Quantum breakthrough enables scientists to reverse the flow of time

In the world of quantum physics, the rules that govern reality behave in ways that challenge what we think we know. Particles can be in two places at once. Actions on one particle can instantly affect another, even across vast distances. Teleportation isn’t science fiction—it’s part of how the universe really works at the tiniest level. Now, researchers have added something even stranger to the mix: quantum time travel.

Not time machines in the classic sci-fi sense. Instead, this is a breakthrough in controlling time within quantum systems—particles so small that regular physics barely applies. A group of scientists from Austria has shown that you can speed up, slow down, and even reverse the “age” of these particles, effectively rewinding time for them.

At the center of this work is a clever device called a quantum switch. Think of it as a remote control for quantum particles. While traditional physics moves like a movie in a theater—playing from start to finish no matter what—quantum systems behave more like watching a movie at home, where you can fast-forward, rewind, or skip scenes altogether.

Miguel Navascués from the Austrian Academy of Sciences compared it to having control over time itself. “We can rewind to a previous scene or skip several scenes ahead,” he said. This is not just theory. Together with University of Vienna physicist Philip Walther, Navascués and colleagues used photons—particles of light—to test their ideas.

Using crystals and a setup based on the quantum switch, they sent a photon on a journey, then used the switch to bring it back to the state it was in before the trip began. This may sound simple, but in the world of quantum mechanics, it’s revolutionary. Normally, just observing a quantum system causes it to change. That’s what makes this discovery so remarkable: they were able to rewind the system without knowing anything about what happened to it.

This method, called a “rewind protocol,” lets any quantum particle—like an electron or photon—go back to a previous state, even if the experimenter doesn’t know what that state was or how it got there. David Trillo, another researcher from the Austrian team, helped show that the process works in both theory and in the lab.

“It was one of the most difficult experiments we’ve ever built for a single photon,” Walther said. But the result was clear: the photon returned to its original state, like hitting rewind on a video without ever watching it.

The team also went beyond rewinding. They found that you can fast-forward time, too. “To make a system age 10 years in one year,” Navascués explained, “you must get the other nine years from somewhere.” In one experiment, they used ten identical systems. By “stealing” one year of aging from each of the first nine, they gave all nine years to the tenth system, which aged 10 years in just one.

These discoveries won’t let anyone travel back in time or undo mistakes in real life. A human body holds massive amounts of information, and it would take millions of years to rewind even one second of someone’s life. But that’s not the point.

The goal is to make quantum processors more powerful and reliable. In quantum computers, which store data in tiny, delicate particles, errors can creep in easily. If you could rewind the system and correct the error, you’d save time and energy.

“We are convinced that it has technological applications,” said Walther. “For example, a rewind protocol in quantum processors can be used to reverse unwanted errors or developments.” Other team members added that future versions could be built using systems beyond just light, and even expanded to more complex setups.

Normally, time only flows forward. This “arrow of time,” first described by astronomer Arthur Eddington, follows the second law of thermodynamics, which says things naturally become more disordered. But this law is statistical—it applies to big systems, not necessarily the smallest ones.

In classical physics, it’s possible to reverse some processes using a trick called phase conjugation. But in quantum mechanics, random noise usually gets in the way. That’s why scientists have long wondered: can you reverse time for a quantum system without knowing anything about it?

Several past studies tried to answer that. But they only worked some of the time and required detailed knowledge of the system or slow, complex procedures. This new protocol is different. It works for any two-level system (known as a qubit), has a high success rate, and runs in real time. That means if you rewind a system by five minutes, it actually takes five minutes.

At the heart of this method lies a key idea from quantum mechanics: the non-commuting nature of quantum operators. This just means the order in which you apply operations matters, unlike in normal math. Using that principle, the scientists created a setup where the target system evolves in a superposition—a mix of different timelines.

Each timeline includes slight changes caused by known physical interactions. By cleverly interfering these timelines, they guided the particle back to a previous state, no matter what path it took. And if something went wrong, they could try again using error correction, boosting the odds of success.

The setup used light particles manipulated by half- and quarter-wave plates to generate different “Hamiltonians”—rules that define how the particle evolves in time. Then, using fast optical switches and the quantum switch, they mixed the time paths together to create the rewind effect.

These results are more than just an idea on paper. The team tested their method on many different quantum processes and reached a rewind accuracy of over 95%. That level of control opens the door to using quantum time reversal in real-world technologies.

The rewind protocol could help develop better quantum computers and new ways to control quantum states in labs. It could even offer insights into how time works in the deepest parts of physics.

Still, Navascués stays realistic. “If we could lock a person in a box with zero external influences, it would be theoretically possible [to rewind them],” he said. “But with our current protocols, the probability of success would be very, very low.”

In other words, it’s not time travel as you know it—but it’s the closest thing physics has come to bending the clock backward.

Research findings are available online in the journal Optica.