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arXiv:2609.37596

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

Chikara Shinei

quant-phSubmitted Sep 29, 2026• Announced Sep 30, 2026• 0 citations• Revised Sep 30, 2026 (v2)
v1 · Sep 29, 2026v2 · Sep 30, 2026
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Abstract

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.

Comments
5 pages, 3 figures, 1 table; Supplementary Material (6 pages, 2 figures, 1 table) included

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Dynamical-mean-field cluster-correlation expansion of NV−^-− spin-echo decoherence in a realistic substitutional-nitrogen bath
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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.
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.
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.
Decoherence is the biggest bottleneck in all quantum technologies. For nitrogen-vacancy (NV) centers in diamond, the loss of coherence is caused by the electron and nuclear spin bath of the diamond lattice. Here, we demonstrate that the spin bath - that typically causes decoherence - entangles the spin states of the NV electron and the host 14^{14}14N nucleus. The many-body interaction between the 14^{14}14N nucleus - electron - bath spins at an energy level anti-crossing occurring for an applied magnetic field orientation perpendicular to the NV axis is responsible for this effect. This is observed experimentally in NV ensembles via electron spin-echo measurements, where the echo envelope is modulated at the frequency of a 14^{14}14N nuclear spin transition. Using numerical simulations, we show that the spin bath coupling to the NV centers is essential for observing this modulation. Due to the zero first-order Zeeman effect at the anti-crossing, the observed oscillations have long spin-echo coherence times, 2--3 times those at the parallel magnetic field orientation. The oscillation frequency is highly stable and robust against environmental fluctuations. These findings provide new opportunities for fundamental studies of many-body physics and quantum sensing.