{
  "abstract": "The demonstration that enhanced yellow fluorescent protein hosts an optically addressable spin-1 qubit in its metastable triplet state raises the prospect of genetically encoded quantum sensing at molecular length scales. We develop a detection-limit theory for using this fluorescent-protein spin qubit (FPSQ) to sense paramagnetic neural signaling radicals by spin relaxometry. We derive the transition-resolved Redfield relaxation matrix of the zero-field-split triplet coupled to a diffusing radical bath, establish the regime in which it collapses to a single exponential, and validate it against Lindblad simulations and nitrogen-vacancy benchmarks. Propagating the effects of photon shot noise, photobleaching-grounded photon budget, and finite measurement bandwidth, we find that the native room-temperature sensor falls short of physiological sensitivity by six to eight orders of magnitude with the bottleneck being the phonon-limited intrinsic $\\Tone$. Analyzing the underlying direct and two-phonon Raman processes, we show that room-temperature relaxation is Raman-dominated by $\\sim\\!720\\!:\\!1$ and that, because the Raman coefficient scales as $v^{-10}$ with sound velocity, a $\\sim\\!2\\times$ stiffening of the chromophore environment recovers $\\Tone\\sim\\SI{100}{\\micro\\second}$, sufficient for micromolar sensing. Nanomolar sensing is obstructed by a direct-process ceiling of \\SI{79}{\\micro\\second} that vibronic decoupling alone cannot breach. We obtain quantitative design rules, identify photon yield as a co-equal bottleneck, and propose a frequency-resolved protocol for chemical specificity.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": null,
  "arxiv": "2609.02792",
  "author_total": 5,
  "authors": [
    {
      "name": "Parul Raghuvanshi"
    },
    {
      "name": "Sagnik Ganguly"
    },
    {
      "name": "Sharika E"
    },
    {
      "name": "Mohana Priya T."
    },
    {
      "name": "Vishvendra S. Poonia"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/es1arzsp9k9df.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2609.02792",
  "fulltext_url": null,
  "id": "es1arzsp9k9df",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2609.02792",
  "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",
      "1304|Electron Spin Resonance Studies"
    ]
  ],
  "openalex": "W7207726051",
  "pdf_url": "https://arxiv.org/pdf/2609.02792",
  "pmcid": null,
  "pmid": null,
  "published": "2026-09-12",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/es1arzsp9k9df.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:21:25Z",
      "native_id": "2609.02792",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2609.02792"
    }
  ],
  "subjects": [
    {
      "field": "physics",
      "origin": "arxiv:quant-ph",
      "subfield": "quantum"
    }
  ],
  "title": "Phonon-limited detection thresholds for genetically encoded fluorescent-protein spin-qubit relaxometry of neural radicals",
  "title_text": "Phonon-limited detection thresholds for genetically encoded fluorescent-protein spin-qubit relaxometry of neural radicals",
  "type": "preprint",
  "updated_at": "2026-10-30T12:21:25Z",
  "url": "https://research.draconitic.com/paper/es1arzsp9k9df",
  "venue": "arXiv",
  "versions": []
}