the specific quantum properties of these systems, such as coherence and entanglement. It is therefore essential that the interaction between a quantum system and the environment is controlled in such a way that the unwanted effects of the environment are suppressed while the necessary interactions are retained. This Colloquium gives an overview, aimed at newcomers to this field, of some of the challenges that need to be overcome to achieve this goal. A number of techniques have been developed for this purpose in different areas of physics including magnetic resonance, optics, and quantum information. They include the application of static or time-dependent fields to the quantum system, which are designed to average the effect of the environmental interactions to zero. Quantum error correction schemes were developed to detect and eliminate certain errors that occur during the storage and processing of quantum information. In many physical systems, it is useful to use specific quantum states that are intrinsically less susceptible to environmental noise for encoding the quantum information. The dominant contribution to the loss of information is pure dephasing, i.e., through the loss of coherence in quantum mechanical superposition states. Accordingly, most schemes for reducing loss of information focus on dephasing processes. This is also the focus of this Colloquium.

[210] Comparative analysis of isotropic diffusion weighted imaging sequences
S. Vellmer, R. Stirnberg, D. Edelhoff, D. Suter, T. Stöcker and I. I. Maximov
Journal of Magnetic Resonance 275, 137 - 147 (2017)
AbstractAbstract Visualisation of living tissue structure and function is a challenging problem of modern imaging techniques. Diffusion \{MRI\} allows one to probe in vivo structures on a micrometer scale. However, conventional diffusion measurements are time-consuming procedures, because they require several measurements with different gradient directions. Considerable time savings are therefore possible by measurement schemes that generate an isotropic diffusion weighting in a single shot. Multiple approaches for generating isotropic diffusion weighting are known and have become very popular as useful tools in clinical research. Thus, there is a strong need for a comprehensive comparison of different isotropic weighting approaches. In the present work we introduce two new sequences based on simple (co)sine modulations and compare their performance to established q-space magic-angle spinning sequences and conventional DTI, using a diffusion phantom assembled from microcapillaries and in vivo experiments at 7 T. The advantages and disadvantages of all compared schemes are demonstrated and discussed.

[211] Single-spin magnetic resonance in the nitrogen-vacancy center of diamond
D. Suter and F. Jelezko
Progress in Nuclear Magnetic Resonance Spectroscopy 98-99, 50 - 62 (2017)
AbstractAbstract Magnetic resonance of single spins has flourished mostly because of the unique properties of the \{NV\} center in diamond. This review covers the basic physics of this defect center, introduces the techniques for working with single spins and gives an overview of some applications like quantum information and sensing.

[212] Defect production in non-equilibrium phase transitions: experimental investigation of the Kibble-Zurek mechanism in a two-qubit quantum simulator
J. Zhang, F. M. Cucchietti, R. Laflamme and D. Suter
New Journal of Physics 19, 043001 (2017)
AbstractSystems passing through quantum critical points at finite rates have a finite probability of undergoing transitions between different eigenstates of the instantaneous Hamiltonian. This mechanism was proposed by Kibble as the underlying mechanism for the formation of topological defects in the early universe and by Zurek for condensed matter systems. Here, we use a system of nuclear spins as an experimental quantum simulator undergoing a non-equilibrium quantum phase transition. The experimental data confirm the validity of the Kibbleâ{\"A}{\`\i}Zurek mechanism of defect formation.

[213] Nonlinear dynamics of a two-level system of a single spin driven beyond the rotating-wave approximation
K. R. K. Rao and D. Suter
Phys. Rev. A 95, 053804 (2017)

[214] Anisotropic diffusion phantoms based on microcapillaries
S. Vellmer, D. Edelhoff, D. Suter and I. I. Maximov
J. Magn. Reson. 279, 1 - 10 (2017)
AbstractAbstract Diffusion \{MRI\} is an efficient and widely used technique for the investigation of tissue structure and organisation in vivo. Multiple phenomenological and biophysical diffusion models are intensively exploited for the analysis of the diffusion experiments. However, the verification of the applied diffusion models remains challenging. In order to provide a â{\"A}{\'u}gold standardâ{\"A}{\`u} and to assess the accuracy of the derived parameters and the limitations of the diffusion models, anisotropic diffusion phantoms with well known architecture are demanded. In the present work we built four anisotropic diffusion phantoms consisting of hollow microcapillaries with very small inner diameters of 5, 10 and 20 {\OE}¼ m and outer diameters of 90 and 150 {\OE}¼ m. For testing the suitability of these phantoms, we performed diffusion measurements on all of them and evaluated the resulting data with a set of popular diffusion models, such as diffusion tensor and diffusion kurtosis imaging, a two compartment model with intra- and extra-capillary water spaces using bi-exponential fitting, and time-dependent diffusion coefficients in Mitraâ{\"A}{\^o}s limit. The perspectives and limitations of these diffusion phantoms are presented and discussed.

[215] Polarizing the electronic and nuclear spin of the NV-center in diamond in arbitrary magnetic fields: analysis of the optical pumping process
T. Chakraborty, J. Zhang and D. Suter
New Journal of Physics 19, 073030 (2017)
AbstractInitializing a set of qubits to a given quantum state is a basic prerequisite for the physical implementation of quantum-information protocols. Here, we discuss the polarization of the electronic and nuclear spin in a single nitrogen vacancy center in diamond. Our initialization scheme uses a sequence of laser, microwave and radio-frequency pulses, and we optimize the pumping parameters of the laser pulse. A rate equation model is formulated that explains the effect of the laser pulse on the spin system. We have experimentally determined the population of the relevant spin states as a function of the duration of the laser pulse by measuring Rabi oscillations and Ramsey-type free-induction decays. The experimental data have been analyzed to determine the pumping rates of the rate equation model.

[216] Quantum Image Processing and Its Application to Edge Detection: Theory and Experiment
X. Yao, H. Wang, Z. Liao, M. Chen, J. Pan, J. Li, K. Zhang, X. Lin, Z. Wang, Z. Luo, W. Zheng, J. Li, M. Zhao, X. Peng and D. Suter
Phys. Rev. X 7, 031041 (2017)

[217] High-efficiency optical pumping of nuclear polarization in a GaAs quantum well
R. W. Mocek, V. L. Korenev, M. Bayer, M. Kotur, R. I. Dzhioev, D. O. Tolmachev, G. Cascio, K. V. Kavokin and D. Suter
Phys. Rev. B 96, 201303 (2017)

[218] Nuclear spin cooling by helicity-alternated optical pumping at weak magnetic fields in $n$-GaAs
P. S. Sokolov, M. Yu. Petrov, K. V. Kavokin, A. S. Kurdyubov, M. S. Kuznetsova, R. V. Cherbunin, S. Yu. Verbin, N. K. Poletaev, D. R. Yakovlev, D. Suter and M. Bayer
Phys. Rev. B 96, 205205 (2017)

[219] Validation of DWI pre-processing procedures for reliable differentiation between human brain gliomas
S. V. a. A. S. Tonoyan, D. Suter, I. N. Pronin and I. I. Maximov
Zeitschrift für Medizinische Physik 28, 14-24 (2018)

[220] Orientation-independent room temperature optical 13C hyperpolarization in powdered diamond
A. Ajoy, K. Liu, R. Nazaryan, X. Lv, P. R. Zangara, B. Safvati, G. Wang, D. Arnold, G. Li, A. Lin, P. Raghavan, E. Druga, S. Dhomkar, D. Pagliero, J. A. Reimer, D. Suter, C. A. Meriles and A. Pines
Science Advances 4, (2018)
AbstractDynamic nuclear polarization via contact with electronic spins has emerged as an attractive route to enhance the sensitivity of nuclear magnetic resonance beyond the traditional limits imposed by magnetic field strength and temperature. Among the various alternative implementations, the use of nitrogen vacancy (NV) centers in diamond{\textemdash}a paramagnetic point defect whose spin can be optically polarized at room temperature{\textemdash}has attracted widespread attention, but applications have been hampered by the need to align the NV axis with the external magnetic field. We overcome this hurdle through the combined use of continuous optical illumination and a microwave sweep over a broad frequency range. As a proof of principle, we demonstrate our approach using powdered diamond with which we attain bulk 13C spin polarization in excess of 0.25\% under ambient conditions. Remarkably, our technique acts efficiently on diamond crystals of all orientations and polarizes nuclear spins with a sign that depends exclusively on the direction of the microwave sweep. Our work paves the way toward the use of hyperpolarized diamond particles as imaging contrast agents for biosensing and, ultimately, for the hyperpolarization of nuclear spins in arbitrary liquids brought in contact with their surface.

