arXiv subjects

V. V. Dobrovitski

Publications and source records attributed to V. V. Dobrovitski.

At least 19 recordsLinked to original sources

arXiv2026

Multimode Jahn-Teller Effect in Negatively Charged Nitrogen-Vacancy Center in Diamond

We present a first-principles study of the multimode Jahn-Teller (JT) effect in the exctied $^{3}E$ state of the negatively charged nitrogen-vacancy (NV) center in diamond. Using density functional theory combined with an intrinsic distortion path (IDP) analysis, we resolve the full activation pathways of the JT distortion and quantitatively decompose the distortion into contributions from individual vibrational modes. We find that multiple vibrational modes participate cooperatively in the JT dynamics, giving rise to a shallow adiabatic potential energy surface with low barriers, consistent with thermally activated pseudorotation. The dominant JT-active modes are found to closely correspond to vibrational features observed in two-dimensional electronic spectroscopy (2DES), in agreement with recent ab initio molecular dynamics simulations. Our results establish a microscopic, mode-resolved picture of vibronic coupling in the excited-state NV center and provide new insight into dephasing, relaxation, and optically driven dynamics relevant to solid-state quantum technologies.

cond-mat.mtrl-sci↗

arXiv2025

Heitler effect and resonance fluorescence in quantum magnonics

We consider a coupled system of a qubit and a magnon mode in which the qubit is weakly driven. We demonstrate that the spectral steady-state responses of both the qubit and the magnon show, in addition to two sidebands split by the coupling, also a peak at the driving frequency with virtually zero linewidth. This phenomenon, which persists at both strong and weak coupling, is an analog of the Heitler effect in atomic physics, and shows the path towards building of coherent magnon sources.

cond-mat.mes-hall↗

arXiv2024

Decoherence induced by a sparse bath of two-level fluctuators: peculiar features of $1/f$ noise in high-quality qubits

Progress in fabrication of semiconductor and superconductor qubits has greatly diminished the number of decohering defects, thus decreasing the devastating low-frequency $1/f$ noise and extending the qubits' coherence times (dephasing time $T_2^*$ and the echo decay time $T_2$). However, large qubit-to-qubit variation of the coherence properties remains a problem, making it difficult to produce a large-scale register where all qubits have a uniformly high quality. In this work we show that large variability is a characteristic feature of a qubit dephased by a sparse bath made of many ($n\gg 1$) decohering defects, coupled to the qubit with similar strength. We model the defects as two-level fluctuators (TLFs) whose transition rates $γ$ are sampled from a log-uniform distribution over an interval $[γ_{m},γ_M]$, which is a standard model for $1/f$ noise. We investigate decoherence by such a bath in the limit of high-quality qubit, i.e.\ when the TLF density $d$ is small (the limit of sparse bath, with $d=n/w\ll 1$, where $n$ is the number of TLFs and $w=\ln{[γ_M/γ_{m}]}$ is the log-width of the distribution). We show that different realizations of the bath produce very similar noise power spectra $S(f)\sim 1/f$, but lead to drastically different coherence times $T_2^*$ and $T_2$. Thus, the spectral density $S(f)$ does not determine coherence of a qubit coupled to a sparse TLF bath, as opposed to a dense bath; instead, decoherence is controlled by only a few exceptional fluctuators, determined by their value of $γ$. We show that removing only two of these TLFs greatly increases $T_2$ and $T_2^*$ times. Our findings help theoretical understanding and further improvements in the coherence properties of semiconductor and superconductor qubits, battling the $1/f$ noise in these platforms.

cond-mat.mes-hall↗

arXiv2023

Long-lived coherences in strongly interacting spin ensembles

Periodic driving has emerged as a powerful tool to control, engineer, and characterize many-body quantum systems. However, the required pulse sequences are often complex, long, or require the ability to control the individual degrees of freedom. In this work, we study how a simple Carr-Purcell Meiboom-Gill (CPMG)-like pulse sequence can be leveraged to enhance the coherence of a large ensemble of spin qubits and serve as an important characterization tool. We implement the periodic drive on an ensemble of dense nitrogen-vacancy (NV) centers in diamond and examine the effect of pulse rotation offset as a control parameter on the dynamics. We use a single diamond sample prepared with several spots of varying NV density, which, in turn, varies the NV-NV dipolar interaction strength. Counter-intuitively, we find that rotation offsets deviating from the ideal π-pulse in the CPMG sequence (often classified as pulse errors) play a critical role in preserving coherence even at nominally zero rotation offset. The cause of the coherence preservation is an emergent effective field that scales linearly with the magnitude of the rotation offset. In addition to extending coherence, we compare the rotation offset dependence of coherence to numerical simulations to measure the disorder and dipolar contributions to the Hamiltonian to quantitatively extract the densities of the constituent spin species within the diamond.

