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Quantum state tomography (QST) 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., Supplementary Material available by email jingfu.zhang@tu-dortmund.de or zhangjfu2000@yahoo.com

Nitrogen-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 nitrogen-vacancy (NV) centers in diamond. It consists of a piece of wide microstrip line which is symmetrically connected to two 50 $\Omega$ microstrip feed lines. In the center of the resonator, an $\Omega$-shaped loop focuses the current and the mw magnetic field. It generates a relatively homogeneous magnetic field over a volume of 0.07mm$^2\times$0.1mm. It can be operated at 2.9 GHz in both transmission and reflection modes with bandwidths of 1000 MHz and 400 MHz, respectively. The high power-to-magnetic field conversion efficiency allows to produce $\pi$-pulses with a duration of 50 ns with only about 200 mW 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 GHz 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.

Shallow 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.

Here, 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., Applied magnetic resonance;Vol . 53, 2022, Issue 7/9, pp 963–977

We 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., Comment: supplementary material available by email jingfu.zhang@tu-dortmund.de

The 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 $${}^{13}{\rm{C}}$$ 13 C 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 $${}^{13}{\rm{C}}$$ 13 C concentrations, we identify the operational relaxation channels for the nuclei at different fields as well as the dominant role played by $${}^{13}{\rm{C}}$$ 13 C 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 $${}^{13}{\rm{C}}$$ 13 C imaging.

Reliable processing of quantum information for developing quantum technologies requires precise control of out-of-equilibrium many-bodysystems. This is a highly challenging task as 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 $K^{\alpha}$. As a function of the perturbation strength, we observe a decoherence scaling transition of the exponent $\alpha$ between two distinct dynamical regimes. In the limiting case below the critical perturbation strength, the exponent $\alpha$ 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., Comment: 5 pages, 3 figures, material and methods

Coherent spin manipulations of spin-$\frac{3}{2}$ 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-$\frac{3}{2}$ color center ensembles in strong magnetic fields. A sharp decrease of the spin-lattice relaxation time ${T}_{1}$, \ensuremath{\sim}40 times, was observed in 6H-SiC at magnetic field of \ensuremath{\sim}3.5 T with increasing temperature from 100 to 300 K, while the spin-spin relaxation time ${T}_{2}$ is only shortened by \ensuremath{\sim}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-$\frac{3}{2}$ center is \ensuremath{\sim}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 $^{29}\mathrm{Si}$ are significantly longer than crystal, with the natural content of isotopes. With a decrease in the $^{29}\mathrm{Si}$ content in our experiments by a factor of \ensuremath{\sim}5, the effective nuclear spin bath in SiC is reduced by a factor of \ensuremath{\sim}2. In a zero magnetic field resonance, transitions are allowed as magnetic dipole transitions with frequency ${\ensuremath{\omega}}_{0}$ 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 ${\ensuremath{\omega}}_{R}=|\ensuremath{\gamma}|{B}_{1}$. 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 ${M}_{S}=\ifmmode\pm\else\textpm\fi{}\frac{3}{2}$ via optically pumped silent ${M}_{S}=\ifmmode\pm\else\textpm\fi{}\frac{1}{2}$ sublevels of the spin-$\frac{3}{2}$ color centers, including a detection of Rabi oscillations.

Silicon 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 $6H\text{\ensuremath{-}}\mathrm{SiC}$. 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.

We report the observation of long-lived Floquet prethermal states in a bulk solid composed of dipolar-coupled $^{13}$C 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 $T_2'\approx$90.9s, extended >60,000-fold over the nuclear free induction decay times. The spins themselves are continuously interrogated for $\sim$10min, corresponding to the application of $\approx$5.8M control pulses. The $^{13}$C nuclei are optically hyperpolarized by lattice Nitrogen Vacancy (NV) centers; the combination of hyperpolarization and continuous spin readout yields significant signal-to-noise 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 towards infinite temperature. This work 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., 5 pages, 5 figures. SI: 2 pages, 4 figures

Low 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.