[221] Superposing pure quantum states with partial prior information
S. Dogra, G. Thomas, S. Ghosh and D. Suter
Phys. Rev. A 97, 052330 (2018)
AbstractThe principle of superposition is an intriguing feature of quantum mechanics, which is regularly exploited in many different circumstances. A recent work [M. Oszmaniec et al., Phys. Rev. Lett. 116, 110403 (2016)] shows that the fundamentals of quantum mechanics restrict the process of superimposing two unknown pure states, even though it is possible to superimpose two quantum states with partial prior knowledge. The prior knowledge imposes geometrical constraints on the choice of input states. We discuss an experimentally feasible protocol to superimpose multiple pure states of a d-dimensional quantum system and carry out an explicit experimental realization for two single-qubit pure states with partial prior information on a two-qubit NMR quantum information processor.

[222] NMR imaging of flow on the microscale
chapter 7
D. Suter and D. Edelhoff
Editors: J. Anders and J. G. Korvink
Wiley-VCH (2018)

[223] All-optical quantum thermometry based on spin-level cross-relaxation and multicenter entanglement under ambient conditions in SiC
A. N. Anisimov, V. A. Soltamov, I. D. Breev, R. A. Babunts, E. N. Mokhov, G. V. Astakhov, V. Dyakonov, D. R. Yakovlev, D. Suter and P. G. Baranov
AIP Advances 8, 085304 (2018)
AbstractAll-optical thermometry technique based on the energy level cross-relaxation in atomic-scale spin centers in SiC is demonstrated. This technique exploits a giant thermal shift of the zero-field splitting for centers in the triplet ground state, S=1, undetected by photoluminescence (so called dark centers) coupling to neighbour- ing spin-3/2 centers which can be optically polarized and read out (bright centers), and does not require radiofrequency fields. EPR was used to identify defects. The width of the cross-relaxation line is almost an order of magnitude smaller than the width of the excited state level-anticrossing line, which was used in all-optical ther- mometry and which can not be significantly reduced since determined by the lifetime of the excited state. With approximately the same temperature shift and the same sig- nal intensities as for excited state level-anticrossing, cross-relaxation signal makes it possible to increase the sensitivity of the temperature measurement by more than an order of magnitude. Temperature sensitivity is estimated to be approximately 10 mK/Hz1/2 within a volume about 1 3, allocated by focused laser excitation in a scanning confocal microscope. Using cross-relaxation in the ground states of bright spin-3/2 centers and dark S=1 centers for temperature sensing and ground state level anti-crossing of bright spin-3/2 centers an integrated magnetic field and tempera- ture sensor with submicron space resolution can be implemented using the same spin system. The coupling of individually addressable bright spin-3/2 centers connected by a chain of dark S=1 spins, could be considered in quantum information pro- cessing and multicenter entanglement under ambient conditions.

[224] Pulse sequences for controlled two- and three-qubit gates in a hybrid quantum register
J. Zhang, S. S. Hegde and D. Suter
Phys. Rev. A 98, 042302 (2018)
AbstractWe propose and demonstrate a quantum control scheme for hybrid quantum registers that can reduce the operation time, and therefore the effects of relaxation, compared to existing implementations. It combines resonant excitation pulses with periods of free precession under the internal Hamiltonian of the qubit system. We use this scheme to implement quantum gates like controlled-NOT operations on electronic and nuclear spins of the nitrogen-vacancy center in diamond. As a specific application, we transfer population between electronic and nuclear spin qubits and use it to measure the Rabi oscillations of a nuclear spin in a system with multiple coupled spins.

[225] Enhanced dynamic nuclear polarization via swept microwave frequency combs
A. Ajoy, R. Nazaryan, K. Liu, X. Lv, B. Safvati, G. Wang, E. Druga, J. A. Reimer, D. Suter, C. Ramanathan, C. A. Meriles and A. Pines
Proceedings of the National Academy of Sciences , (2018)
AbstractDynamic nuclear polarization (DNP) can lead to rapidly accelerated NMR spectroscopy and MRI imaging via significant magnetic resonance signal gains. We develop a technique of microwave frequency comb DNP that is especially suited to broad-spectrum electron radicals widely used for DNP in a variety of contexts. The frequency comb allows one to overcome the bottleneck set by slow electron sweeps, increasing the effective number of polarization transfer events and resulting in a multiplicative gain in DNP signal enhancement. We demonstrate the technique by increasing the DNP enhancement from 30 to 100 measured with respect to the thermal signal at 7T in powdered diamond with optically polarizable defect centers and discuss its applications more widely for a variety of DNP radicals.Dynamic nuclear polarization (DNP) has enabled enormous gains in magnetic resonance signals and led to vastly accelerated NMR/MRI imaging and spectroscopy. Unlike conventional cw-techniques, DNP methods that exploit the full electron spectrum are appealing since they allow direct participation of all electrons in the hyperpolarization process. Such methods typically entail sweeps of microwave radiation over the broad electron linewidth to excite DNP but are often inefficient because the sweeps, constrained by adiabaticity requirements, are slow. In this paper, we develop a technique to overcome the DNP bottlenecks set by the slow sweeps, using a swept microwave frequency comb that increases the effective number of polarization transfer events while respecting adiabaticity constraints. This allows a multiplicative gain in DNP enhancement, scaling with the number of comb frequencies and limited only by the hyperfine-mediated electron linewidth. We demonstrate the technique for the optical hyperpolarization of 13C nuclei in powdered microdiamonds at low fields, increasing the DNP enhancement from 30 to 100 measured with respect to the thermal signal at 7T. For low concentrations of broad linewidth electron radicals [e.g., TEMPO ((2,2,6,6-tetramethylpiperidin-1-yl)oxyl)], these multiplicative gains could exceed an order of magnitude.

[226] Single-beam resonant spin amplification of electrons interacting with nuclei in a GaAs/(Al,Ga)As quantum well
M. Kotur, F. Saeed, R. W. Mocek, V. L. Korenev, I. A. Akimov, A. S. Bhatti, D. R. Yakovlev, D. Suter and M. Bayer
Phys. Rev. B 98, 205304 (2018)
AbstractThe dynamic polarization of nuclear spins interacting with resident electrons under resonant excitation of trions is studied in a nominally undoped GaAs/(Al,Ga)As quantum well. Unlike in common time-resolved pump-probe techniques, we used a single-beam approach in which the excitation light is modulated between the circular and linear polarization states. The time-integrated intensity of the excitation laser reflected from the sample surface, proportional to the optical generation rate and changing due to the pumping of the resident electrons, is detected. Polarized electrons, on the other hand, transfer their spin to the lattice nuclei via the hyperfine interaction. Exciting the sample with a train of pulses in an external magnetic field leads to resonant spin amplification observed when the Larmor precession frequency is synchronized with the laser pulse repetition rate. A buildup of the nuclear spin polarization causes a shifting of the resonant spin amplification peaks since the resulting nuclear field alters the strength of the external magnetic field experienced by the electrons. It was established that the nuclear spin polarization time T1 is temperature dependent and, owing to the electron localization at lower temperatures, becomes shorter. Locking of the nuclear (Overhauser) field in the oblique external magnetic field, related to the anisotropy of the electron g factor, was observed. The g factor ratio between the in-plane (g) and out-of-plane (g) components was estimated to be g/g = 1.3.