quant-ph↗

arXiv2022

Efficient and robust estimation of many-qubit Hamiltonians

Characterizing the interactions and dynamics of quantum mechanical systems is an essential task in the development of quantum technologies. We propose an efficient protocol based on the estimation of the time derivatives of few qubit observables using polynomial interpolation for characterizing the underlying Hamiltonian dynamics and Markovian noise of a multi-qubit device. For finite range dynamics, our protocol exponentially relaxes the necessary time resolution of the measurements and quadratically reduces the overall sample complexity compared to previous approaches. Furthermore, we show that our protocol can characterize the dynamics of systems with algebraically decaying interactions. The implementation of the protocol requires only the preparation of product states and single-qubit measurements. Furthermore, we develop a shadow tomography method for quantum channels that is of independent interest. This protocol can be used to parallelize to learn the Hamiltonian, rendering it applicable for the characterization of both current and future quantum devices.

quant-ph↗

arXiv2021

Long-lived coherence in driven spin systems: from two to infinite spatial dimensions

Long-lived coherences, emerging under periodic pulse driving in the disordered ensembles of strongly interacting spins, offer immense advantages for future quantum technologies, but the physical origin and the key properties of this phenomenon remain poorly understood. We theoretically investigate this effect in ensembles of different dimensionality, and predict existence of the long-lived coherences in two-dimensional and infinite-dimensional (where every spin is coupled to all others) systems, which are of particular importance for quantum sensing and quantum information processing. We explore the transition from two to infinite dimensions, and show that the long-time coherence dynamics in all dimensionalities is qualitatively similar, although the short-time behavior is drastically different, exhibiting dimensionality-dependent singularity. Our study establishes the common physical origin of the long-lived coherences in different dimensionalities, and suggests that this effect is a generic feature of the strongly coupled spin systems with positional disorder. Our results lay out foundation for utilizing the long-lived coherences in a range of application, from quantum sensing with two-dimensional spin ensembles, to quantum information processing with the infinitely-dimensional spin systems in the cavity-QED settings.

quant-ph↗

arXiv2021

Probing spin dynamics on diamond surfaces using a single quantum sensor

Understanding the dynamics of a quantum bit's environment is essential for the realization of practical systems for quantum information processing and metrology. We use single nitrogen-vacancy (NV) centers in diamond to study the dynamics of a disordered spin ensemble at the diamond surface. Specifically, we tune the density of "dark" surface spins to interrogate their contribution to the decoherence of shallow NV center spin qubits. When the average surface spin spacing exceeds the NV center depth, we find that the surface spin contribution to the NV center free induction decay can be described by a stretched exponential with variable power n. We show that these observations are consistent with a model in which the spatial positions of the surface spins are fixed for each measurement, but some of them reconfigure between measurements. In particular, we observe a depth-dependent critical time associated with a dynamical transition from Gaussian (n=2) decay to n=2/3, and show that this transition arises from the competition between the small decay contributions of many distant spins and strong coupling to a few proximal spins at the surface. These observations demonstrate the potential of a local sensor for understanding complex systems and elucidate pathways for improving and controlling spin qubits at the surface.

quant-ph↗

arXiv2020

Orbital and Spin Dynamics of Single Neutrally-Charged Nitrogen-Vacancy Centers in Diamond

The neutral charge state plays an important role in quantum information and sensing applications based on nitrogen-vacancy centers. However, the orbital and spin dynamics remain unexplored. Here, we use resonant excitation of single centers to directly reveal the fine structure, enabling selective addressing of spin-orbit states. Through pump-probe experiments, we find the orbital relaxation time (430ns at 4.7K) and measure its temperature-dependence up to 11.8K. Finally we reveal the spin relaxation time (1.5s), and realize projective high-fidelity single-shot readout of the spin state ($\geq98\%$).

quant-ph↗

arXiv2019

Thermalization in Solid-State NMR Controlled by Quantum Chaos in Spin Bath

We theoretically investigate thermalization and spin diffusion driven by a quantum spin bath for a realistic solid-state NMR experiment. We consider polycrystalline L-alanine, and investigate how the spin polarization spreads among several $^{13}$C nuclear spins, which interact via dipole-dipole coupling with the bath of strongly dipolar-coupled $^1$H nuclear (proton) spins. We do this by using direct numerical simulation of the many-spin time-dependent Schrödinger equation. We find that, although the proton spins located near the carbon sites interact most strongly with the $^{13}$C spins, this interaction alone is not enough to drive spin diffusion and thermalize the $^{13}$C nuclear spins. We demonstrate that the thermalization within the $^{13}$C subsystem is driven by the collective many-body dynamics of the proton spin bath, and specifically, that the onset of thermalization among the $^{13}$C spins is directly related to the onset of chaotic behavior in the proton spin bath. Therefore, thermalization and spin diffusion within the $^{13}$C subsystem is controlled by the proton spins located far from the C sites. In spite of their weak coupling to the $^{13}$C spins, these far-away protons help produce a network of strongly coupled proton spins with collective dynamics, that drives thermalization.

