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.
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.
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.
Dynamical-mean-field cluster-correlation expansion of NV− spin-echo decoherence in a realistic substitutional-nitrogen bath
The Hahn-echo time T2 of nitrogen-vacancy (NV−) ensembles in nitrogen-rich diamond follows the empirical relation T2⋅[N] = 160 μs 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 Sz, 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 14N P1 bath it reduces T2⋅[N] at 30 mT from 264 ± 16 to 188 ± 12 μs ppm and gives 167 ± 11 μs ppm at 10 mT, 1.7 and 0.4 standard deviations from experiment, with no dark spins. Letting each centre jump independently gives 147 ± 9 μs ppm at 30 mT; the correlation of the flip-flop partners lengthens T2 by 20-30 percent at high field. The Overhauser field is neither frozen nor motionally narrowed: 68-83 percent of its autocorrelation survives 200 μs. At 2.5 mT, where the P1 electron and nucleus hybridise, T2 remains below experiment for any choice of rates, so part of the field dependence remains (T2 rises by a factor 1.69 ± 0.07 instead of 2.15 ± 0.08 from 2.5 to 30 mT).
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 T2. We determine a crossover depth at which T2 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 T2 on orientation for specific surfaces. We also find that the modulation of T2 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 T2 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.
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 ms = 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 ms = 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 1017 cm−3 the dephasing time T2∗ of a separately addressed 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 (T1 = 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.
We investigate nitrogen-vacancy center (NV) ensembles in diamond under the influence of strongly correlated electron-spin baths. We thoroughly calculate the decoherence properties of the NV central spin for bath concentrations of 0.1–100 ppm using the cluster-correlation expansion (CCE) method. We systematically analyze possible origins of the significant deviations in the values of the T2 coherence time reported in the literature. We demonstrate that significant variations can originate from the choice of averaging and fitting procedures used for the ensemble average and we point out the respective aspects that need to be considered when comparing the various theoretical studies. Our study may inspire readers to perform reliable simulations on the central spin problem. It provides an understanding and interpretation of the outcome parameters describing the dynamics of the local bath spins.
We present the experimental results of nitrogen-vacancy (NV) electron spin decoherence, which are linked to the coexistence of electron spin bath of nitrogen impurity (P1 center) and 13 C nuclear spin bath. In previous works, only one dominant decoherence source is studied: P1 electron spin bath for type-Ib diamond; or 13 C nuclear spin bath for type-IIa diamond. In general, the thermal fluctuation from both spin baths can be eliminated by the Hahn echo sequence, resulting in a long coherence time ( T 2 ) of about 400 μs. However, in a high-purity type-IIa diamond where 13 C nuclear spin bath is the dominant decoherence source, dramatic decreases of NV electron spin T 2 time caused by P1 electron spin bath are observed under certain magnetic field. We further apply the engineered Hahn echo sequence to confirm the decoherence mechanism of multiple spin baths and quantitatively estimate the contribution of P1 electron spin bath. Our results are helpful to understand the NV decoherence mechanisms, which will benefit quantum computing and quantum metrology.