Physicists Say an Old Photon Experiment Hides a Qubit That Runs Two Ways in Time

A physicist in South Korea says an experiment his team ran in 2018 for a different purpose turns out to have been a working example of a "time qubit," according to ScienceAlert. Despite the headlines, nothing travels backward in time. The qubit is a quantum system that evolves in two opposite temporal directions at the same time.

The work, by Tai Hyun Yoon of Korea University, is a preprint that has not yet been peer-reviewed. It could eventually matter for quantum computing, but for now it is a proof of concept built on old data.

What a time qubit actually is

The idea is fairly new. ScienceAlert reported that it comes from a preprint by Yakov Bloch of Tel Aviv University, posted to arXiv in November 2025. Bloch describes a quantum system that can evolve in two opposite directions at once. ScienceAlert compares it to a clock whose hands turn clockwise and counterclockwise simultaneously.

The qubit does not rewind time itself. It lets physicists control the direction in which a quantum process unfolds, which could offer new ways to handle information.

Bloch proposed a test that splits the path of a quantum particle in two and applies opposing magnetic fields along the two paths. When the paths are brought back together, the interference pattern could show how the two opposing evolutions are linked. According to ScienceAlert, that proposal remains theoretical.

How Yoon used light instead of magnets

Yoon took a different route. Rather than reversing the evolution of a spinning particle with magnetic fields, he used light. His setup relies on two crystal sources that each generate entangled pairs of photons, and one photon from each pair carries the single-photon qubit. The technique is called entangled nonlinear biphoton source interferometry.

The direction of a given quantum interaction is set by adjusting the phase of the light that generates the photons. Shifting the phase by half a cycle reverses the interaction. ScienceAlert stressed that time is not flowing backward; the mathematical process governing the interaction simply runs the opposite way.

In quantum mechanics, a mathematical description called a Hamiltonian governs how a system's state changes from one moment to the next. Flip its sign, and the system evolves as if time were running in reverse. Yoon's method does not flip the whole Hamiltonian, only the part tied to the interaction he controls. That yields two opposing directions of evolution.

Two directions alone do not make a qubit, though. The qubit also needs superposition, meaning both directions exist together. Normally, creating a photon pair leaves behind information that could reveal which crystal produced it, and that information breaks the coherence a qubit needs. By feeding both crystals from the same coherent laser, the distinguishing information is suppressed, so the two opposing evolutions can interfere. The result, as ScienceAlert described it, is a single-photon qubit that encodes two paths and two opposing directions of quantum evolution.

Why it matters

For readers who will never touch a quantum lab, the practical stakes are long-term. The ability to manipulate the direction of quantum evolution could open new ways to process information, with implications especially for quantum computing, according to ScienceAlert. Yoon himself points to possible routes toward error correction, which is one of the central difficulties of building reliable quantum computers, as well as the processing of higher-dimensional quantum information and tabletop simulations of certain relativistic quantum effects.

Those are possibilities, not products. Nothing in the report suggests a device is close to use.

The caveats

Yoon did not build a new experiment. He went back to measurements he and colleagues had already collected, including from the 2018 work, which showed the phase-dependent behavior and interference his theory requires. By reexamining that data, he argued that the apparatus could be understood as an implementation of a single-photon time qubit.

ScienceAlert noted that using older data to test a new theory makes this a less than perfect demonstration. It is also a preprint, so other physicists have not yet reviewed the claims.

What could come next

According to ScienceAlert, a more specialized experiment with tighter control over the quantum states could test the full range of operations the theory predicts. Whether such an experiment is planned was not disclosed. Until one is run and the work passes peer review, the time qubit remains an intriguing reading of old results rather than an established tool.

Around Prague, based on reporting by ScienceAlert