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arXiv:2610.29279

Low-field dephasing time of an optically polarized NV$^-$ spin bath beyond the P1 limit in diamond: a cluster-correlation-expansion and exact spin-bath study

Chikara Shinei

quant-phSubmitted Oct 21, 2026• Announced Oct 22, 2026• 0 citations
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Abstract

Dense ensembles of nitrogen-vacancy (NV$^-$) centres in nitrogen-rich diamond dephase on the electron spins of the substitutional nitrogen (P1) centres. If the paramagnetic nitrogen is instead converted into NV$^-$ centres, the laser that initializes the sensor also pumps the bath into $m_s$ = 0, a state with no moment along its own axis. Using cluster-correlation-expansion (CCE) calculations, combined with an exact solver for probe-bath exchange and with no adjustable parameter, this paper asks whether such an optically polarized NV$^-$ bath dephases a sensor more slowly than a P1 bath of the same density. The answer is set by the field. In a field along [111], three quarters of the bath centres lie off the field axis, and the moment of their $m_s$ = 0 state along the field grows from 0.04 at 25 G to 0.46 at 300 G and 0.81 at 800 G. At 25 G the polarized bath is therefore nearly silent: at $10^{17}$ cm$^{-3}$ the dephasing time $T_2^*$ of a separately addressed $^{15}$NV$^-$ probe is 112 us, six times the 18.1 us of a P1 bath, and still 2.4 times at 90 % polarization; an ensemble measurement gains a factor of two. At 300 G the advantage over P1 shrinks to 1.4, and at 800 G the polarized bath dephases faster than P1. The field dependence of every bath, P1 included, follows the mean moment of its centres along the field. For the Hahn echo, exchange with on-axis bath centres and bath relaxation ($T_1$ = 6 ms) limit the gain to 1.2 times the P1 value at 25 G. Optical polarization of an NV$^-$ bath at low field therefore offers dephasing times beyond the P1 limit of nitrogen-rich diamond.

Related papers

Dynamical-mean-field cluster-correlation expansion of NV$^-$ spin-echo decoherence in a realistic substitutional-nitrogen bath

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The Hahn-echo time $T_2$ of nitrogen-vacancy (NV$^-$) ensembles in nitrogen-rich diamond follows the empirical relation $T_2\cdot$[N] = 160 $μ$s ppm, independent of field between 2 and 30 mT. Cluster-correlation-expansion (CCE) calculations that treat the Overhauser field of the surrounding P1 centres as a frozen, sampled mean field overestimate it at high field by up to a factor of two, an excess recently attributed to additional dark defects. We show that the excess is largely an artefact of freezing the field. The P1 centres outside each cluster flip-flop in pairs that conserve their total $S_z$, generated by kinetic Monte Carlo at golden-rule rates from the dipolar Hamiltonian without adjustable parameters, and each cluster is propagated in the resulting time-dependent field. This dynamical mean field interpolates between the static-mean-field and mean-field-free limits. For a $^{14}$N P1 bath it reduces $T_2\cdot$[N] at 30 mT from 264 $\pm$ 16 to 188 $\pm$ 12 $μ$s ppm and gives 167 $\pm$ 11 $μ$s ppm at 10 mT, 1.7 and 0.4 standard deviations from experiment, with no dark spins. Letting each centre jump independently gives 147 $\pm$ 9 $μ$s ppm at 30 mT; the correlation of the flip-flop partners lengthens $T_2$ by 20-30 percent at high field. The Overhauser field is neither frozen nor motionally narrowed: 68-83 percent of its autocorrelation survives 200 $μ$s. At 2.5 mT, where the P1 electron and nucleus hybridise, $T_2$ remains below experiment for any choice of rates, so part of the field dependence remains ($T_2$ rises by a factor 1.69 $\pm$ 0.07 instead of 2.15 $\pm$ 0.08 from 2.5 to 30 mT).

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cond-mat.mtrl-sciCross-listedarXiv:2609.24295•Sep 22, 2026•0 citations

Understanding Surface-Induced Decoherence of NV Centers in Diamond

Jonah Nagura, Mykyta Onizhuk, Giulia Galli

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Semiclassical spin-bath calculation of the nitrogen-isotope effect on NV$^-$ ensemble coherence in diamond

