Single-photon time qubit: Quantum-vacuum control and complementarity of the arrow of time

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 ℋ_T=span{ket+,ket−} that selects between opposite Hamiltonian evolutions Ĥ_±=Ĥ₀±V̂, 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 V̂→ −V̂ on top of the spontaneous two-mode squeezing background Ĥ₀, reversing the stimulated evolution time [Û_stim,−(t)=Û_stim,+(−t)] and enabling universal single-qubit 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 |r|=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.

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BibTeX RIS

BibTeX

@misc{yoon2026singlephoton,
  title = {{Single-photon time qubit: Quantum-vacuum control and complementarity of the arrow of time}},
  author = {Tai Hyun Yoon},
  howpublished = {arXiv},
  year = {2026},
  month = oct,
  doi = {10.48550/arxiv.2610.27639},
  eprint = {2610.27639},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2610.27639},
  note = {CC BY 4.0},
}

RIS

TY  - UNPB
TI  - Single-photon time qubit: Quantum-vacuum control and complementarity of the arrow of time
AU  - Tai Hyun Yoon
PY  - 2026
DA  - 2026/10/20
PB  - arXiv
DO  - 10.48550/arxiv.2610.27639
UR  - https://arxiv.org/abs/2610.27639
AB  - 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 ℋ_T=span{ket+,ket−} that selects between opposite Hamiltonian evolutions Ĥ_±=Ĥ_0±V̂, 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 V̂→ −V̂ on top of the spontaneous two-mode squeezing background Ĥ_0, reversing the stimulated evolution time [Û_stim,−(t)=Û_stim,+(−t)] and enabling universal single-qubit 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 |r|=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.
ER  -

Abstract

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