arXiv subjects

Dieter Suter

Publications and source records attributed to Dieter Suter.

At least 37 records · Page 2Linked to original sources

arXiv2020

Interference in Atomic Magnetometry

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.

quant-ph↗

arXiv2020

Efficient Implementation of a Quantum Algorithm in a Single Nitrogen Vacancy Center of Diamond

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.

quant-ph↗

arXiv2020

Efficient quantum gates for individual nuclear spin qubits by indirect control

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.

quant-ph↗

arXiv2020

Experimental characterization of spin-3/2 silicon vacancy centers in 6H-SiC

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.

quant-ph↗

arXiv2020

Optimal photon energies for initialization of hybrid spin quantum registers of NV centers in diamond

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

quant-ph↗

arXiv2019

Precision limits of tissue microstructure characterization by Magnetic Resonance Imaging

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.

quant-ph↗

arXiv2019

Improved indirect control of nuclear spins in diamond NV centers

Hybrid quantum registers consisting of different types of qubits offer a range of advantages as well as challenges. The main challenge is that some types of qubits 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 an NV 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.

quant-ph↗

arXiv2019

Level anti-crossings of an NV center in diamond: Decoherence-free subspaces and 3D sensors of microwave magnetic fields

Nitrogen-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 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 (T_2* ) are 5-7 times longer than those at other orientations. Another interesting property at these level anti-crossings 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.

quant-ph↗

arXiv2019

Dynamics of frequency-swept nuclear spin optical pumping in powdered diamond at low magnetic fields

A broad effort is underway to improve the sensitivity of nuclear magnetic resonance 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 hyper-polarization of target fluids in contact with the diamond surface or the use of hyperpolarized particles as contrast agents for in-vivo imaging.

cond-mat.mes-hall↗

arXiv2019

Hyperpolarized relaxometry based nuclear T1 noise spectroscopy in hybrid diamond quantum registers

Understanding the origins of spin lifetimes in hybrid quantum systems is a matter of current importance in several areas of quantum information and sensing. Methods that spectrally map spin relaxation processes provide insight into their origin and can motivate methods to mitigate them. In this paper, using a combination of hyperpolarization and precision field cycling over a wide range (1mT-7T), we map frequency dependent relaxation in a prototypical hybrid system of 13C nuclear spins in diamond coupled to Nitrogen Vacancy centers. Nuclear hyperpolarization through the optically pumped NV electrons allows signal time savings for the measurements exceeding million-fold over conventional methods. We observe that 13C lifetimes show a dramatic field dependence, growing rapidly with field up to 100mT and saturating thereafter. Through a systematic study with increasing substitutional electron (P1 center) concentration as well as 13C enrichment levels, we identify the operational relaxation channels for the nuclei in different field regimes. In particular, we demonstrate the dominant role played by the 13C nuclei coupling to the interacting P1 electronic spin bath. These results pave the way for quantum control techniques for dissipation engineering to boost spin lifetimes in diamond, with applications ranging from engineered quantum memories to hyperpolarized 13C imaging.

quant-ph↗

arXiv2018

Room temperature "Optical Nanodiamond Hyperpolarizer": physics, design and operation

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.

quant-ph↗

arXiv2018

Pulse sequences for controlled 2- and 3-qubit gates in a hybrid quantum register

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.

quant-ph↗

arXiv2018

Bloch-Siegert Shift in a Hybrid Quantum Register: Quantification and Compensation

Quantum registers that combine the attractive properties of different types of qubits are useful for many different applications. They also pose a number of challenges, often associated with the large differences in coupling strengths between the different types of qubits. One example is the non-resonant effect that alternating electromagnetic fields have on the transitions of qubits that are not targeted by the specific gate operation. The example being studied here is 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. Here we study this effect quantitatively in the important example of the nitrogen vacancy (NV) center in diamond and demonstrate how it can be eliminated.

quant-ph↗

arXiv2018

Superposing pure quantum states with partial prior information

The principle of superposition is an intriguing feature of Quantum Mechanics, which is regularly exploited at various instances. A recent work [PRL \textbf{116}, 110403 (2016)] shows that the fundamentals of Quantum Mechanics restrict the superposition of two arbitrary pure states of a quantum system, even though it is possible to superpose two quantum states with partial prior knowledge. The prior knowledge imposes geometrical constraints on the choice of input pure states. We discuss an experimentally feasible protocol to superpose multiple pure states of a $d$ dimensional quantum system and carry out an explicit experimental realization to superpose two single-qubit pure states on a two-qubit NMR quantum information processor.

quant-ph↗

arXiv2018

Quantum Image Processing and Its Application to Edge Detection: Theory and Experiment

Processing of digital images is continuously gaining in volume and relevance, with concomitant demands on data storage, transmission and processing power. Encoding the image information in quantum-mechanical systems instead of classical ones and replacing classical with quantum information processing may alleviate some of these challenges. By encoding and processing the image information in quantum-mechanical systems, we here demonstrate the framework of quantum image processing, where a pure quantum state encodes the image information: we encode the pixel values in the probability amplitudes and the pixel positions in the computational basis states. Our quantum image representation reduces the required number of qubits compared to existing implementations, and we present image processing algorithms that provide exponential speed-up over their classical counterparts. For the commonly used task of detecting the edge of an image, we propose and implement a quantum algorithm that completes the task with only one single-qubit operation, independent of the size of the image. This demonstrates the potential of quantum image processing for highly efficient image and video processing in the big data era.

quant-ph↗

arXiv2017

Polarizing the electronic and nuclear spin of the NV-center in diamond in arbitrary magnetic fields: analysis of the optical pumping process

Initializing 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.

quant-ph↗

arXiv2017

Nonlinear Dynamics of a two-level system of a single spin driven beyond the rotating-wave approximation

Quantum systems driven by strong oscillating fields are the source of many interesting physical phenomena. In this work, we experimentally study the dynamics of a two-level system of a single spin driven in the strong-driving regime where the rotating-wave approximation is not valid. This two-level system is a subsystem of a single Nitrogen-Vacancy center coupled to a first-shell $^{13}$C nuclear spin in diamond at a level anti-crossing point that occurs in the $m_{s}=\pm1$ manifold when the energy level splitting between the $m_{s}$ = $+1$ and $-1$ spin states due to the static magnetic field is $\approx$ 127 MHz, which is roughly equal to the spectral splitting due to the $^{13}$C hyperfine interaction. The transition frequency of this electron spin two-level system in a static magnetic field of 28.9 G is 1.7 MHz and it can be driven only by the $z$-component of the RF field. Electron spin Rabi frequencies in this system can reach tens of MHz even for moderate RF powers. The simple sinusoidal Rabi oscillations that occur when the amplitude of the driving field is much smaller than the transition frequency become complex when the driving field strength is comparable or greater than the energy level splitting. We observe that the system oscillates faster than the amplitude of the driving field and the response of the system shows multiple frequencies.

quant-ph↗

arXiv2017

Defect production in nonequilibrium phase transitions: Experimental investigation of the Kibble-Zurek mechanism in a two-qubit quantum simulator

Systems 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 Kibble-Zurek mechanism of defect formation.

quant-ph↗