arXiv subjects

Mykyta Onizhuk

Publications and source records attributed to Mykyta Onizhuk.

18 recordsLinked to original sources

arXiv2026

Minute-long quantum coherence enabled by electrical depletion of magnetic noise

Integrating solid-state spin defects into classical electronic devices can enable new opportunities for quantum information processing that benefit from existing semiconductor technology. Here, we investigate the impact of bias control of an isotopically purified silicon carbide (SiC) p-i-n diode on the coherence of embedded spins. We show that the diode allows for the depletion of not only the electrical, but also the magnetic noise sources. This results in extended relaxation and coherence times of individual electronic and nuclear spins, with Hahn echo times exceeding values reported for single spins in any platform (> 100 seconds). These results demonstrate the importance of materials control and electronic device integration to create highly coherent solid-state quantum technology.

quant-ph↗

arXiv2025

Understanding Surface-Induced Decoherence of NV Centers in Diamond

Nitrogen vacancy centers (NV) in proximity to diamond surfaces are promising nanoscale quantum sensors. However, their coherence properties are negatively affected by magnetic and electric surface noise, whose origin and detailed impact have remained elusive. Using atomistic models of diamond surfaces derived with density functional theory, together with decoherence time calculations with cluster correlation expansion methods, we quantify the effects of surface crystallographic orientation and functionalization, and of the density of unpaired electrons on the NV Hahn-echo time $T_2$. We determine a crossover depth at which $T_2$ ceases to be limited by surface nuclear spins and recovers the bulk-limited value. We find that for static surface-electron baths, the ratio between the NV depth and the separation between surface electron spins determines a transition from fast-fluctuating to quasi-static noise, leading to a dependence of $T_2$ on orientation for specific surfaces. We also find that the modulation of $T_2$ by spin-phonon relaxations leads to motional-narrowing at sub-microsecond relaxation times. Importantly, our calculations show that it is only when accounting for surface-spin in-sequence hopping that measured $T_2$ values as a function of depth can be reproduced, thus highlighting the importance of hopping-mediated models to describe the surface spin noise affecting NV sensors. Overall, our work provides clear guidelines for engineering diamond surfaces to achieve enhanced NV coherence for quantum sensing and information processing applications.

quant-ph↗

arXiv2025

Probing aqueous interfaces with spin defects

Understanding the physical and chemical properties of aqueous interfaces is important in diverse fields of science, ranging from biology and chemistry to materials science. In spite of crucial progress in surface sensitive spectroscopic techniques over the past decades, the microscopic properties of aqueous interfaces remain difficult to measure. Here we explore the use of noise spectroscopy to characterize interfacial properties, specifically of quantum sensors hosted in two-dimensional materials in contact with water. We combine molecular dynamics simulations of water/graphene interfaces and the calculations of the spin dynamics of an NV-like color center, and we investigate the impact of interfacial water and simple ions on the decoherence time of the defect. We show that the Hahn echo coherence time of the NV center is sensitive to motional narrowing and to the hydrogen bonding arrangement and the dynamical properties of water and ions at the interface. We present results as a function of the liquid temperature, strength of the water-surface interaction, and for varied mono-valent and di-valent ions, highlighting the broad applicability of near-surface qubits to gain insight into the properties of aqueous interfaces.

physics.chem-ph↗

arXiv2025

High-throughput spin-bath characterization of spin-defects in semiconductors

Detailed knowledge of the local environments of spin-defects in semiconductors, such as nitrogen vacancy (NV) centers in diamond or divacancies in silicon carbide, is crucial for optimizing control and entanglement protocols in quantum sensing and information applications. However, a direct experimental characterization of individual defect environments is not scalable, as spin bath measurements are extremely time consuming. In this work, we address the ill-posed inverse problem of recovering the atomic positions and hyperfine couplings of random nuclei surrounding spin-defects from sparse experimental coherence signals, which can be obtained in hours. To address the challenge to determine the number of isotopic nuclear spins along with their hyperfine couplings, we employ a trans-dimensional Bayesian approach that incorporates ab initio data. This approach provides posterior distributions of the numbers, hyperfine couplings, and locations of nuclear spins present in the sample. In addition to enabling high-throughput screening of spin-defects, we demonstrate how this trans-dimensional Bayesian approach can guide experimental design for dynamical decoupling experiments to detect nuclear spins within targeted hyperfine coupling regimes. While the primary focus is on accelerating spin-defect characterization, this Bayesian approach also lays the foundation for digital twin studies of spin-defects, where a virtual model of the spin-defect system evolves in real time with ongoing experimental measurements. Together, the set of tools we designed and applied paves the way for scalable deployment of spin-defects in semiconductors for quantum sensing and information applications.