quant-ph↗

arXiv2018

Spin lifetime and charge noise in hot silicon quantum dot qubits

We investigate the magnetic field and temperature dependence of the single-electron spin lifetime in silicon quantum dots and find a lifetime of 2.8 ms at a temperature of 1.1 K. We develop a model based on spin-valley mixing and find that Johnson noise and two-phonon processes limit relaxation at low and high temperature respectively. We also investigate the effect of temperature on charge noise and find a linear dependence up to 4 K. These results contribute to the understanding of relaxation in silicon quantum dots and are promising for qubit operation at elevated temperatures.

cond-mat.mes-hall↗

arXiv2017

Absorption Spectrum of a Two-Level System Subjected to a Periodic Pulse Sequence

We investigate how the quantum control of a two-level system (TLS) coupled to photons can modify and tune the TLS's photon absorption spectrum. Tuning and controlling the emission and the absorption is of much interest e.g.\ for the development of efficient interfaces between stationary and flying qubits in modern architectures for quantum computation and quantum communication. We consider the periodic pulse control, where the TLS is subjected to a periodic sequence of the near-resonant Rabi driving pulses, each pulse implementing a 180$^\circ$ rotation. For small inter-pulse delays, the absorption spectrum features a pronounced peak of stimulated emission at the pulse frequency, as well as equidistant satellite peaks with smaller spectral weights. As long as the detuning between the carrier frequency of the driving and the TLS transition frequency remains moderate, this spectral shape shows little change. Therefore, the quantum control allows shifting the absorption peak to a desired position, and locks the absorption peak to the carrier frequency of the driving pulses. Detailed description of the spectrum, and its evolution as a function time, the inter-pulse spacing and the detuning, is presented.

quant-ph↗

arXiv2016

Polaron spin echo envelope modulations in an organic semiconducting polymer

Theoretical treatment of the electron spin echo envelope modulation (ESEEM) spectra from polarons in a semiconducting $π$- conjugated polymer is presented. The contact hyperfine coupling and the dipolar interaction between the polaron and proton spins are found to have distinct contributions in the ESEEM spectra. However, since the two contributions are spaced very closely, and the dipolar contribution is dominant, the detection of the contact hyperfine interaction is difficult. To resolve this problem, a recipe of probing the contact hyperfine and dipolar interactions selectively is proposed, and a method for detecting the polaron contact hyperfine interaction is formulated. The ESEEM decay due to the polaron random hopping is analyzed, and the robustness of the method against this decay is verified. Moreover, this decay is linked to the transport properties of polarons, providing an auxiliary probe for the polaron transport.

cond-mat.mes-hall↗

arXiv2016

Quantum dynamics of nuclear spins and spin relaxation in organic semiconductors

We investigate the role of the nuclear spin quantum dynamics in hyperfine-induced spin relaxation of hopping carriers in organic semiconductors. The fast hopping regime with a small carrier spin precession during a waiting time between hops is typical for organic semiconductors possessing long spin coherence times. We consider this regime and focus on a carrier random walk diffusion in one dimension, where the effect of the nuclear spin dynamics is expected to be the strongest. Exact numerical simulations of spin systems with up to 25 nuclear spins are performed using the Suzuki-Trotter decomposition of evolution operator. Larger nuclear spin systems are modeled utilizing the spin-coherent state $P$-representation approach developed earlier. We find that the nuclear spin dynamics strongly influences the carrier spin relaxation at long times. If the random walk is restricted to a small area, it leads to the quenching of carrier spin polarization at a non-zero value at long times. If the random walk is unrestricted, the carrier spin polarization acquires a long-time tail, decaying as $ 1/\sqrt{t}$. Based on the numerical results, we devise a simple formula describing the effect quantitatively.

cond-mat.mes-hall↗

arXiv2016

Gate fidelity and coherence of an electron spin in a Si/SiGe quantum dot with micromagnet

