Physicists May Have Inadvertently Built a Qubit That Can Run Backward in Time

Key Points:

• Korean physicists, led by Tai Hyun Yoon, have developed a theory and demonstrated a "quantum vacuum-seeded single-photon time qubit," which allows quantum processes to evolve in two opposite temporal directions simultaneously, akin to a clock running both forwards and backwards.
• This time qubit concept, initially proposed by Yakov Bloch in 2025, does not involve actual backward time travel but enables manipulation of the direction of quantum evolution by reversing parts of the governing Hamiltonian through phase adjustments in entangled photon pairs.
• Yoon's approach uses entangled nonlinear biphoton source (ENBS) interferometry with two crystal sources and a shared coherent laser to suppress path-distinguishing information, allowing superposition of opposing quantum evolutions and creating the time qubit.
• Rather than designing a new experiment, Yoon reanalyzed data from earlier experiments conducted in 2018, showing that existing measurements already demonstrated the phase-dependent interference consistent with a single-photon time qubit, serving as a proof of concept.
• This breakthrough opens potential pathways for advanced quantum computing applications, including fault-tolerant temporal quantum error correction and high-dimensional qudit processing, suggesting that dedicated experiments could further explore and harness time qubits.