[227] Bloch-Siegert shift in a hybrid quantum register: Quantification and compensation
J. Zhang, S. Saha and D. Suter
Phys. Rev. A 98, 052354 (2018)
AbstractQuantum information processing relies on unitary transformations applied to specific qubits. In most cases, these gate operations are driven by alternating electromagnetic fields that are near resonant with specific transitions between eigenstates of the system Hamiltonian. For single-qubit gate operations, they should implement the operation on the target qubit, while all other qubits should be left invariant. It is typically assumed that this goal can be achieved if the amplitude of the control field is small compared to the frequency difference between the field and the transition frequency of the passive qubits. However, in many cases, even qubits whose energy-level differences are very far from the frequency of the applied control field can be affected by nonresonant effects, which are normally nonlinear in the amplitude of the control field. A typical example is the effect known as Bloch-Siegert shift. Unless these shifts are accounted for and, if possible, compensated, they can completely destroy the information contained in the quantum register. Therefore, we study this effect quantitatively in the important example of the nitrogen vacancy center in diamond and demonstrate how it can be eliminated.

[228] Floquet engineering to entanglement protection of distant nitrogen vacancy centers
W. L. Yang, W. L. Song, J. An, M. Feng, D. Suter and J. Du
New J. Phys. 21, 013007 (2019)
AbstractIt remains challenging to preserve entanglement between distant solid-state qubits with high-fidelity, such as nitrogen vacancy centers (NVCs). We propose a Floquet engineering strategy to protect the maximal entanglement between two weakly interacting NVCs separated in long spatial distance by locally applying periodic strong driving on the NVCs. It is found that entanglement of the Floquet states of the NVCs resonantly reaches its maximum during the whole driving period at certain values of the driving parameters. Our analysis reveals that it is the occurrence of the avoided level crossing in the Floquet quasienergy spectrum which results in such entanglement resonance. The dissipation effect on the generated entanglement has also been analyzed. Our results may be of both theoretical and experimental interests in exploring the long-lasting entanglement between weakly interacting spins to long distances in realistic environments.

[229] Dynamics of frequency-swept nuclear spin optical pumping in powdered diamond at low magnetic fields
P. R. Zangara, S. Dhomkar, A. Ajoy, K. Liu, R. Nazaryan, D. Pagliero, D. Suter, J. A. Reimer, A. Pines and C. A. Meriles
Proceedings of the National Academy of Sciences 116, 2512--2520 (2019)
AbstractWe articulate theory and experimental observations to understand the dynamic polarization of 13C spins in nitrogen vacancy (NV)-hosting diamond particles. The explored powder geometry is ideally suited to hyperpolarize fluids, as its inherently large surface-to-volume ratio is much greater than that possible in systems based on single crystals. Given the low-field magnets and low-intensity microwave and light sources we use, our technique promises to serve as a platform for low-cost, portable spin polarizers operating under ambient conditions. Alternatively, the diamond particles themselves could be exploited as background-free contrast agents for magnetic resonance imaging, a modality that builds on the biocompatibility of diamond and thus is complementary to the use of NV-hosting diamond particles as in vivo, fluorescent labels and drug delivery agents.A broad effort is underway to improve the sensitivity of NMR through the use of dynamic nuclear polarization. Nitrogen vacancy (NV) centers in diamond offer an appealing platform because these paramagnetic defects can be optically polarized efficiently at room temperature. However, work thus far has been mainly limited to single crystals, because most polarization transfer protocols are sensitive to misalignment between the NV and magnetic field axes. Here we study the spin dynamics of NV-13C pairs in the simultaneous presence of optical excitation and microwave frequency sweeps at low magnetic fields. We show that a subtle interplay between illumination intensity, frequency sweep rate, and hyperfine coupling strength leads to efficient, sweep-direction-dependent 13C spin polarization over a broad range of orientations of the magnetic field. In particular, our results strongly suggest that finely tuned, moderately coupled nuclear spins are key to the hyperpolarization process, which makes this mechanism distinct from other known dynamic polarization channels. These findings pave the route to applications where powders are intrinsically advantageous, including the hyperpolarization of target fluids in contact with the diamond surface or the use of hyperpolarized particles as contrast agents for in vivo imaging.

[230] Neumanns Beweis, verborgene Variablen und die Unvollständigkeit der Quantenmechanik
D. Suter
Editors: K. Herrmann
Springer Nature (2019)

[231] CVD growth of ultrapure diamond, generation of NV centers by ion implantation, and their spectroscopic characterization for quantum technological applications
T. Chakraborty, F. Lehmann, J. Zhang, S. Borgsdorf, N. Wöhrl, R. Remfort, V. Buck, U. Köhler and D. Suter
Phys. Rev. Materials 3, 065205 (2019)
AbstractApplications of nitrogen-vacancy (NV) centers in diamond in quantum technology have attracted considerable attention in recent years. Deterministic generation of ensembles of NV centers can advance the research on quantum sensing, many-body quantum systems, multipartite entanglement, and so on. Here we report the complete process of controlled generation of NV centers in diamond as well as their characterization: growing diamond films through chemical vapor deposition (CVD), ion implantation, and spectroscopic characterization of the defect centers using a confocal microscope. A microwave-assisted CVD setup is presented which we constructed for the preparation of single-crystalline homoepitaxial diamond films. The films were prepared with minimized nitrogen concentration, which is confirmed through photoluminescence measurements. We demonstrate an in situ ultrahigh vacuum (UHV) implantation and heating process for creation of NV centers using a novel experimental setup. For the first time hot implantation has been shown which prevents surface charging effects. We do not observe graphitization due to UHV heating. By optimizing the implantation parameters it has been possible to implant NV centers in a precise way. We present large area mapping of the samples to determine the distribution of the centers and describe the characterization of the centers by spectroscopic techniques. Reducing the decoherence caused by environmental noise is of primary importance for many applications in quantum technology. We demonstrate improvement on coherence time T2 of the NV spins by suppression of their interaction with the surrounding spin bath using robust dynamical decoupling sequences.

[232] Extending electron paramagnetic resonance to nanoliter volume protein single crystals using a self-resonant microhelix
J. W. Sidabras, J. Duan, M. Winkler, T. Happe, R. Hussein, A. Zouni, D. Suter, A. Schnegg, W. Lubitz and E. J. Reijerse
Science Advances 5, (2019)
AbstractElectron paramagnetic resonance (EPR) spectroscopy on protein single crystals is the ultimate method for determining the electronic structure of paramagnetic intermediates at the active site of an enzyme and relating the magnetic tensor to a molecular structure. However, crystals of dimensions typical for protein crystallography (0.05 to 0.3mm) provide insufficient signal intensity. In this work, we present a microwave self-resonant microhelix for nanoliter samples that can be implemented in a commercial X-band (9.5 GHz) EPR spectrometer. The self-resonant microhelix provides a measured signal-to-noise improvement up to a factor of 28 with respect to commercial EPR resonators. This work opens up the possibility to use advanced EPR techniques for studying protein single crystals of dimensions typical for x-ray crystallography. The technique is demonstrated by EPR experiments on single crystal [FeFe]-hydrogenase (Clostridium pasteurianum; CpI) with dimensions of 0.3 mm by 0.1 mm by 0.1 mm, yielding a proposed g-tensor orientation of the Hox state.

[233] Magnetization Dynamics of an Individual Single-Crystalline Fe-Filled Carbon Nanotube
K. Lenz, R. Narkowicz, K. Wagner, C. F. Reiche, J. Körner, T. Schneider, A. K\'akay, H. Schultheiss, U. Weissker, D. Wolf, D. Suter, B. Büchner, J. Fassbender, T. Mühl and J. Lindner
Small 2019, 1904315 (2019)
AbstractAbstract The magnetization dynamics of individual Fe-filled multiwall carbon-nanotubes (FeCNT), grown by chemical vapor deposition, are investigated by microresonator ferromagnetic resonance (FMR) and Brillouin light scattering (BLS) microscopy and corroborated by micromagnetic simulations. Currently, only static magnetometry measurements are available. They suggest that the FeCNTs consist of a single-crystalline Fe nanowire throughout the length. The number and structure of the FMR lines and the abrupt decay of the spin-wave transport seen in BLS indicate, however, that the Fe filling is not a single straight piece along the length. Therefore, a stepwise cutting procedure is applied in order to investigate the evolution of the ferromagnetic resonance lines as a function of the nanowire length. The results show that the FeCNT is indeed not homogeneous along the full length but is built from 300 to 400 nm long single-crystalline segments. These segments consist of magnetically high quality Fe nanowires with almost the bulk values of Fe and with a similar small damping in relation to thin films, promoting FeCNTs as appealing candidates for spin-wave transport in magnonic applications.