Chikara Shinei

The ratio $T_2$/$T_2^*$ of the Hahn-echo and Ramsey times of nitrogen-vacancy (NV$^-$) ensembles is independent of the nitrogen concentration [N] and is $\approx$ 16 in diamond of natural isotopic abundance ($^{14}$N), yet a recent $^{15}$N-doped ensemble magnetometer gives only 8.05 $\pm$ 0.18 in the single-quantum convention. We ask whether the nitrogen nuclear isotope alone can cause such a reduction. Using a semiclassical spin-bath model with the Jahn-Teller-resolved P1 hyperfine tensor, we show that the fraction of P1 pairs that are hyperfine-degenerate, and hence free to flip-flop, is 1/4 for $^{14}$N (I = 1) but 5/16 for $^{15}$N (I = 1/2). Simulations over [N] = 0.1--100 ppm confirm that this shortens $T_2$ while leaving $T_2^*$ exactly unchanged: $T_2$($^{14}$N)/$T_2$($^{15}$N) = 1.118 $\pm$ 0.011, independent of [N] and 11$σ$ above unity. The measured contrast, 2.07 $\pm$ 0.25, is 3.8$σ$ larger, identifying resonant-channel counting as a real but partial contribution and setting a quantitative benchmark for many-body calculations.

quant-pharXiv:2609.37596•Sep 30, 2026•0 citations
Low-field dephasing time of an optically polarized NV−^-− spin bath beyond the P1 limit in diamond: a cluster-correlation-expansion and exact spin-bath study
Dense ensembles of nitrogen-vacancy (NV−^-−) centres in nitrogen-rich diamond dephase on the electron spins of the substitutional nitrogen (P1) centres. If the paramagnetic nitrogen is instead converted into NV−^-− centres, the laser that initializes the sensor also pumps the bath into msm_sms​ = 0, a state with no moment along its own axis. Using cluster-correlation-expansion (CCE) calculations, combined with an exact solver for probe-bath exchange and with no adjustable parameter, this paper asks whether such an optically polarized NV−^-− bath dephases a sensor more slowly than a P1 bath of the same density. The answer is set by the field. In a field along [111], three quarters of the bath centres lie off the field axis, and the moment of their msm_sms​ = 0 state along the field grows from 0.04 at 25 G to 0.46 at 300 G and 0.81 at 800 G. At 25 G the polarized bath is therefore nearly silent: at 101710^{17}1017 cm−3^{-3}−3 the dephasing time T2∗T_2^*T2∗​ of a separately addressed 15^{15}15NV−^-− probe is 112 us, six times the 18.1 us of a P1 bath, and still 2.4 times at 90 % polarization; an ensemble measurement gains a factor of two. At 300 G the advantage over P1 shrinks to 1.4, and at 800 G the polarized bath dephases faster than P1. The field dependence of every bath, P1 included, follows the mean moment of its centres along the field. For the Hahn echo, exchange with on-axis bath centres and bath relaxation (T1T_1T1​ = 6 ms) limit the gain to 1.2 times the P1 value at 25 G. Optical polarization of an NV−^-− bath at low field therefore offers dephasing times beyond the P1 limit of nitrogen-rich diamond.
Dynamical-mean-field cluster-correlation expansion of NV−^-− spin-echo decoherence in a realistic substitutional-nitrogen bath
The Hahn-echo time T2T_2T2​ of nitrogen-vacancy (NV−^-−) ensembles in nitrogen-rich diamond follows the empirical relation T2⋅T_2\cdotT2​⋅[N] = 160 μμμs ppm, independent of field between 2 and 30 mT. Cluster-correlation-expansion (CCE) calculations that treat the Overhauser field of the surrounding P1 centres as a frozen, sampled mean field overestimate it at high field by up to a factor of two, an excess recently attributed to additional dark defects. We show that the excess is largely an artefact of freezing the field. The P1 centres outside each cluster flip-flop in pairs that conserve their total SzS_zSz​, generated by kinetic Monte Carlo at golden-rule rates from the dipolar Hamiltonian without adjustable parameters, and each cluster is propagated in the resulting time-dependent field. This dynamical mean field interpolates between the static-mean-field and mean-field-free limits. For a 14^{14}14N P1 bath it reduces T2⋅T_2\cdotT2​⋅[N] at 30 mT from 264 ±\pm± 16 to 188 ±\pm± 12 μμμs ppm and gives 167 ±\pm± 11 μμμs ppm at 10 mT, 1.7 and 0.4 standard deviations from experiment, with no dark spins. Letting each centre jump independently gives 147 ±\pm± 9 μμμs ppm at 30 mT; the correlation of the flip-flop partners lengthens T2T_2T2​ by 20-30 percent at high field. The Overhauser field is neither frozen nor motionally narrowed: 68-83 percent of its autocorrelation survives 200 μμμs. At 2.5 mT, where the P1 electron and nucleus hybridise, T2T_2T2​ remains below experiment for any choice of rates, so part of the field dependence remains (T2T_2T2​ rises by a factor 1.69 ±\pm± 0.07 instead of 2.15 ±\pm± 0.08 from 2.5 to 30 mT).