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, e.g., in NMR and in biomagnetism. In this work, we uncover a hitherto unexplained interference effect in atomic magnetometers, which causes an important systematic effect to greatly deteriorate the accuracy of measuring magnetic fields. We present a standard approach to detecting and characterizing the interference effect in, but not limited to, atomic magnetometers. As applications of our work, we consider the effect of the interference in NMR structural determination and locating the brain electrophysiological symptom, and show that it will help to improve the measurement accuracy by taking interference effects into account. Through our experiments, we indeed find good agreement between our prediction and the asymmetric amplitudes of resonant lines in ultralow-field NMR spectra -- an effect that has not been understood so far. We anticipate that our work will stimulate interesting new researches for magnetic interference phenomena in a wide range of magnetometers and their applications., 29 pages, 4 figures

Quantum 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 Grover's quantum search in a single NV center, whose electron is coupled to a carbon nuclear spin. The results clearly demonstrate the advantage of the quantum algorithm over the classical case., Comment: Supplemental material added; Accepted for publication in PRL

The 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 polarisation 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 summarises some of the examples in different physical system, including atomic and molecular systems, dielectric solids composed of rare earth and transition metal ions and semiconductors., Magnetic resonance;Vol.1. 2020, 1, p.115-139

Silicon 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.

Diffusion magnetic resonance imaging (dMRI) is a powerful tool in clinical applications, in particular, in oncology screening. dMRI demonstrated its benefit and efficiency in the localisation and detection of different types of human brain tumours. Clinical dMRI data suffer from multiple artefacts such as motion and eddy-current distortions, contamination by noise, outliers etc. In order to increase the image quality of the derived diffusion scalar metrics and the accuracy of the subsequent data analysis, various pre-processing approaches are actively developed and used. In the present work we assess the effect of different pre-processing procedures such as a noise correction, different smoothing algorithms and spatial interpolation of raw diffusion data, with respect to the accuracy of brain glioma differentiation. As a set of sensitive biomarkers of the glioma malignancy grades we chose the derived scalar metrics from diffusion and kurtosis tensor imaging as well as the neurite orientation dispersion and density imaging (NODDI) biophysical model. Our results show that the application of noise correction, anisotropic diffusion filtering, and cubic-order spline interpolation resulted in the highest sensitivity and specificity for glioma malignancy grading. Thus, these pre-processing steps are recommended for the statistical analysis in brain tumour studies.

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 "gold standard" 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 20μm and outer diameters of 90 and 150μm. 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 Mitra's limit. The perspectives and limitations of these diffusion phantoms are presented and discussed.

Dynamic Nuclear Polarization (DNP) is a powerful suite of techniques that deliver multifold signal enhancements in 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 use of cryogens, large magnetic fields, and high power microwaves, 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. Indeed the realization of a miniature "optical nanodiamond hyperpolarizer", where 13C nuclei are optically hyperpolarized has been a longstanding goal but has been technically challenging to achieve. Here, unravelling new physics underlying an optical DNP mechanism first introduced in [Ajoy et al., Sci. Adv. 4, eaar5492 (2018)], we report the realization of such a device in an ultracompact footprint and working fully at room temperature. Instrumental requirements are very modest: low polarizing fields, extremely low optical and microwave irradiation powers, and convenient frequency ranges that enable device miniaturization. We obtain best reported optical 13C hyperpolarization in diamond particles exceeding 720 times of the thermal 7T value (0.86% bulk polarization), corresponding to a ten-million-fold gain in NMR averaging time. In addition the hyperpolarization signal can be background-suppressed by over two-orders of magnitude and retained for multiple-minute long periods. Besides compelling applications in quantum sensing, and bright-contrast MRI imaging, this work paves the way for low-cost DNP platforms that relay the 13C polarization to liquids in contact with the high surface-area particles. This will ultimately allow development of miniature "quantum-assisted" NMR spectrometers for chemical analysis., Contains supplementary info

Initializing 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 $^{14}\mathrm{N}$ 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 ${\mathrm{NV}}^{0}$ into ${\mathrm{NV}}^{\ensuremath{-}}$ and therefore suppresses one source of noise acting on the nuclear spin.