[234] Hyperpolarized relaxometry based nuclear T1 noise spectroscopy in diamond
A. Ajoy, B. Safvati, R. Nazaryan, J. T. Oon, B. Han, P. Raghavan, R. Nirodi, A. Aguilar, K. Liu, X. Cai, X. Lv, E. Druga, C. Ramanathan, J. A. Reimer, C. A. Meriles, D. Suter and A. Pines
Nature Communications 10, 5160 (2019)
AbstractThe origins of spin lifetimes in quantum systems is a matter of importance in several areas of quantum information. Spectrally mapping spin relaxation processes provides insight into their origin and motivates methods to mitigate them. In this paper, we map nuclear relaxation in a prototypical system of 13C nuclei in diamond coupled to Nitrogen Vacancy (NV) centers over a wide field range (1 mT-7 T). Nuclear hyperpolarization through optically pumped NV electrons allows signal measurement savings exceeding million-fold over conventional methods. Through a systematic study with varying substitutional electron (P1 center) and 13C concentrations, we identify the operational relaxation channels for the nuclei at different fields as well as the dominant role played by 13C coupling to the interacting P1 electronic spin bath. These results motivate quantum control techniques for dissipation engineering to boost spin lifetimes in diamond, with applications including engineered quantum memories and hyperpolarized 13C imaging.

[235] Improved Indirect Control of Nuclear Spins in Diamond NV Centers
J. Zhang, S. S. Hegde and D. Suter
Phys. Rev. Applied 12, 064047 (2019)
AbstractHybrid quantum registers consisting of different types of qubit offer a range of advantages as well as challenges. The main challenge is that some types of qubit react only slowly to external control fields, thus considerably slowing down the information-processing operations. One promising approach that has been tested in a number of cases is to use indirect control, where external fields are applied only to qubits that interact strongly with resonant excitation pulses. Here, we use this approach to indirectly control the nuclear spins of a nitrogen-vacancy center, using microwave pulses to drive the electron spin, combined with free precession periods optimized for generating logical gate operations on the nuclear spins. The scheme provides universal control and we present two typical applications: polarizing the nuclear spin and measuring nuclear-spin free-induction-decay signals, both without applying radio-frequency pulses. This scheme is versatile, as it can be implemented over a wide range of magnetic field strengths and at any temperature.

[236] Optimal photon energies for initialization of hybrid spin quantum registers of nitrogen-vacancy centers in diamond
K. Rama Koteswara Rao, Y. Wang, J. Zhang and D. Suter
Phys. Rev. A 101, 013835 (2020)
AbstractInitializing quantum registers with high fidelity is a fundamental precondition for many applications like quantum information processing and sensing. The electronic and nuclear spins of a nitrogen-vacancy (NV) center in diamond form an interesting hybrid quantum register that can be initialized by a combination of laser, microwave, and radio-frequency pulses. However, the laser illumination, which is necessary for achieving electron spin polarization, also has the unwanted side effect of depolarizing the nuclear spin. Here we study how the depolarization dynamics of the 14N nuclear spin depends on the laser wavelength. We show experimentally that excitation with an orange laser (594 nm) causes significantly less nuclear spin depolarization compared to the green laser (532 nm) typically used for excitation and hence leads to higher nuclear spin polarization. This could be because orange light excitation inhibits ionization of NV0 into NV and therefore suppresses one source of noise acting on the nuclear spin.

[237] Room temperature optical nanodiamond hyperpolarizer: Physics, design, and operation
A. Ajoy, R. Nazaryan, E. Druga, K. Liu, A. Aguilar, B. Han, M. Gierth, J. T. Oon, B. Safvati, R. Tsang, J. H. Walton, D. Suter, C. A. Meriles, J. A. Reimer and A. Pines
Review of Scientific Instruments 91, 023106 (2020)
AbstractDynamic Nuclear Polarization (DNP) is a powerful suite of techniques that deliver multifold signal enhancements in nuclear magnetic reso- nance (NMR) and MRI. The generated athermal spin states can also be exploited for quantum sensing and as probes for many-body physics. Typical DNP methods require the use of cryogens, large magnetic fields, and high power microwave excitation, which are expensive and unwieldy. Nanodiamond particles, rich in Nitrogen-Vacancy (NV) centers, have attracted attention as alternative DNP agents because they can potentially be optically hyperpolarized at room temperature. Here, unraveling new physics underlying an optical DNP mechanism first introduced by Ajoy et al. [Sci. Adv. 4, eaar5492 (2018)], we report the realization of a miniature optical nanodiamond hyperpolarizer, where 13C nuclei within the diamond particles are hyperpolarized via the NV centers. The device occupies a compact footprint and operates at room temperature. Instrumental requirements are very modest: low polarizing fields, low optical and microwave irradiation powers, and conve- nient frequency ranges that enable miniaturization. We obtain the best reported optical 13C hyperpolarization in diamond particles exceeding 720 times of the thermal 7 T value (0.86% bulk polarization), corresponding to a ten-million-fold gain in averaging time to detect them by NMR. In addition, the hyperpolarization signal can be background-suppressed by over two-orders of magnitude, retained for multiple-minute long periods at low fields, and deployed efficiently even to 13C enriched particles. Besides applications in quantum sensing and bright-contrast MRI imaging, this work opens possibilities for low-cost room-temperature DNP platforms that relay the 13C polarization to liquids in contact with the high surface-area particles.

[238] Experimental characterization of spin-3/2 silicon vacancy centers in 6H-SiC
H. Singh, A. N. Anisimov, S. S. Nagalyuk, E. N. Mokhov, P. G. Baranov and D. Suter
Phys. Rev. B 101, 134110 (2020)
AbstractSilicon carbide (SiC) hosts many interesting defects that can potentially serve as qubits for a range of advanced quantum technologies. Some of them have very interesting properties, making them potentially useful, e.g., as interfaces between stationary and flying qubits. Here we present a detailed overview of the relevant properties of the spins in silicon vacancies of the 6H-SiC polytype. This includes the temperature-dependent photoluminescence, optically detected magnetic resonance, and the relaxation times of the longitudinal and transverse components of the spins during free precession as well as under the influence of different refocusing schemes.

[239] Efficient Quantum Gates for Individual Nuclear Spin Qubits by Indirect Control
S. S. Hegde, J. Zhang and D. Suter
Phys. Rev. Lett. 124, 220501 (2020)
AbstractHybrid quantum registers, such as electron-nuclear spin systems, have emerged as promising hardware for implementing quantum information and computing protocols in scalable systems. Nevertheless, the coherent control of such systems still faces challenges. Particularly, the lower gyromagnetic ratios of the nuclear spins cause them to respond slowly to control fields, resulting in gate times that are generally longer than the coherence time of the electron. Here, we demonstrate a scheme for circumventing this problem by indirect control: we apply a small number of short pulses only to the electron and let the full system undergo free evolution under the hyperfine coupling between the pulses. Using this scheme, we realize robust quantum gates in an electron-nuclear spin system, including a Hadamard gate on the nuclear spin and a controlled-NOT gate with the nuclear spin as the target qubit. The durations of these gates are shorter than the electron coherence time, and thus additional operations to extend the system coherence time are not needed. Our demonstration serves as a proof of concept for achieving efficient coherent control of electron-nuclear spin systems, such as nitrogen vacancy centers in diamond. Our scheme is still applicable when the nuclear spins are only weakly coupled to the electron.

[240] Optical detection of magnetic resonance
D. Suter
Magnetic Resonance 1, 115--139 (2020)
AbstractThe combination of magnetic resonance with laser spectroscopy provides some interesting options for increasing the sensitivity and information content of magnetic resonance. This review covers the basic physics behind the relevant processes, such as angular momentum conservation during absorption and emission. This can be used to enhance the polarization of the spin system by orders of magnitude compared to thermal polarization as well as for detection with sensitivities down to the level of individual spins. These fundamental principles have been used in many different fields. This review summarizes some of the examples in different physical systems, including atomic and molecular systems, dielectric solids semiconductors.

