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This study experimentally realizes a "time qubit," treating the arrow of time as a controllable quantum variable rather than a fixed parameter. By manipulating single-photon interferometry from entangled sources, researchers successfully reverse stimulated evolution and measure temporal-direction information without postselection. The work establishes a fundamental complementarity law between vacuum fluctuations and coherent seeds, advancing foundational quantum optics and potential applications in quantum metrology.
Single-photon time qubit: Quantum-vacuum control and complementarity of the arrow of time
arXiv:2610.27639v1 Announce Type: new 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 $\pi$ 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 $|\bm{r}|=\mathcal{V}=|\alpha|^2/(1+|\alpha|^2)$ for the purity of the arrow of time and enabling direct first-order measurement of the Loschmidt time-reversal echo.
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 $\pi$ 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 $|\bm{r}|=\mathcal{V}=|\alpha|^2/(1+|\alpha|^2)$ for the purity of the arrow of time and enabling direct first-order measurement of the Loschmidt time-reversal echo.
Quantum Physics (quant-ph); Optics (physics.optics)
arXiv:2610.27639 [quant-ph]
(or arXiv:2610.27639v1 [quant-ph] for this version)
https://doi.org/10.48550/arXiv.2610.27639 Focus to learn more arXiv-issued DOI via DataCite (pending registration)
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