quant-ph↗

arXiv2025

Decoherence of solid-state spin qubits: a computational perspective

The usefulness of solid-state spins in quantum technologies depends on how long they can remain in a coherent superposition of quantum states. This Colloquium discusses how first-principles simulations can predict spin dynamics for different types of solid-state electron spins, helping design novel and improved platforms for quantum computing, networking, and sensing. We begin by outlining the necessary concepts of the noise affecting generic quantum systems. We then focus on recent advances in predicting spin-phonon relaxation of the spin-defect qubits. Next, we discuss cluster methods as a means of simulating quantum decoherence induced by spin-spin interactions, emphasizing the critical role of validation in ensuring the accuracy of these simulations. We highlight how validated cluster methods can be instrumental in interpreting recent experimental results and, more importantly, predicting the coherence properties of novel spin-based quantum platforms, guiding the development of next-generation quantum technologies.

quant-ph↗

arXiv2025

Quantum decoherence of nitrogen-vacancy spin ensembles in a nitrogen spin bath in diamond under dynamical decoupling

The negatively charged nitrogen-vacancy (NV) center in diamond has emerged as a leading qubit platform for quantum technology applications. One of the key challenges for NV-based quantum applications is building an accurate model to predict its decoherence properties and their quantum nature. In this study, we combine theory and experiment to investigate NV decoherence dynamics in the presence of nitrogen donor (P1 center) baths, which is one of the dominant decoherence sources in diamond. We employ a cluster-correlation expansion (CCE) method to compute the NV decoherence under the Hahn-echo (HE) and Carr-Purcell-Meiboom-Gill (CPMG) pulse sequences at various P1 concentrations from 1ppm to 300 ppm. We show that the coherence time (T2) increases with the number of pi pulses applied, indicating that the NV spin is decoupled from the P1 bath. Notably, we find that T2 scales quadratically as a function of the pulse number, on a logarithmic scale, as opposed to the linear scaling predicted by widely accepted semi-classical theories in the literature. In our experiment, we measure the CPMG signal for two diamond samples with high P1 concentrations of 0.8ppm and 13ppm. We demonstrate that the T2 scaling is indeed quadratic, thus confirming our theoretical predictions. Our results show that the quantum bath model combined with the CCE method can accurately capture the quantum nature of the P1-driven NV decoherence. Our study opens a new avenue for developing a complete noise model that could be used to optimize the performance of NV-based quantum devices.

quant-ph↗

arXiv2024

Coherent Erbium Spin Defects in Colloidal Nanocrystal Hosts

We demonstrate nearly a microsecond of spin coherence in Er3+ ions doped in cerium dioxide nanocrystal hosts, despite a large gyromagnetic ratio and nanometric proximity of the spin defect to the nanocrystal surface. The long spin coherence is enabled by reducing the dopant density below the instantaneous diffusion limit in a nuclear spin-free host material, reaching the limit of a single erbium spin defect per nanocrystal. We observe a large Orbach energy in a highly symmetric cubic site, further protecting the coherence in a qubit that would otherwise rapidly decohere. Spatially correlated electron spectroscopy measurements reveal the presence of Ce3+ at the nanocrystal surface that likely acts as extraneous paramagnetic spin noise. Even with these factors, defect-embedded nanocrystal hosts show tremendous promise for quantum sensing and quantum communication applications, with multiple avenues, including core-shell fabrication, redox tuning of oxygen vacancies, and organic surfactant modification, available to further enhance their spin coherence and functionality in the future.

cond-mat.mes-hall↗

arXiv2024

Discovery of Atomic Clock-Like Spin Defects in Simple Oxides from First Principles

Virtually noiseless due to the scarcity of spinful nuclei in the lattice, simple oxides hold promise as hosts of solid-state spin qubits. However, no suitable spin defect has yet been found in these systems. Using high-throughput first-principles calculations, we predict spin defects in calcium oxide with electronic properties remarkably similar to those of the NV center in diamond. These defects are charged complexes where a dopant atom -- Sb, Bi, or I -- occupies the volume vacated by adjacent cation and anion vacancies. The predicted zero phonon line shows that the Bi complex emits in the telecommunication range, and the computed many-body energy levels suggest a viable optical cycle required for qubit initialization. Notably, the high-spin nucleus of each dopant strongly couples to the electron spin, leading to many controllable quantum levels and the emergence of atomic clock-like transitions that are well protected from environmental noise. Specifically, the Hanh-echo coherence time increases beyond seconds at the clock-like transition in the defect with \ch{^{209}Bi}. Our results pave the way to designing quantum states with long coherence times in simple oxides, making them attractive platforms for quantum technologies.