[241] Efficient Implementation of a Quantum Algorithm in a Single Nitrogen-Vacancy Center of Diamond
J. Zhang, S. S. Hegde and D. Suter
Phys. Rev. Lett. 125, 030501 (2020)
AbstractQuantum computers have the potential to speed up certain problems that are hard for classical computers. Hybrid systems, such as the nitrogen-vacancy (NV) center in diamond, are among the most promising systems to implement quantum computing, provided the control of the different types of qubits can be efficiently implemented. In the case of the NV center, the anisotropic hyperfine interaction allows one to control the nuclear spins indirectly, through gate operations targeting the electron spin, combined with free precession. Here, we demonstrate that this approach allows one to implement a full quantum algorithm, using the example of Grovers quantum search in a single NV center, whose electron is coupled to a carbon nuclear spin.

[242] Precision Limits of Tissue Microstructure Characterization by Magnetic Resonance Imaging
A. Zwick, D. Suter, G. Kurizki and G. A. \'Alvarez
Phys. Rev. Applied 14, 024088 (2020)
AbstractCharacterization of microstructures in living tissues is one of the keys to diagnosing early stages of pathology and understanding disease mechanisms. However, the extraction of reliable information on biomarkers based on microstructure details is still a challenge, as the size of features that can be resolved with noninvasive magnetic resonance imaging (MRI) is orders of magnitude larger than the relevant structures. Here we derive from quantum information theory the ultimate precision limits for obtaining such details by MRI probing of water-molecule diffusion. We show that currently available MRI pulse sequences can be optimized to attain the ultimate precision limits by choosing control parameters that are uniquely determined by the expected size, the diffusion coefficient, and the spin relaxation time T2. By attaining the ultimate precision limit per measurement, the number of measurements and the total acqui- sition time may be drastically reduced compared to the present state of the art. These results are expected to open alternative avenues towards unraveling diagnostic information by quantitative MRI.

[243] Level anti-crossings of an NV center in diamond: Decoherence-free subspaces and 3D sensors of microwave magnetic fields
R. K. R. Kamineni and D. Suter
New Journal of Physics 22, 103065 (2020)
AbstractNitrogen-vacancy (NV) centers in diamond have become an important tool for quantum technologies. All of these applications rely on long coherence times of electron and nuclear spins associated with these centers. Here, we study the energy level anti-crossings of an NV center in diamond coupled to a first-shell 13C nuclear spin in a small static magnetic field. These level anti-crossings (LACs) occur for specific orientations of the static magnetic field due to the strong non-secular components of the Hamiltonian. At these orientations we observe decoherence-free subspaces, where the electron spin coherence times () are 5â{\"A}{\`\i}7 times longer than those at other orientations. Another interesting property at these LACs is that individual transition amplitudes are dominated by a single component of the magnetic dipole moment. Accordingly, this can be used for vector detection of microwave magnetic fields with a single NV center. This is particularly important to precisely control the center using numerical optimal control techniques.

[244] Interference in Atomic Magnetometry
M. Jiang, W. Xu, Q. Li, Z. Wu, D. Suter and X. Peng
Advanced Quantum Technologies 3, 2000078 (2020)
AbstractAbstract Atomic magnetometers are highly sensitive detectors of magnetic fields that monitor the evolution of the macroscopic magnetic moment of atomic vapors, and opening new applications in biological, physical, and chemical science. However, the performance of atomic magnetometers is often limited by hidden systematic effects that may cause misdiagnosis for a variety of applications, for example, in nuclear magnetic resonance (NMR) and in biomagnetism. In this work, a hitherto unexplained interference effect in atomic magnetometers is uncovered, which causes an important systematic effect to greatly deteriorate the accuracy of measuring magnetic fields. A standard approach to detecting and characterizing the interference effect in, but not limited to, atomic magnetometers is presented. As applications of the work, the effect of the interference in NMR structural determination and locating the brain electrophysiological symptom is considered, and it is shown that it will help to improve the measurement accuracy by taking interference effects into account. Through the experiments, good agreement is indeed found between the prediction and the asymmetric amplitudes of resonant lines in ultralow-field NMR spectraâ{\"A}{\^\i}an effect that has not been understood so far. It is anticipated that the work will stimulate interesting new researches for magnetic interference phenomena in a wide range of magnetometers and their applications.

[245] Optical spin initialization of spin-3/2 silicon vacancy centers in 6H SiC at room temperature
H. Singh, A. N. Anisimov, I. D. Breev, P. G. Baranov and D. Suter
Phys. Rev. B 103, 104103 (2021)
AbstractSilicon vacancies in silicon carbide have been proposed as an alternative to nitrogen vacancy centers in diamonds for spintronics and quantum technologies. An important precondition for these applications is the initialization of the qubits into a specific quantum state. In this work, we study the optical alignment of the spin 3/2 negatively charged silicon vacancy in 6H-SiC. Using time-resolved optically detected magnetic resonance, we coherently control the silicon vacancy spin ensemble and measure Rabi frequencies, spin-spin and spin-lattice relaxation times of all three transitions. Then to study the optical initialization process of the silicon vacancy spin ensemble, the vacancy spin ensemble is prepared in different ground states and optically excited. We describe a simple rate equation model that can explain the observed behavior and determine the relevant rate constants.

[246] Integrated and Portable Magnetometer Based on Nitrogen-Vacancy Ensembles in Diamond
F. M. Stürner, A. Brenneis, T. Buck, J. Kassel, R. Rölver, T. Fuchs, A. Savitsky, D. Suter, J. Grimmel, S. Hengesbach, M. Förtsch, K. Nakamura, H. Sumiya, S. Onoda, J. Isoya and F. Jelezko
Advanced Quantum Technologies 4, 2000111 (2021)
AbstractAbstract Magnetic field sensors that exploit quantum effects have shown that they can outperform classical sensors in terms of sensitivity enabling a range of novel applications in future, such as a brain machine interface. Negatively charged nitrogen-vacancy (NV) centers in diamond have emerged as a promising high sensitivity platform for measuring magnetic fields at room temperature. Transferring this technology from laboratory setups into products and applications, the total size of the sensor, the overall power consumption, and the costs need to be reduced and optimized. Here, a fiber-based NV magnetometer featuring a complete integration of all functional components is demonstrated without using any bulky laboratory equipment. This integrated prototype allows portable measurement of magnetic fields with a sensitivity of 344 pT Hzâ{\`a}{\'\i}1/2.

[247] Relaxation processes and high-field coherent spin manipulation in color center ensembles in 6H-SiC
V. A. Soltamov, B. V. Yavkin, A. N. Anisimov, H. Singh, A. P. Bundakova, G. V. Mamin, S. B. Orlinskii, E. N. Mokhov, D. Suter and P. G. Baranov
Phys. Rev. B 103, 195201 (2021)
AbstractCoherent spin manipulations of spin- 3 2 color center ensembles in 6H-SiC crystal have been studied in high magnetic fields using methods of pulsed electron paramagnetic resonance, Rabi oscillations, and pulsed electron-electron double resonance under optical alignment conditions of the spin level populations. Rabi oscillation experiments show room temperature coherent control of these spin- 3/2 color center ensembles in strong magnetic fields. A sharp decrease of the spin-lattice relaxation time T 1 , 40 times, was observed in 6H-SiC at magnetic field of 3.5 T with increasing temperature from 100 to 300 K, while the spin-spin relaxation time T 2 is only shortened by 1.3 times. With an increase in the magnetic field, the times T 1 and T 2 were shown to decrease. The relaxation time T 1 in the case of magnetic field directed along the axis of the spin- 3 2 center is 2 times longer than T 1 in magnetic field perpendicular to this axis. Relaxation times of the spin center in crystal grown with a reduced concentration of an isotope 29 Si are significantly longer than crystal, with the natural content of isotopes. With a decrease in the 29 Si content in our experiments by a factor of 5, the effective nuclear spin bath in SiC is reduced by a factor of 2. In a zero magnetic field resonance, transitions are allowed as magnetic dipole transitions with frequency 0 which correspond to the zero-field splitting. In zero magnetic field and in fixed magnetic fields, the Rabi frequency was shown, using so-called Feynman-Vernon-Hellwarth transformation, to be R = | | B 1 . In pulsed electron-electron double resonance experiments, a change in the intensity of the electron spin echo signal corresponding to one of the spin-allowed fine structure transitions is recorded depending on the sweep of the second frequency. The experiments show the possibility to coherently detect the optical spin alignment between M S = 3/2 via optically pumped silent M S = 1/2 sublevels of the spin- 3 2 color centers, including a detection of Rabi oscillations.

