A quantum experiment that sees a particle of light travel both forwards and backwards in time, at the same moment, is yet another example of the weirdness of the quantum realm. Although this experiment, which has been demonstrated by two research groups, has no immediate practical use, it could eventually have implications for quantum computers or even help develop a theory of quantum gravity.
Unlike our one-way sense of time, the laws of quantum physics are are run. You can’t un-cook an egg, but you can reverse the state of a photon travelling through a crystal, which is mathematically equivalent to the photon going back in time.
Another quirk of the quantum world is that a quantum state can have multiple values at the same time, called a superposition, until you measure it. Schrödinger’s cat is a famous example, in which a theoretical cat can be in a quantum state of both alive and dead inside a box until you open it.
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Last year, at the University of Oxford and his colleagues combined these two ideas, of what a superposition of processes going forwards and backwards in time might look like. They call it a quantum time flip.
“In a certain sense, you can say that the quantum time flip is Schrödinger’s cat for the direction of time,” says Chiribella. “Instead of alive or dead, you are a forward agent or a backward agent, and some quantum particle that can be in a superposition state could control whether you are forward or backward. So like the cat, you would end up being in a coherent superposition of forward and backward, which means that you are neither forward nor backward, or that you are both forward and backward at the same time.”
“This is not completely crazy, if you think about it,” says at the University of Stuttgart, Germany, who wasn’t involved in the research. “Quantum mechanics allows for a superposition of states — why not a superposition of processes?”
Now, Chribella and his colleagues have experimentally demonstrated that a quantum time flip is really possible. The team split a photon, a particle of light, into a superposition of two separate paths that go through a crystal, one from right-to-left and the other from left-to-right. While the right-to-left path sees the photon travel normally, the left-to-right path through the crystal affects the photon’s polarisation, or orientation, in exactly the way it would be affected if it was going backwards in time.
The photon splitter means the researchers can’t tell which route the photon has taken, because it is in a superposition of both paths. But at the end of the path, they recombine the split photon and measure its polarisation. By repeating this experiment enough times, the team could statistically prove that the photon must have been in a superposition of time processes.
at the University of Vienna, Austria, and his colleagues have also used a similar set-up to demonstrate a quantum time flip, and both teams wanted to see what it could be used for. In 2021, Chiribella’s team devised a task that involved examining the relationship between two quantum logic gates in which the inputs and outputs can be reversed – that is, computed in either direction. When attempting this experimentally, both teams found that this task was only possible for every pair of logic gates if you have access to a quantum time flip.
“We show that if you can superpose a process and its time reversal then there are computational tasks for which you can outperform processes in which time flows in a definite direction,” says Strömberg.
“That’s really exciting, because now you have a tool, something that was buried there since the very beginning of quantum mechanics,” says Lutz. While it isn’t immediately obvious how it might be useful, the importance of quantum superposition also wasn’t immediately obvious when it was discovered, but it now plays a crucial role in making quantum computers work, says Lutz. “The prospect of doing something similar, with a superposition of forwards and backwards time directions, is exciting.”
It could also help us tackle one of the biggest outstanding problems in physics, namely combining quantum mechanics with our understanding of gravity to create a quantum theory of gravity, says at the Perimeter Institute in Canada. “This new theory of quantum gravity, we would expect it to have causal indefiniteness,” he says, meaning that superpositions of different directions of time would crop up. “It makes us think more carefully about those concepts, about what we mean by time direction.”
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