{
  "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.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": null,
  "arxiv": "2609.37596",
  "author_total": 1,
  "authors": [
    {
      "name": "Chikara Shinei"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/fcecv0n9amfns.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2609.37596",
  "fulltext_url": null,
  "id": "fcecv0n9amfns",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2609.37596",
  "language": null,
  "licence": "CC-BY-4.0",
  "licence_label": "CC BY 4.0",
  "licence_source": "article",
  "licence_url": "http://creativecommons.org/licenses/by/4.0/",
  "metadata_licence": "CC0-1.0",
  "native_subjects": [
    [
      "arxiv",
      "quant-ph"
    ],
    [
      "openalex",
      "2505|Diamond and Carbon-based Materials Research"
    ]
  ],
  "openalex": "W7214964755",
  "pdf_url": "https://arxiv.org/pdf/2609.37596",
  "pmcid": null,
  "pmid": null,
  "published": "2026-09-30",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/fcecv0n9amfns.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:25:48Z",
      "native_id": "2609.37596",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2609.37596"
    }
  ],
  "subjects": [
    {
      "field": "physics",
      "origin": "arxiv:quant-ph",
      "subfield": "quantum"
    }
  ],
  "title": "Semiclassical spin-bath calculation of the nitrogen-isotope effect on NV$^-$ ensemble coherence in diamond",
  "title_text": "Semiclassical spin-bath calculation of the nitrogen-isotope effect on NV^- ensemble coherence in diamond",
  "type": "preprint",
  "updated_at": "2026-10-30T12:25:48Z",
  "url": "https://research.draconitic.com/paper/fcecv0n9amfns",
  "venue": "arXiv",
  "versions": []
}