[248] Optimal control pulses for subspectral editing in low field NMR
A. Bahti, A. Telfah, J. Lambert, R. Hergenröder and D. Suter
Journal of Magnetic Resonance 328, 106993 (2021)
AbstractLow field NMR is an inexpensive and low footprint technique to obtain physical, chemical, electronic and structural information on small molecules, but suffers from poor spectral dispersion, especially when applied to the analysis of mixtures. Subspectral editing employing optimal control pulses is a suitable approach to cope with the severe signal superpositions in complex mixture spectra at low field. In this contribution, the use of optimal control pulses is demonstrated to be feasible at a field strength as low as 0.5 T. The optimal control pulse shapes were calculated using the Krotov algorithm. Downsizing the complexity of the algorithm from exponential to polynomial is shown to be possible by using a system approach with each system corresponding to a (small) molecule. In this way compound selective excitation pulses can be calculated. The signals of substructures of the cyclopentenone molecule were excited using optimal control pulses calculated by the Krotov algorithm demonstrating the feasibility of subspectral editing. Likewise, for a mixture of benzoic acid and alanine, editing of the signals of either benzoic acid or alanine employing optimal control pulses was shown to be possible. The obtained results are very promising and can be extended to the targeted analysis of complex mixtures such as biofluids or metabolic samples at low field strengths opening access for benchtop NMR to point of care settings.

[249] Decoherence scaling transition in the dynamics of quantum information scrambling
F. D. Dominguez, M. C. Rodriguez, R. Kaiser, D. Suter and G. A. Alvarez
Phys. Rev. A 104, 012402 (2021)
AbstractReliable processing of quantum information for developing quantum technologies requires precise control of out-of-equilibrium many-body systems. This is a highly challenging task because the fragility of quantum states to external perturbations increases with the system size. Here, we report on a series of experimental quantum simulations that quantify the sensitivity of a controlled Hamiltonian evolution to perturbations that drive the system away from the targeted evolution. Based on out-of-time ordered correlations, we demonstrate that the decay rate of the process fidelity increases with the effective number K of correlated qubits as K. As a function of the perturbation strength, we observe a decoherence scaling transition of the exponent between two distinct dynamical regimes. In the limiting case below the critical perturbation strength, the exponent drops sharply below 1, and there is no inherent limit to the number of qubits that can be controlled. This resilient quantum feature of the controlled dynamics of quantum information is promising for reliable control of large quantum systems.

[250] Dielectric Coupler for General Purpose Q-Band EPR Cavity
M. Judd, G. Jolley, D. Suter, N. Cox and A. Savitsky
Applied Magnetic Resonance , (2021)
AbstractHere, we report on a robust and efficient mechanism for tuning the microwave coupling of a Q-band (34 GHz), general purpose, cylindrical EPR cavity operating in the TE011 mode. This novel mechanism allows for both the adjustment of the cavity's coupling over a wide frequency range, as well as its bandwidth from that of a high-Q cavity (about 10 MHz), to a broadband cavity (above 1 GHz). The coupling element consists of a dielectric plate fixed onto a movable waveguide short that allows for two modes of operation. In the first mode, the dielectric plate does not influence the resonance properties of the coupling iris and allows for precise, critical coupling of the high-Q cavity. In the second mode, the dielectric plate is positioned in front of the coupling iris, varying the iris'resonance properties and allowing very strong overcoupling to be achieved. This mechanism can be generalized for other types of EPR cavities, in particular at high microwave frequencies.

[251] Ultra-deep optical cooling of coupled nuclear spin-spin and quadrupole reservoirs in a GaAs/(Al,Ga)As quantum well
M. Kotur, D. O. Tolmachev, V. M. Litvyak, K. V. Kavokin, D. Suter, D. R. Yakovlev and M. Bayer
Communications Physics 4, 193 (2021)
AbstractThe physics of interacting nuclear spins in solids is well interpreted within the nuclear spin temperature concept. A common approach to cooling the nuclear spin system is adiabatic demagnetization of the initial, optically created, nuclear spin polarization. Here, the selective cooling of 75As spins by optical pumping followed by adiabatic demagnetization in the rotating frame is realized in a nominally undoped GaAs/(Al,Ga)As quantum well. The lowest nuclear spin temperature achieved is 0.54 {\OE}¼K. The rotation of 6 kG strong Overhauser field at the 75As Larmor frequency of 5.5 MHz is evidenced by the dynamic Hanle effect. Despite the presence of the quadrupole induced nuclear spin splitting, it is shown that the rotating 75As magnetization is uniquely determined by the spin temperature of coupled spin-spin and quadrupole reservoirs. The dependence of heat capacity of these reservoirs on the external magnetic field direction with respect to crystal and structure axes is investigated.

[252] Floquet Prethermalization with Lifetime Exceeding 90 s in a Bulk Hyperpolarized Solid
W. Beatrez, O. Janes, A. Akkiraju, A. Pillai, A. Oddo, P. Reshetikhin, E. Druga, M. McAllister, M. Elo, B. Gilbert, D. Suter and A. Ajoy
Phys. Rev. Lett. 127, 170603 (2021)
AbstractWe report the observation of long-lived Floquet prethermal states in a bulk solid composed of dipolar- coupled 13C nuclei in diamond at room temperature. For precessing nuclear spins prepared in an initial transverse state, we demonstrate pulsed spin-lock Floquet control that prevents their decay over multiple- minute-long periods. We observe Floquet prethermal lifetimes T02 90.9 s, extended > 60 000-fold over the nuclear free induction decay times. The spins themselves are continuously interrogated for 10 min, corresponding to the application of 5.8 106 control pulses. The 13C nuclei are optically hyperpolarized by lattice nitrogen vacancy centers; the combination of hyperpolarization and continuous spin readout yields significant signal-to-noise ratio in the measurements. This allows probing the Floquet thermalization dynamics with unprecedented clarity. We identify four characteristic regimes of the thermalization process, discerning short-time transient processes leading to the prethermal plateau and long-time system heating toward infinite temperature. This Letter points to new opportunities possible via Floquet control in networks of dilute, randomly distributed, low-sensitivity nuclei. In particular, the combination of minutes- long prethermal lifetimes and continuous spin interrogation opens avenues for quantum sensors constructed from hyperpolarized Floquet prethermal nuclei.

[253] Magnetic-Field-Assisted Spectral Decomposition and Imaging of Charge States of $\mathrmN$-$V$ Centers in Diamond
T. Chakraborty, R. Bhattacharya, V. S. Anjusha, M. Nesladek, D. Suter and T. S. Mahesh
Phys. Rev. Applied 17, 024046 (2022)
AbstractWith the advent of quantum technology, nitrogen vacancy (N-V) centers in diamond turn out to be a frontier that provides an efficient platform for quantum computation, communication, and sensing applications. Due to the coupled spin-charge dynamics of the N-V system, knowledge of N-V charge- state dynamics can help to formulate efficient spin-control sequences strategically. Here, we report two spectroscopy-based deconvolution methods to create charge-state mapping images of ensembles of N-V centers in diamond. First, relying on the fact that an off-axis external magnetic field mixes the electronic spins and selectively modifies the photoluminescence (PL) of N-V, we perform decomposition of the optical spectrum for an ensemble of N-V and extract the spectra for N-V and N-V0 states. Next, we introduce an optical-filter-based decomposition protocol and perform PL imaging for N-V and N-V0. Previously obtained spectra for N-V and N-V0 states are used to calculate their transmissivities through a long-pass optical filter. These results help us to determine the spatial distribution of the N-V charge states in a diamond sample.