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.

Self-resonant microhelix provides sensitivity for X-band electron paramagnetic resonance on protein crystals with, Electron 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.

Hybrid 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 spin. Here, we demonstrate a scheme for circumventing this problem by indirect control: We apply a small number of short pulses only to the electron spin 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 spin 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 NV centers in diamond. Our scheme is still applicable when the nuclear spins are only weakly coupled to the electron spin., Supplementary material added; Accepted for publication in PRL

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.

OBJECTIVE: Ultrahigh field MRI provides great opportunities for medical diagnostics and research. However, ultrahigh field MRI also brings challenges, such as larger magnetic susceptibility induced field changes. Parallel-transmit radio-frequency pulses can ameliorate these complications while performing advanced tasks in routine applications. To address one class of such pulses, we propose an optimal-control algorithm as a tool for designing advanced multi-dimensional, large flip-angle, radio-frequency pulses. We contrast initial conditions, constraints, and field correction abilities against increasing pulse trajectory acceleration factors.MATERIALS AND METHODS: On an 8-channel 7T system, we demonstrate the quasi-Newton algorithm with pulse designs for reduced field-of-view imaging with an oil phantom and in vivo with scans of the human brain stem. We used echo-planar imaging with 2D spatial-selective pulses. Pulses are computed sufficiently rapid for routine applications.RESULTS: Our dataset was quantitatively analyzed with the conventional mean-square-error metric and the structural-similarity index from image processing. Analysis of both full and reduced field-of-view scans benefit from utilizing both complementary measures.CONCLUSION: We obtained excellent outer-volume suppression with our proposed method, thus enabling reduced field-of-view imaging using pulse trajectory acceleration factors up to 4.

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 Hz−1/2.

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− 13 C 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 13 C 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. Fil: Zangara, Pablo René. City University of New York; Estados Unidos. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Física Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; Argentina Fil: Dhomkar, Siddharth. City University of New York; Estados Unidos Fil: Ajoy, Ashok. University of California at Berkeley; Estados Unidos. Lawrence Berkeley National Laboratory; Estados Unidos Fil: Liu, Kristina. University of California at Berkeley; Estados Unidos. Lawrence Berkeley National Laboratory; Estados Unidos Fil: Nazaryan, Raffi. Lawrence Berkeley National Laboratory; Estados Unidos. University of California at Berkeley; Estados Unidos Fil: Pagliero, Daniela. City University of New York; Estados Unidos Fil: Suter, Dieter. Universität Dortmund; Alemania Fil: Reimer, Jeffrey A.. University of California at Berkeley; Estados Unidos. Lawrence Berkeley National Laboratory; Estados Unidos Fil: Pines, Alexander. Lawrence Berkeley National Laboratory; Estados Unidos. University of California at Berkeley; Estados Unidos Fil: Meriles, Carlos A.. City University of New York; Estados Unidos

Die Erforschung der Natur kombiniert empirische Erkenntnisse mit Interpretationen, mit dem Ziel, eine moglichst umfassende Beschreibung unserer Umwelt aus moglichst wenigen Grundvoraussetzungen herzuleiten. Dieser Prozess sollte idealerweise objektiv und fur andere nachvollziehbar sein. Trotzdem spielen immer auch personliche Vorlieben der beteiligten Personen eine wichtige Rolle.

Characterization of microstructures in live 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 non-invasive 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 already 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 $T_{2}$. By attaining the ultimate precision limit per measurement, the number of measurements and the total acquisition time may be drastically reduced compared to the present state of the art. These results will therefore allow MRI to advance towards unravelling a wealth of diagnostic information., Comment: 7 pages, 3 figures

Quantum 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.

We 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.