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Two separate research groups have managed to make a photon travel forward and backward in time simultaneously – a phenomenon called “quantum time reversal.” This experiment may have implications for the design of quantum computers, but also help develop a theory of quantum gravity.
On our scale, time only flows in one direction; Events that occur, often in a well-ordered causal sequence, cannot be reversed. In quantum physics, on the other hand, this kind of thing is very possible: at the subatomic scale, there is no asymmetrical difference between the past and the future and the two events can be truly simultaneous. In addition, each quantum state can correspond to many values at the same time, until a measurement is made – this is called the principle of superposition, which is described mainly by the famous Schrödinger’s cat, which remains dead and live until the box is opened.
Researchers at the University of Oxford, led by physicist Giulio Chiribella, combined these two basic concepts in an attempt to describe what a superposition of forward and backward processes in time looks like. Quantum physics allows the superposition of states, so it is possible that it allows the superposition of processes. For Chiribella, who described this concept earlier this year, it seems like Schrödinger’s cat for the direction of time.
The arrow of time is seen as a superposition of temporal processes
” The experience of time flowing in a definite direction, from the past to the future, is deeply rooted in our thinking. At the microscopic level, however, the laws of nature seem indifferent to the difference between the past and the future. “, wrote Chiribella and Zixuan Liu in an article published this summer in Physics of Communication. In this study, they established a mathematical framework for quantum operations that follow an indefinite temporal direction, as well as an indefinite causal sequence.
Now, they explain how they have shown experimentally that quantum time reversal is indeed possible. To do this, they use a photon splitter, where each particle of light is split as a superposition of two different paths through a crystal; one runs from right to left, the other from left to right. Through the right-to-left path, the photon moves normally; in the other direction, on the other hand, based on the characteristics of the polarization of light through the crystal, the polarization of the photon is affected in the same way as if it goes back in time.
During the experiment, it is impossible to know which path the photons are passing, because they are all the time in a superposition of two paths. At the end of the journey, researchers can recombine the split photon to measure its polarization. With enough tests, they confirmed statistically that photons must be in a superposition of paths and therefore in two temporal processes.
” Our results establish input-output indefiniteness as a new resource for quantum information protocols and allow tabletop simulation of hypothetical scenarios where the arrow of time may be in quantum superposition. “, they concluded in their preprint article. At the same time, a team from the University of Vienna, led by Teodor Strömberg, conducted similar experiments, which also demonstrated quantum time reversal.
Towards a new theory of quantum gravity
The researchers turned their attention to possible applications of their discovery. Chiribella and his colleagues set out to examine the relationship between two quantum logic gates where the inputs and outputs can be flipped (ie computed) repeatedly. Then they discovered that this is only possible for each pair of logic gates if there is access to a quantum time reversal.
The two teams finally showed that the superposition of a process and its temporal inverse implies the existence of computational tasks through which it is possible to surpass processes where time flows in a specific direction.
The utility of this discovery is not yet clear, but experts point out that the importance of quantum superposition was not clear when it was discovered, while today it plays a major role in the operation of quantum computers. So it is possible that the superposition of temporal directions will one day lead to other innovations.
It could also lead to a new theory of quantum gravity, which would finally be able to unify quantum mechanics with general relativity. Some experts, including Lucien Hardy, a theoretical physicist at the Perimeter Institute, hope that this theory is based on “causal indefiniteness”, which includes overlaps in different directions of time. More broadly, this discovery calls into question the very definition of time direction, which is certainly not fundamental at the quantum level.