cond-mat.mtrl-sci↗

arXiv2023

Quantum spin probe of single charge dynamics

Electronic defects in semiconductors form the basis for many emerging quantum technologies. Understanding defect spin and charge dynamics in solid state platforms is crucial to developing these building blocks, but many defect centers are difficult to access at the single-particle level due to the lack of sensitive readout techniques. A method for probing optically inactive spin defects would reveal semiconductor physics at the atomic scale and advance the study of new quantum systems. We exploit the intrinsic correlation between the charge and spin states of defect centers to measure defect charge populations and dynamics through the steady-state spin population, read-out at the single-defect level with a nearby optically active qubit. We directly measure ionization and charge relaxation of single dark defects in diamond, effects we do not have access to with traditional coherence-based quantum sensing. These spin resonance-based methods generalize to other solid state defect systems in relevant materials.

quant-ph↗

arXiv2023

Understanding central spin decoherence due to interacting dissipative spin baths

We propose a new approach to simulate the decoherence of a central spin coupled to an interacting dissipative spin bath with cluster-correlation expansion techniques. We benchmark the approach on generic 1D and 2D spin baths and find excellent agreement with numerically exact simulations. Our calculations show a complex interplay between dissipation and coherent spin exchange, leading to increased central spin coherence in the presence of fast dissipation. Finally, we model near-surface NV centers in diamond and show that accounting for bath dissipation is crucial to understanding their decoherence. Our method can be applied to a variety of systems and provides a powerful tool to investigate spin dynamics in dissipative environments.

quant-ph↗

arXiv2023

Guiding Diamond Spin Qubit Growth with Computational Methods

The nitrogen vacancy (NV) center in diamond, a well-studied, optically active spin defect, is the prototypical system in many state of the art quantum sensing and communication applications. In addition to the enticing properties intrinsic to the NV center, its diamond host's nuclear and electronic spin baths can be leveraged as resources for quantum information, rather than considered solely as sources of decoherence. However, current synthesis approaches result in stochastic defect spin positions, reducing the technology's potential for deterministic control and yield of NV-spin bath systems, as well as scalability and integration with other technologies. Here, we demonstrate the use of theoretical calculations of electronic central spin decoherence as an integral part of an NV-spin bath synthesis workflow, providing a path forward for the quantitative design of NV center-based quantum sensing systems. We use computationally generated coherence data to characterize the properties of single NV center qubits across relevant growth parameters to find general trends in coherence time distributions dependent on spin bath dimensionality and density. We then build a maximum likelihood estimator with our theoretical model, enabling the characterization of a test sample through NV T2* measurements. Finally, we explore the impact of dimensionality on the yield of strongly coupled electron spin systems. The methods presented herein are general and applicable to other qubit platforms that can be appropriately simulated.

quant-ph↗

arXiv2023

Detecting spin bath polarization with quantum quench phase shifts of single spins in diamond

Single-qubit sensing protocols can be used to measure qubit-bath coupling parameters. However, for sufficiently large coupling, the sensing protocol itself perturbs the bath, which is predicted to result in a characteristic response in the sensing measurements. Here, we observe this bath perturbation, also known as a quantum quench, by preparing the nuclear spin bath of a nitrogen-vacancy (NV) center in polarized initial states and performing phase-resolved spin echo measurements on the NV electron spin. These measurements reveal a time-dependent phase determined by the initial state of the bath. We derive the relationship between sensor phase and Gaussian spin bath polarization, and apply it to reconstruct both the axial and transverse polarization components. Using this insight, we optimize the transfer efficiency of our dynamic nuclear polarization sequence. This technique for directly measuring bath polarization may assist in preparing high-fidelity quantum memory states, improving nanoscale NMR methods, and investigating non-Gaussian quantum baths.

quant-ph↗

arXiv2023

Decoherence of Nuclear Spins in the Proximity of Nitrogen Vacancy Centers in Diamond

Nuclear spins in the proximity of electronic spin defects in solids are promising platforms for quantum information processing due to their ability to preserve quantum states for a remarkably long time. Here we report a comprehensive study of the nuclear decoherence processes in the vicinity of the nitrogen-vacancy (NV) center in diamond. We simulate from first principles the change in the dynamics of nuclear spins as a function of distance and state of the NV center and validate our results with experimental data. Our simulations reveal nontrivial oscillations in the Hahn echo signal, pointing to a new sensing modality of dynamical-decoupling spectroscopy, and show how hybridization of the electronic states suppresses the coherence time of strongly coupled nuclear spins. The computational framework developed in our work is general and can be broadly applied to predict the dynamical properties of nuclear spins.

quant-ph↗

arXiv2021

PyCCE: A Python Package for Cluster Correlation Expansion Simulations of Spin Qubit Dynamic

