Physicist says an old experiment built a qubit that runs both ways in time

[LokiCurrent]

A strange new idea in quantum physics suggests a qubit can evolve forwards and backwards in time at once. ScienceAlert reports on a preprint by Tai Hyun Yoon of Korea University describing a quantum vacuum seeded single photon time qubit

The concept builds on a theoretical proposal from Yakov Bloch at Tel Aviv University in November 2025. Bloch suggested splitting a particle's path under opposing magnetic fields so its evolution would run in opposite time directions at the same time, in superposition

Yoon's version uses two crystals that produce entangled pairs of photons, with one photon from each pair carrying the qubit. Shifting the light's phase by half a cycle flips the sign of part of the system's Hamiltonian, which reverses the interaction mathematically. Pumping both crystals with the same laser removes any information about which path was taken, so the two opposite evolutions interfere

The surprising part is that Yoon reanalysed data from a 2018 experiment by his own group, originally built for a different purpose, and argues it already implemented the time qubit. That is why ScienceAlert says physicists may have built it inadvertently. Yoon believes the approach could lead towards fault tolerant temporal quantum error correction

Nothing is being sent back in time here, which is worth stressing. Running backwards means the system's evolution is mathematically reversed. The work is not peer reviewed yet, and a dedicated experiment will be needed to confirm it

Does reanalysing old data make this result more or less convincing? And could time based qubits ever be useful for computing?

Physicists May Have Inadvertently Built a Qubit That Can Run Backward in Timewww.sciencealert.com