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
HT=span{∣+⟩,∣−⟩} that selects between opposite Hamiltonian evolutions
H^±=H^0±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
H^0, reversing the stimulated evolution time [
U^stim,−(t)=U^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.
A temporal magnon-qubit Mach-Zehnder (MZ) interferometer is proposed. The interferometer is based on controllable entanglement of a microwave qubit and a magnonic state, achieved by application of a pulsed magnetic field playing the role of a magnon-qubit temporal "beam splitter". Analogous to a typical MZ interferometer, the generated interference pattern of the final qubit population carries information about the magnon dynamics. One important application of the proposed scheme is the study of single magnon decoherence. Interestingly, this scheme allows one to independently determine rates of two possible decoherence channels. This may help enable single magnon state applications and answer fundamental questions of quasi-particle decoherence at single quantum levels.
The interaction of light and matter provides a `clock' which allows us to extract characteristic timescales of quantum motion. Two-level atoms temporarily subjected to a radiation field will undergo Rabi oscillations, and the rotation angle of the corresponding Bloch vector serves as a measure of the interaction time. The approach is closely related to proposed `Larmor clock' experiments exploiting particle spin precession in a magnetic field, and gives some insight into how the usual notion of time emerges from quantum mechanics in its classical limit. The usage of the micromaser device for an experimental determination of quantum reflection, transmission, and dwell times is discussed.
Abstract Time in physics is conventionally treated as an external real parameter that measures change without participating in it.This study introduces a framework in which time is represented as an intrinsic complex coordinate whose real component corresponds to observable (external) time and whose imaginary component represents the internal phase of physical existence. Within this complex-time structure, temporal evolution is described by a continuous phase rotation on a complex manifold. The amplitude–phase duality of this rotation may provide a unified perspective in which amplitude persistence corresponds to energy and probability conservation, while phase evolution appears to govern the reversible internal oscillation that gives rise to the perception of temporal continuity. The framework is formulated through a minimal variational principle that embeds the phase rotation within a consistent Lagrangian–Hamiltonian structure, linking internal oscillation with relativistic spacetime geometry. It can reproduce the dual relativistic limits of pure temporal rotation () and phase freezing (), and suggests that the coexistence of spatial motion and finite phase evolution may underlie the continuity perceived as the flow of time. Possible experimental implications—such as relativistic interferometry, quantum-clock comparisons, and condensed-matter analogs—are discussed as potential tests of the model. Although the theory remains conceptual and exploratory, it offers a mathematically coherent basis for viewing time as an internal oscillatory degree of freedom. The results suggest that temporal continuity may originate from internal phase coherence, and that macroscopic irreversibility could emergefrom its gradual loss—topics that warrant further investigation. Running Title: Complex-Time Structure and Unified Temporal Framework Keywords: Complex time; phase rotation; quantum–relativistic unification; intrinsic time coordinate; reversible temporal dynamics; phase coherence; variational principle; proper time; phase freezing; arrow of time; internal oscillation.
Although time is one of the fundamental notions in physics, it does not have a unique description.In quantum theory time is a parameter ordering the succession of the probability amplitudes of a quantum system, while according to relativity theory each system experiences in general a different proper time, depending on the systemʼs world line, due to time dilation.It is therefore of fundamental interest to test the notion of time in the regime where both quantum and relativistic effects play a role, for example, when different amplitudes of a single quantum clock experience different magnitudes of time dilation.Here we propose a realization of such an experiment with a single electron in a Penning trap.The clock can be implemented in the electronic spin precession and its time dilation then depends on the radial (cyclotron) state of the electron.We show that coherent manipulation and detection of the electron can be achieved already with present day technology.A single electron in a Penning trap is a technologically ready platform where the notion of time can be probed in a hitherto untested regime, where it requires a relativistic as well as quantum description.
We report a test of the universality of free fall by comparing the gravity acceleration of the ^{87}Rb atoms in m_{F}=+1 versus those in m_{F}=-1, of which the corresponding spin orientations are opposite. A Mach-Zehnder-type atom interferometer is exploited to alternately measure the free fall acceleration of the atoms in these two magnetic sublevels, and the resultant Eötvös ratio is η_{S}=(0.2±1.2)×10^{-7}. This also gives an upper limit of 5.4×10^{-6} m^{-2} for a possible gradient field of the spacetime torsion. The interferometer using atoms in m_{F}=±1 is highly sensitive to the magnetic field inhomogeneity. A double differential measurement method is developed to alleviate the inhomogeneity influence, of which the effectiveness is validated by a magnetic field modulating experiment.