[254] Optimization of a quantum control sequence for initializing a nitrogen-vacancy spin register
T. Chakraborty, J. Zhang and D. Suter
Phys. Rev. A 105, 022622 (2022)
AbstractImplementation of many quantum information protocols requires an efficient initialization of the quantum register. In the present paper, we optimize a population trapping protocol for initializing a hybrid spin register associated with a single nitrogen-vacancy (NV) center in diamond. We initialize the quantum register by polarizing the electronic and nuclear spins of the NV with a sequence of microwave, radio-frequency, and optical pulses. We use a rate equation model to explain the distribution of population under the effect of the optical pulses. The model is compared to the experimental data obtained by performing partial quantum state tomography. To further increase the spin polarization, we propose a recursive protocol with optimized optical pulses. We also discuss the role of the relative values of the nuclear- and electronic-spin pumping rate in achieving the maximum degree of spin polarization.

[255] Inverted fine structure of a 6H-SiC qubit enabling robust spin-photon interface
I. D. Breev, Z. Shang, A. V. Poshakinskiy, H. Singh, Y. Berencen, M. Hollenbach, S. S. Nagalyuk, E. N. Mokhov, R. A. Babunts, P. G. Baranov, D. Suter, S. A. Tarasenko, G. V. Astakhov and A. N. Anisimov
npj Quantum Information 8, 23 (2022)
AbstractControllable solid-state spin qubits are currently becoming useful building blocks for applied quantum technologies. Here, we demonstrate that in a specific type of silicon-vacancy in the 6H-SiC polytype the excited-state fine structure is inverted, compared to 4H-SiC. From the angular polarization dependencies of the emission, we reconstruct the spatial symmetry and determine the optical selection rules depending on the local deformation and spinorbit interaction. We show that this system is well suited for the implementation of robust spinphoton entanglement schemes. Furthermore, the inverted fine structure leads to unexpected behavior of the spin readout contrast. It vanishes and recovers with lattice cooling due to two competing optical spin pumping mechanisms. Our experimental and theoretical approaches provide a deep insight into the optical and spin properties of atomic-scale qubits in SiC required for quantum communication and distributed quantum information processing.

[256] Multi-photon multi-quantum transitions in the spin-3/2 silicon-vacancy centers of SiC
H. Singh, M. A. Hollberg, A. N. Anisimov, P. G. Baranov and D. Suter
Phys. Rev. Research 4, 023022 (2022)
AbstractSilicon vacancy centers in silicon carbide are promising candidates for storing and manipulating quantum information. Implementation of fast quantum gates is an essential requirement for quantum information process- ing. In a low magnetic field, the resonance frequencies of silicon vacancy spins are in the range of a few MHz, the same order of magnitude as the Rabi frequencies of typical control fields. As a consequence, the rotating wave approximation becomes invalid and nonlinear processes like the absorption and emission of multiple photons become relevant. This paper focuses on multi-photon transitions of negatively charged silicon vacancies driven by a strong RF field. We present continuous-wave optically detected magnetic resonance (ODMR) spectra measured at different RF powers to identify the 1-, 2-, and 3 RF photon transitions of different types of the silicon vacancy in the 6H-SIC polytype. Time-resolved experiments of Rabi oscillations and free induction decays of these multiple RF photon transitions were observed. Apart from zero-field data, we also present spectra in magnetic fields with different strength and orientation with respect to the systems symmetry axis.

[257] Toward the Speed Limit of High-Fidelity Two-Qubit Gates
S. S. Hegde, J. Zhang and D. Suter
Phys. Rev. Lett. 128, 230502 (2022)
AbstractMost implementations of quantum gate operations rely on external control fields to drive the evolution of the quantum system. Generating these control fields requires significant efforts to design the suitable control Hamiltonians. Furthermore, any error in the control fields reduces the fidelity of the implemented control operation with respect to the ideal target operation. Achieving sufficiently fast gate operations at low error rates remains therefore a huge challenge. In this Letter, we present a novel approach to overcome this challenge by eliminating, for specific gate operations, the time-dependent control fields entirely. This approach appears useful for maximizing the speed of the gate operation while simultaneously eliminating relevant sources of errors. We present an experimental demonstration of the concept in a single nitrogen-vacancy center in diamond at room temperature.

[258] Small Armed Aircraft and Missiles -- Dangers for International Security
J. Altmann, M. Pilch and D. Suter
Die Friedens-Warte 95, 316--339 (2022)
AbstractArmed forces show interest in small and very small aircraft and missiles; research, development, and deployment are increasing since about a decade and will likely accelerate further. We have collected information on small (size <2 m) and very small (<0.2 m) uninhabited air vehicles (UAVs). Our UAV database contains 152 types from 30 countries, 24 types are armed. Much less military effort has gone into small and very small missiles (diameter < 69 mm and < 40 mm, respectively). Some types have existed since decades, new ones could be used, e. g., as weapons of small UAVs. Our missile database has 50 types from 17 countries. Basic properties of UAVs and missiles, major results from the databases, and a short assessment of technological trends are given. Both kinds of armed systems are considered under criteria of preventive arms control. Options for preventive limitations, for confidence building, and for export control are discussed and recommended.

[259] Erratum: Spin Hamiltonian characterization and refinement for Pr3+:YAlO3 and Pr3+:Y2SiO5 [Phys. Rev. B 85, 014429 (2012)]
M. Lovric, P. Glasenapp and D. Suter
Phys. Rev. B 107, 019903 (2023)

[260] Variable bandwidth, high efficiency microwave resonator for control of spin-qubits in nitrogen-vacancy centers
A. Savitsky, J. Zhang and D. Suter
Review of Scientific Instruments 94, 023101 (2023)
AbstractNitrogen-Vacancy (NV) centers in diamond are attractive tools for sensing and quantum information. Realization of this potential requires effective tools for controlling the spin degree of freedom by microwave (mw) magnetic fields. In this work, we present a planar microwave resonator optimized for microwave-optical double resonance experiments on single NV centers in diamond. It consists of a piece of wide microstrip line, which is symmetrically connected to two 50 microstrip feed lines. In the center of the resonator, an -shaped loop focuses the current and the mw magnetic field. It generates a relatively homogeneous magnetic field over a volume of 0.07 0.1 mm3. It can be operated at 2.9 GHz in both transmission and reflection modes with bandwidths of 1000 and 400 MHz, respectively. The high power-to- magnetic field conversion efficiency allows us to produce -pulses with a duration of 50 ns with only about 200 and 50 mW microwave power in transmission and reflection, respectively. The transmission mode also offers capability for efficient radio frequency excitation. The resonance frequency can be tuned between 1.3 and 6 GHz by adjusting the length of the resonator. This will be useful for experiments on NV-centers at higher external magnetic fields and on different types of optically active spin centers.

[261] Fast Quantum State Tomography in the Nitrogen Vacancy Center of Diamond
J. Zhang, S. S. Hegde and D. Suter
Phys. Rev. Lett. 130, 090801 (2023)
AbstractQuantum state tomography is the procedure for reconstructing unknown quantum states from a series of measurements of different observables. Depending on the physical system, different sets of observables have been used for this procedure. In the case of spin qubits, the most common procedure is to measure the transverse magnetization of the system as a function of time. Here, we present a different scheme that relies on time-independent observables and therefore does not require measurements at different evolution times, thereby greatly reducing the overall measurement time. To recover the full density matrix, we use a set of unitary operations that transform the density operator elements into the directly measurable observable. We demonstrate the performance of this scheme in the electron-nuclear spin system of the nitrogen vacancy center in diamond.

