Korean Preprint Claims a Photon 'Time Qubit' Running Both Ways
A physicist in Seoul says an experiment run years ago for another purpose has already produced a "time qubit": a quantum system that evolves in two opposite directions at the same time. Tai Hyun Yoon of Korea University describes it in an arXiv preprint posted on 20 October, which has not yet passed peer review.
The qubit does not travel back in time. In the concept, it is a system that can unfold in two opposite temporal directions in superposition, the quantum state in which both options exist at once.
The idea behind a time qubit
The notion comes from a preprint by Yakov Bloch of Tel Aviv University, uploaded to arXiv in November 2025. In quantum mechanics, a Hamiltonian is the mathematical description that determines how a system's state changes from one moment to the next. Bloch proposed a two-level control space that selects between two opposite versions of that evolution. His proposed test splits a particle's path in two, applies opposing magnetic fields along each path and recombines them; the interference pattern would show how the two opposing evolutions are connected. That idea remains theoretical.
What Yoon did differently
Yoon used light instead of magnetic fields. His setup has two crystal sources that each generate entangled photon pairs, and one photon from each pair carries the single-photon qubit. The method is called entangled nonlinear biphoton source (ENBS) interferometry. The direction of the interaction is set by the phase of the light used to create the photons: shifting the phase by half a cycle reverses the part of the interaction he controls. As the paper describes it, time itself is not running backward; the mathematical process behind that interaction runs the opposite way. His method does not reverse the whole Hamiltonian.
Two opposing directions alone do not make a qubit. The system also needs a superposition of the two. Normally, creating a photon pair leaves information that could reveal which crystal produced it, which would prevent coherent superposition. By feeding both crystals with light from the same coherent laser, the which-source information is suppressed and the two evolutions can interfere. The result is a single-photon qubit that encodes two paths and two opposing directions of evolution.
Old data, new theory
Yoon did not build a new experiment. He re-examined measurements he and colleagues had collected earlier, including an experiment from 2018, which had shown the phase-dependent behaviour and interference his theory requires. On that basis he argues the apparatus can be understood as an implementation of a single-photon time qubit. Because old data are used to test a new theory, it is not a perfect demonstration; it works as a proof of concept. A dedicated experiment with tighter control of the quantum states could test the full set of operations the theory predicts.
Yoon points to uses in fault-tolerant temporal quantum error correction, high-dimensional qudit processing and tabletop simulations of relativistic quantum detector dynamics, but these remain proposals. Science and technology news is gathered in our Science section.