{
  "abstract": "Treating the arrow of time as a controllable quantum variable rather than a fixed external parameter is a central challenge in quantum physics. Recently, Bloch~\\cite{Bloch2025} proposed the theoretical concept of a \\emph{time qubit}, a two-level control space $\\mathcal{H}_T=\\mathrm{span}\\{\\ket{+},\\ket{-}\\}$ that selects between opposite Hamiltonian evolutions $\\hat{H}_{\\pm}=\\hat{H}_0\\pm\\hat{V}$, but this paradigm has remained experimentally unrealized. Here we formulate the quantum-optical theory of a single-photon time qubit---uncovering quantum-vacuum control and complementarity of the arrow of time---and experimentally probe and validate its predictions using phase-resolved single-photon interferometry from two coherently seeded entangled nonlinear biphoton sources~\\cite{Lee2018,Yoon2021}. First, we show theoretically and verify experimentally that a controlled $π$ pump--seed phase inversion during pair creation from the quantum vacuum flips the stimulated displacement generator $\\hat{V}\\to -\\hat{V}$ on top of the spontaneous two-mode squeezing background $\\hat{H}_0$, reversing the stimulated evolution time [$\\hat{U}_{\\mathrm{stim},-}(t)=\\hat{U}_{\\mathrm{stim},+}(-t)$] and enabling universal single-qubit $\\mathrm{SU}(2)$ control and postselection-free first-order tomography on the temporal Bloch sphere. Second, we prove that spontaneous idler vacuum fluctuations record complete which-branch (temporal-direction) information whereas the shared coherent seed erases it without cascading or aligning the idler beams, establishing an exact wave--particle complementarity law $|\\bm{r}|=\\mathcal{V}=|α|^2/(1+|α|^2)$ for the purity of the arrow of time and enabling direct first-order measurement of the Loschmidt time-reversal echo.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": null,
  "arxiv": "2610.27639",
  "author_total": 1,
  "authors": [
    {
      "name": "Tai Hyun Yoon"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/ej0pqx3e59ymt.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2610.27639",
  "fulltext_url": null,
  "id": "ej0pqx3e59ymt",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2610.27639",
  "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",
      "physics.optics"
    ],
    [
      "arxiv",
      "quant-ph"
    ],
    [
      "openalex",
      "1702|Quantum Information and Cryptography"
    ]
  ],
  "openalex": "W7223088826",
  "pdf_url": "https://arxiv.org/pdf/2610.27639",
  "pmcid": null,
  "pmid": null,
  "published": "2026-10-20",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/ej0pqx3e59ymt.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:34:04Z",
      "native_id": "2610.27639",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2610.27639"
    }
  ],
  "subjects": [
    {
      "field": "physics",
      "origin": "arxiv:physics.optics",
      "subfield": "optics-photonics"
    },
    {
      "field": "physics",
      "origin": "arxiv:quant-ph",
      "subfield": "quantum"
    }
  ],
  "title": "Single-photon time qubit: Quantum-vacuum control and complementarity of the arrow of time",
  "title_text": "Single-photon time qubit: Quantum-vacuum control and complementarity of the arrow of time",
  "type": "preprint",
  "updated_at": "2026-10-30T12:34:04Z",
  "url": "https://research.draconitic.com/paper/ej0pqx3e59ymt",
  "venue": "arXiv",
  "versions": []
}