What if a quantum device could make time flow in two opposite directions at once? Physicists are exploring this mind-bending possibility after discovering a way for a quantum bit, the tiny unit of information used in quantum computers, to evolve both forward and backward in time. While this doesn’t mean scientists have invented a time machine, the discovery challenges our everyday understanding of how time works and could open new possibilities in quantum technology. But how can a single quantum system behave as though time is moving in two directions at once? Read on to discover the science behind this fascinating breakthrough and what it could mean for the future of computing.
A quantum device that can evolve in two directions
Time moves in one direction in our everyday experience, but the rules of quantum physics allow scientists to explore some unusual possibilities. Researchers are investigating whether quantum systems can evolve in two opposite directions of time simultaneously, a concept that could eventually lead to new ways of processing information.
Physicist Tai Hyun Yoon of Korea University has developed a theoretical framework describing how a quantum bit, or qubit, could achieve this behavior using light. Even more surprisingly, Yoon found that an experiment he and his colleagues conducted in 2018 may have already demonstrated the underlying process, years before he recognized its potential.
What does it mean for a qubit to run backward in time?
The idea of a time qubit emerged in a preprint published by Tel Aviv University physicist Yakov Bloch on arXiv in November 2025.
Despite the name, a time qubit cannot travel into the past or reverse time itself. Instead, it is a quantum system that can exist in a superposition of two opposite directions of quantum evolution.
Think of a clock whose hands can move clockwise and counterclockwise simultaneously. A time qubit works in a somewhat similar way, allowing a quantum process to evolve in opposing temporal directions.
Bloch proposed testing this concept by splitting a quantum particle into two paths and exposing each path to an opposing magnetic field. When the paths reunite, the resulting interference pattern could reveal how the two directions of evolution interact.
Yoon explored a different approach, using light rather than magnetic fields to control quantum interactions.
How light can reverse a quantum process
Yoon’s method uses two crystal sources that produce pairs of entangled photons. Entanglement is a quantum connection between particles that allows their properties to be linked, even when they are considered separately.
The technique, known as entangled nonlinear biphoton source (ENBS) interferometry, uses one photon from each pair to create a single-photon qubit.
The key is controlling the phase of the light used to generate the photons. Phase describes a wave’s position within its repeating cycle and can influence how different waves interact.
In quantum mechanics, a mathematical function called the Hamiltonian describes how a system’s quantum state changes over time. Reversing the sign of a Hamiltonian reverses the direction of the evolution it describes.
Yoon’s approach does not reverse the entire Hamiltonian. Instead, it reverses the part responsible for a particular interaction, allowing the system to evolve in two opposing directions.
Adjusting the light’s phase by half a cycle can reverse this interaction. This does not make time itself run backward; it changes the mathematical process governing how the quantum system evolves.
The secret to making both directions exist together
Being able to reverse a quantum interaction is only part of the challenge. To create a time qubit, both directions must exist in a superposition, meaning the quantum system can occupy a combination of two possible states simultaneously.
This is where the shared laser source becomes important.
Normally, information left behind during photon generation could reveal which crystal produced a particular photon pair. That information would distinguish the two possible origins, making it harder for the quantum states to combine coherently.
By illuminating both crystals with light from the same coherent laser source, Yoon’s method can suppress this distinguishing information. This allows the opposing quantum evolutions to interfere and combine into a superposition.
The resulting single-photon qubit therefore represents more than two different paths. It encodes two opposing directions of quantum evolution, bringing the proposed time-qubit concept within experimental reach.
An old experiment reveals a new possibility
Rather than building an entirely new device, Yoon revisited measurements from experiments he and his colleagues had already conducted, including one in 2018.
Those experiments had demonstrated phase-dependent behavior and interference consistent with the principles described in his new theoretical framework. By analyzing the existing data, Yoon showed how the earlier apparatus could be interpreted as an implementation of a single-photon time qubit.
However, the findings are not yet a definitive experimental demonstration. The researchers originally conducted the experiments for a different purpose, meaning the existing measurements were not specifically designed to test every aspect of the proposed time qubit.
A dedicated experiment with greater control over quantum states could provide a stronger test of the theory and establish whether the system can perform the full range of operations predicted by Yoon’s framework.
Could this lead to better quantum computers?
If researchers can develop the approach further, time qubits could offer new ways to encode and process quantum information
In his preprint, Yoon describes a possible route toward more advanced quantum computing techniques, including methods for correcting errors and processing information in higher-dimensional quantum systems.
He writes, “Coupling the discrete ENBS time qubit via controlled time-frequency displacements to hardware-native Gottesman-Kitaev-Preskill (TF-GKP) bosonic grid states in chronocyclic phase space establishes a robust pathway toward fault-tolerant temporal quantum error correction, high-dimensional qudit processing, and tabletop simulations of relativistic quantum detector dynamics.”
In simpler terms, Yoon suggests that connecting time qubits with specialized quantum states could help researchers develop more reliable quantum computers, process more complex information, and simulate certain effects involving quantum physics and relativity
These possibilities remain theoretical and will require further investigation before their practical value can be established.
For now, the research offers a new way to investigate how quantum systems evolve and interact. The study has not yet been peer-reviewed, and its findings are available as a preprint on arXiv.
Sources:
Science Alert
arXiv