Abstract Nitrogen-vacancy (NV) centers in diamond have been developed into essential hardware units for a wide range of solid-state-based quantum technology applications. While such applications require the long spin coherence times of the NV centers, they are often limited due to decoherence. In this study, we theoretically investigate the decoherence of NV-spin ensembles induced by nitrogen impurities (P1 centers), which are one of the most dominant and inevitable magnetic field noise sources in diamond. We combined cluster correlation expansion and density functional theory to compute the Hahn-echo spin-coherence time of the NV centers for a broad range of P1 concentrations. Results indicate a clear linear dependence of T2 on P1 concentrations on a log scale with a slope of −1.06, which is in excellent agreement with previous experimental results. The interplay between the Jahn–Teller effect and the hyperfine interaction in the P1 center plays a critical role in determining the bath dynamics and the resulting NV decoherence. Our results provide a theoretical upper bound for the NV-spin T2 over a wide range of P1 densities, serving as a key reference for materials optimization and spin bath characterization to develop highly coherent NV-based devices for quantum information technology.
Annealing a semiconductor makes its vacancies mobile; they aggregate into multi-vacancy complexes that often carry spin. Such centres are a magnetic-noise source for any spin qubit among them, in silicon and silicon carbide as well as in diamond, and they are accordingly blamed for NV- decoherence in irradiated and implanted diamond, with two coherence records credited to removing them. Cluster-correlation-expansion simulations driven by published electron-paramagnetic-resonance parameters invert that attribution. At a fixed paramagnetic spin density a multi-vacancy bath gives a Hahn-echo coherence time 3.1-7.1 times longer than a bath of isolated negative vacancies, over three decades of concentration. A bath dephases the qubit because its spins exchange spin projections with one another, and two can exchange only if their transition frequencies match. A fine-structure splitting shifts those frequencies. Were every defect on the same crystallographic site, all would shift alike and still match: worth only a factor 1.15. Real defects occupy symmetry-equivalent sites pointing in different directions, so neighbours land at different frequencies and stop exchanging: a further 2.7. What governs the coherence time is therefore the fraction of bath pairs sharing a transition frequency, not the zero-field splitting. Vacancy aggregation extends NV- spin coherence rather than shortening it.
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 T2T_2T2​. We determine a crossover depth at which T2T_2T2​ 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 T2T_2T2​ on orientation for specific surfaces. We also find that the modulation of T2T_2T2​ 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 T2T_2T2​ 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.
Abstract Nitrogen-vacancy (NV) center ensembles in diamond are one of the most promising solid-state quantum platforms for various sensing applications. Achieving ultimate sensitivity requires simultaneously long spin dephasing times ( T2∗{T}_{2}^{* }T2∗​ T 2 * ) and high NV center concentrations. In this work, we propose a systematic measurement approach to quantify the electron spin dephasing in NV center ensembles and analyze the contributions of various sources to the dephasing time, including NV-NV interactions, strain and electric field distributions, 13 C nuclear spins, and P1 electron spins. Our method is validated using a series of high-performance diamond samples, providing a comprehensive understanding of dephasing mechanisms and revealing correlations between NV concentration and different dephasing sources. Building on these insights, we outline strategies to further enhance the achievable sensitivity for DC magnetic field measurements.
Semiclassical spin-bath calculation of the nitrogen-isotope effect on NV−^-− ensemble coherence in diamond
The ratio T2T_2T2​/T2∗T_2^*T2∗​ of the Hahn-echo and Ramsey times of nitrogen-vacancy (NV−^-−) ensembles is independent of the nitrogen concentration [N] and is ≈\approx≈ 16 in diamond of natural isotopic abundance (14^{14}14N), yet a recent 15^{15}15N-doped ensemble magnetometer gives only 8.05 ±\pm± 0.18 in the single-quantum convention. We ask whether the nitrogen nuclear isotope alone can cause such a reduction. Using a semiclassical spin-bath model with the Jahn-Teller-resolved P1 hyperfine tensor, we show that the fraction of P1 pairs that are hyperfine-degenerate, and hence free to flip-flop, is 1/4 for 14^{14}14N (I = 1) but 5/16 for 15^{15}15N (I = 1/2). Simulations over [N] = 0.1--100 ppm confirm that this shortens T2T_2T2​ while leaving T2∗T_2^*T2∗​ exactly unchanged: T2T_2T2​(14^{14}14N)/T2T_2T2​(15^{15}15N) = 1.118 ±\pm± 0.011, independent of [N] and 11σσσ above unity. The measured contrast, 2.07 ±\pm± 0.25, is 3.8σσσ larger, identifying resonant-channel counting as a real but partial contribution and setting a quantitative benchmark for many-body calculations.