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 H±=H0±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 V→−V^ on top of the spontaneous two-mode squeezing background H0, reversing the stimulated evolution time [Ustim,−(t)=Ustim,+(−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 $|\bm{r}|=\mathcal{V}=|\alpha|2/(1+|\alpha|2)$ for the purity of the arrow of time and enabling direct first-order measurement of the Loschmidt time-reversal echo.
Single-photon time qubit: Quantum-vacuum control and complementarity of the arrow of time - Oct 2026
https://arxiv.org/abs/2610.27639
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 H±=H0±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 V→−V^ on top of the spontaneous two-mode squeezing background H0, reversing the stimulated evolution time [Ustim,−(t)=Ustim,+(−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 $|\bm{r}|=\mathcal{V}=|\alpha|2/(1+|\alpha|2)$ for the purity of the arrow of time and enabling direct first-order measurement of the Loschmidt time-reversal echo.