We present PyCCE, an open-source Python library to simulate the dynamics of spin qubits in a spin bath, using the cluster-correlation expansion (CCE) method. PyCCE includes modules to generate realistic spin baths, employing coupling parameters computed from first principles with electronic structure codes, and enables the user to run simulations with either the conventional or generalized CCE method. We illustrate three use cases of the Python library: the calculation of the Hahn-echo coherence time of the nitrogen vacancy in diamond; the calculation of the coherence time of the basal divacancy in silicon carbide at avoided crossings; and the magnetic field orientation-dependent dynamics of a shallow donor in silicon. The complete documentation and installation instructions are available at https://pycce.readthedocs.io/en/latest/.

quant-ph↗

arXiv2021

Substrate-controlled dynamics of spin qubits in low dimensional van-der-Waals materials

We report a theoretical study of the coherence dynamics of spin qubits in two-dimensional materials (2DMs) and van-der-Waals heterostructures, as a function of the host thickness and the composition of the surrounding environment. We focus on MoS$_2$ and WS$_2$, two promising systems for quantum technology applications, and we consider the decoherence arising from the interaction of the spin qubit with nuclear spins. We show that the Hahn-echo coherence time is determined by a complex interplay between the source of decoherence in the qubit host and in the environment, which in turn determines whether the noise evolution is in a classical or quantum mechanical regime. We suggest that the composition and thickness of van-der-Waals heterostructures encapsulating a qubit host can be engineered to maximize coherence times. Finally, we discuss how quantum sensors may be able to probe the dynamics of the nuclear bath in 2DMs.

quant-ph↗

arXiv2021

Spin-spin interactions in solids from mixed all-electron and pseudopotential calculations $-$ a path to screening materials for spin qubits

Understanding the quantum dynamics of spin defects and their coherence properties requires accurate modeling of spin-spin interaction in solids and molecules, for example by using spin Hamiltonians with parameters obtained from first-principles calculations. We present a real-space approach based on density functional theory for the calculation of spin-Hamiltonian parameters, where only selected atoms are treated at the all-electron level, while the rest of the system is described with the pseudopotential approximation. Our approach permits calculations for systems containing more than 1000 atoms, as demonstrated for defects in diamond and silicon carbide. We show that only a small number of atoms surrounding the defect needs to be treated at the all-electron level, in order to obtain an overall all-electron accuracy for hyperfine and zero-field splitting tensors. We also present results for coherence times, computed with the cluster correlation expansion method, highlighting the importance of accurate spin-Hamiltonian parameters for quantitative predictions of spin dynamics.

cond-mat.mtrl-sci↗

arXiv2020

Probing the coherence of solid-state qubits at avoided crossings

Optically addressable paramagnetic defects in wide-band-gap semiconductors are promising platforms for quantum communications and sensing. The presence of avoided crossings between the electronic levels of these defects can substantially alter their quantum dynamics and be both detrimental and beneficial for quantum information applications. Avoided crossings give rise to clock transitions, which can significantly improve protection from magnetic noise and favorably increase coherence time. However, the reduced coupling between electronic and nuclear spins at an avoided crossing may be detrimental to applications where nuclear spins act as quantum memories. Here we present a combined theoretical and experimental study of the quantum dynamics of paramagnetic defects interacting with a nuclear spin bath at avoided crossings. We develop a computational approach based on a generalization of the cluster expansion technique, which can account for processes beyond pure dephasing and describe the dynamics of any solid-state spin-qubits near avoided crossings. Using this approach and experimental validation, we determine the change in nature and source of noise at avoided crossings for divacancies in SiC. We find that we can condition the clock transition of the divacancies in SiC on multiple adjacent nuclear spins states. In our experiments, we demonstrate that one can suppress the effects of fluctuating charge impurities with depletion techniques, leading to an increased coherence time at clock transition, limited purely by magnetic noise. Combined with ab-initio predictions of spin Hamiltonian parameters, the proposed theoretical approach paves the way to designing the coherence properties of spin qubits from first principles.

quant-ph↗

arXiv2020

Entanglement and control of single quantum memories in isotopically engineered silicon carbide

Nuclear spins in the solid state are both a cause of decoherence and a valuable resource for spin qubits. In this work, we demonstrate control of isolated 29Si nuclear spins in silicon carbide (SiC) to create an entangled state between an optically active divacancy spin and a strongly coupled nuclear register. We then show how isotopic engineering of SiC unlocks control of single weakly coupled nuclear spins and present an ab initio method to predict the optimal isotopic fraction which maximizes the number of usable nuclear memories. We bolster these results by reporting high-fidelity electron spin control (F=99.984(1)%), alongside extended coherence times (T2=2.3 ms, T2DD>14.5 ms), and a >40 fold increase in dephasing time (T2*) from isotopic purification. Overall, this work underlines the importance of controlling the nuclear environment in solid-state systems and provides milestone demonstrations that link single photon emitters with nuclear memories in an industrially scalable material.

quant-ph↗