The gate fidelity and the coherence time of a qubit are important benchmarks for quantum computation. We construct a qubit using a single electron spin in a Si/SiGe quantum dot and control it electrically via an artificial spin-orbit field from a micromagnet. We measure an average single-qubit gate fidelity of $\approx$ 99$\%$ using randomized benchmarking, which is consistent with dephasing from the slowly evolving nuclear spins in substrate. The coherence time measured using dynamical decoupling extends up to $\approx$ 400 $μ$s for 128 decoupling pulses, with no sign of saturation. We find evidence that the coherence time is limited by noise in the 10 kHz $-$ 1 MHz range, possibly because charge noise affecting the spin via the micromagnet gradient. This work shows that an electron spin in a Si/SiGe quantum dot is a good candidate for quantum information processing as well as for a quantum memory, even without isotopic purification.

cond-mat.mes-hall↗

arXiv2015

Suppressing Spectral Diffusion of the Emitted Photons with Optical Pulses

In many quantum architectures the solid-state qubits, such as quantum dots or color centers, are interfaced via emitted photons. However, the frequency of photons emitted by solid-state systems exhibits slow uncontrollable fluctuations over time (spectral diffusion), creating a serious problem for implementation of the photon-mediated protocols. Here we show that a sequence of optical pulses applied to the solid-state emitter can stabilize the emission line at the desired frequency. We demonstrate efficiency, robustness, and feasibility of the method analytically and numerically. Taking nitrogen-vacancy (NV) center in diamond as an example, we show that only several pulses, with the width of 1 ns, separated by few ns (which is not difficult to achieve) can suppress spectral diffusion. Our method provides a simple and robust way to greatly improve the efficiency of photon-mediated entanglement and/or coupling to photonic cavities for solid-state qubits.

quant-ph↗

arXiv2015

Hyperfine-induced spin relaxation of a diffusively moving carrier in low dimensions: implications for spin transport in organic semiconductors

The hyperfine coupling between the spin of a charge carrier and the nuclear spin bath is a predominant channel for the carrier spin relaxation in many organic semiconductors. We theoretically investigate the hyperfine-induced spin relaxation of a carrier performing a random walk on a d-dimensional regular lattice, in a transport regime typical for organic semiconductors. We show that in d=1 and d=2 the time dependence of the space-integrated spin polarization, P(t), is dominated by a superexponential decay, crossing over to a stretched exponential tail at long times. The faster decay is attributed to multiple self-intersections (returns) of the random walk trajectories, which occur more often in lower dimensions. We also show, analytically and numerically, that the returns lead to sensitivity of P(t) to external electric and magnetic fields, and this sensitivity strongly depends on dimensionality of the system (d=1 vs. d=3). Furthermore, we investigate in detail the coordinate dependence of the time-integrated spin polarization, $σ(r)$, which can be probed in the spin transport experiments with spin-polarized electrodes. We demonstrate that, while $σ(r)$ is essentially exponential, the effect of multiple self-intersections can be identified in transport measurements from the strong dependence of the spin decay length on the external magnetic and electric fields.

cond-mat.mes-hall↗

arXiv2015

Highly selective detection of individual nuclear spins using the rotary echo on an electron spin as a probe

We consider an electronic spin, such as a nitrogen-vacancy (NV) center in diamond, weakly coupled to a large number (bath) of nuclear spins, and subjected to the Rabi driving with a periodically alternating phase (multiple rotary echo). We show that by switching the driving phase synchronously with the precession of a given nuclear spin, the interaction to this spin is selectively enhanced, while the rest of the bath remains decoupled. The enhancement is of resonant character. The key feature of the suggested scheme is that the width of the resonance is adjustable, and can be greatly decreased by increasing the driving strength. Thus, the resonance can be significantly narrowed, by a factor of 10--100 in comparison with the existing detection methods. Significant improvement in selectivity is explained analytically and confirmed by direct numerical many-spin simulations. The method can be applied to a wide range of solid-state systems.

cond-mat.mes-hall↗

arXiv2014

Decay of the rotary echoes for the spin of a nitrogen-vacancy center in diamond

We study dynamics of the electron spin of a nitrogen-vacancy (NV) center subjected to a strong driving field with periodically reversed direction (train of rotary echoes). We use analytical and numerical tools to analyze in detail the form and timescales of decay of the rotary echo train, modeling the decohering spin environment as a random magnetic field. We demonstrate that the problem can be exactly mapped onto a model of spin 1 coupled to a single bosonic mode with imaginary frequency. This mapping allows comprehensive analytical investigation beyond the standard Bloch-Redfield-type approaches. We explore the decay of the rotary echo train under assumption of strong driving, and identify the most important regimes of the decay. The analytical results are compared with the direct numerical simulations to confirm quantitative accuracy of our study. We present the results for realistic environment of substitutional nitrogen atoms (P1 centers), and provide a simplified but accurate description for decay of the rotary echo train of the NV center's spin. The approach presented here can also be used to study decoherence and longitudinal relaxation of other spin systems under conditions of strong driving.

cond-mat.mes-hall↗