[262] Quantum pattern recognition on real quantum processing units
S. Das, J. Zhang, S. Martina, D. Suter and F. Caruso
Quantum Machine Intelligence 5, 16 (2023)
AbstractOne of the most promising applications of quantum computing is the processing of graphical data like images. Here, we investigate the possibility of realizing a quantum pattern recognition protocol based on swap test, and use the IBMQ noisy intermediate-scale quantum (NISQ) devices to verify the idea. We find that with a two-qubit protocol, swap test can efficiently detect the similarity between two patterns with good fidelity, though for three or more qubits, the noise in the real devices becomes detrimental. To mitigate this noise effect, we resort to destructive swap test, which shows an improved performance for three-qubit states. Due to limited cloud access to larger IBMQ processors, we take a segment-wise approach to apply the destructive swap test on higher dimensional images. In this case, we define an average overlap measure which shows faithfulness to distinguish between two very different or very similar patterns when run on real IBMQ processors. As test images, we use binary images with simple patterns, grayscale MNIST numbers and fashion MNIST images, as well as binary images of human blood vessel obtained from magnetic resonance imaging (MRI). We also present an experimental set up for applying destructive swap test using the nitrogen vacancy (NVs) center in diamond. Our experimental data show high fidelity for single qubit states. Lastly, we propose a protocol inspired from quantum associative memory, which works in an analogous way to supervised learning for performing quantum pattern recognition using destructive swap test.

[263] Characterization of single shallow silicon-vacancy centers in 4H-SiC
H. Singh, M. A. Hollberg, M. Ghezellou, J. Ul-Hassan, F. Kaiser and D. Suter
Phys. Rev. B 107, 134117 (2023)
AbstractShallow negatively charged silicon-vacancy centers have applications in magnetic quantum sensing and other quantum applications. Vacancy centers near the surface (within 100 nm) have different spin relaxation rates and optical spin polarization, affecting the optically detected magnetic resonance (ODMR) signal. This makes it essential to characterize these centers. Here we present the relevant spin properties of such centers. ODMR with a contrast of up to 6%, which is better than the state of the art, allowed us to determine the zero-field splitting, which is relevant for most sensing applications. We also present intensity-correlation data to verify that the signal originates from a single center and to extract transition rates between different electronic states.

[264] Electron Induced Nanoscale Nuclear Spin Relaxation Probed by Hyperpolarization Injection
W. Beatrez, A. Pillai, O. Janes, D. Suter and A. Ajoy
Phys. Rev. Lett. 131, 010802 (2023)
AbstractWe report on experiments that quantify the role of a central electronic spin as a relaxation source for nuclear spins in its nanoscale environment. Our strategy exploits hyperpolarization injection from the electron as a means to controllably probe an increasing number of nuclear spins in the bath and subsequently interrogate them with high fidelity. Our experiments are focused on a model system of a nitrogen vacancy center electronic spin surrounded by several hundred 13C nuclear spins. We observe that the 13C transverse spin relaxation times vary significantly with the extent of hyperpolarization injection, allowing the ability to measure the influence of electron-mediated relaxation extending over several nanometers. These results suggest interesting new means to spatially discriminate nuclear spins in a nanoscale environment and have direct relevance to dynamic nuclear polarization and quantum sensors and memories constructed from hyperpolarized nuclei.

[265] Single NV centers as sensors for radio-frequency fields
J. Zhang and D. Suter
Phys. Rev. Res. 5, L022026 (2023)
AbstractWe show that a single electron spin can serve as a sensor for radio-frequency (rf) magnetic fields. The longitudinal and transverse components of the rf field can be extracted from the phase acquired during free evolution of the spin coherence. In our experimental demonstration, a single electron spin of an NV center in diamond serves as an atomic size of two components of an rf field.

[266] Zero-Field ODMR and relaxation of Si-vacancy centers in 6H-SiC
H. Singh, A. N. Anisimov, P. G. Baranov and D. Suter
Materials Research Express 10, 116201 (nov.2023)
AbstractSilicon vacancies in silicon carbide (SiC) have been proposed as interesting candidates for quantum technology applications such as quantum sensing and quantum repeaters. SiC exists in many polytypes with different plane stacking sequences, and in each polytype, the vacancies can occupy a variety of different lattice sites. In this work, we focus on the three important charged silicon vacancies in the 6H-SiC polytype. We record the photoluminescence and continuous-wave optically detected magnetic resonance (ODMR) spectra at different radio-frequency power levels and different temperatures. We individually select the zero-phonon lines of the different silicon vacancies at low temperatures and record the corresponding ODMR spectra. ODMR allows us to correlate optical and magnetic resonance spectra and thereby separate signals from V 1 and V 3. The results also explain the observed sign change of the ODMR signal as a function of temperature.

[267] Low-field NMR with multilayer Halbach magnet and NMR selective excitation
A. Telfah, A. Bahti, K. Kaufmann, E. Ebel, R. Hergenröder and D. Suter
Scientific Reports 13, 21092 (2023)
Abstract{This study introduces a low-field NMR spectrometer (LF-NMR) featuring a multilayer Halbach magnet supported by a combined mechanical and electrical shimming system. This setup offers improved field homogeneity and sensitivity compared to spectrometers relying on typical Halbach and dipole magnets. The multilayer Halbach magnet was designed and assembled using three nested cylindrical magnets, with an additional inner Halbach layer that can be rotated for mechanical shimming. The coils and shim-kernel of the electrical shimming system were constructed and coated with layers of zirconia, thermal epoxy, and silver-paste resin to facilitate passive heat dissipation and ensure mechanical and thermal stability. Furthermore, the 7-channel shim coils were divided into two parts connected in parallel, resulting in a reduction of joule heating temperatures from 96.2 to 32.6 $\,^{\circ}$C. Without the shimming system, the Halbach magnet exhibits a field inhomogeneity of approximately 140 ppm over the sample volume. The probehead was designed to incorporate a solenoidal mini coil, integrated into a single planar board. This design choice aimed to enhance sensitivity, minimize B1 inhomogeneity, and reduce impedance discrepancies, transmission loss, and signal reflections. Consequently, the resulting linewidth of water within a 3 mm length and 2.4 mm inner diameter sample volume was 4.5 Hz. To demonstrate the effectiveness of spectral editing in LF-NMR applications at 29.934 MHz, we selectively excited hydroxyl and/or methyl protons in neat acetic acid using optimal control pulses calculated through the Krotov algorithm.}

[268] Vector detection of ac magnetic fields by nitrogen vacancy centers of single orientation in diamond
P. Lamba, A. Rana, S. Halder, S. Dhomkar, D. Suter and R. K. Kamineni
Phys. Rev. B 109, 195424 (May.2024)

[269] Spin-noise-induced synchronization between non-overlapping laser beams
S. Song, M. Jiang, Y. Qin, Z. Wu, H. Su, R. Liu, D. Suter and X. Peng
Phys. Rev. A 110, 033105 (Sep.2024)

[270] NMR Spectral Editing, Water Suppression, and Dipolar Decoupling in Low-Field NMR Spectroscopy Using Optimal Control Pulses and Multiple-Pulse Sequence
A. Bahti, A. Telfah, R. Hergenröder and D. Suter
Analytical Chemistry 97, 1983--1991 (02.2025)
AbstractSpectral dispersion in low-field nuclear magnetic resonance (NMR) can significantly affect NMR spectral analysis, particularly when studying complex mixtures like metabolic profiling of biological samples. To address signal superposition in these spectra, we employed spectral editing with selective excitation pulses, proving it to be a suitable approach. Optimal control pulses were implemented in low-field NMR and demonstrated their capability to selectively excite and eliminate specific amino acids, such as phenylalanine and taurine, either individually or simultaneously. The broadening of NMR signals in viscous samples, like bio samples, due to homonuclear dipolar coupling often leads to loss of spectral details, impacting spectral assignments. Therefore, in this work, the multiple-pulse WAHUHA sequence at both high and low field NMR was employed resulting in approximately 63 and 25{\%} reduction in line widths respectively, evident from line width changes in the NMR spectra. The effectiveness of this process was validated by comparing its performance with that of magic angle spinning NMR. Additionally, water suppression was achieved through selective excitation by adding a term representing the water signal to the overall Hamiltonian, expressing the water signal peak frequency, and covering adjacent frequencies on both sides of the water peak within the water signal.