Time reversal in the quantum realm

Reverse time: Physicists have developed and experimentally implemented a protocol that allows quantum processes to be “rewound” in time. In principle, a locally limited time reversal occurs – a process that is impossible in the macroscopic world. Unlike previous approaches, the new “rewinding” protocol is also universally applicable and enables this reversal with a very high probability, as the researchers report. That makes it practically broadly applicable.

In our everyday life and in classical physics, time is a one-way street: it can only run forward. Because time and entropy are closely linked and the increasing thermodynamic “disorder” cannot be reversed without external influence. A fallen cup will not reassemble itself, and a dandelion will not revert to a dandelion flower.

Overlay allows local “rewind”

But different rules apply in quantum physics. Because probabilities rule in the world of the smallest particles, different states can exist simultaneously in the phenomenon of superposition – as in the famous thought experiment of Schrödinger’s cat, which is dead and alive at the same time. This duality even applies to time, because the directions of the arrow of time also overlap. Theoretically, it should therefore be possible to reverse time in a quantum system – similar to how the phase of a light wave can be reversed using special mirrors.

In fact, there is already some evidence that such localized violations of the laws of thermodynamics are possible. Physicists have also observed this , for example with qubits returning from an increasingly “fuzzy” position to the localized, known initial state. The catch: Such reversals occur extremely rarely and randomly in these protocols, and this “rewinding” also takes three times as long as the normal forward running process.

A universal rewinding protocol

This has now changed: physicists around Peter Schiansky from the University of Vienna have developed a quantum physical rewinding protocol that can take place with almost any degree of probability and is therefore quasi deterministic. “It is remarkable that for this protocol it is not even necessary to know the nature of the interactions with the quantum system,” says Schiansky.

The advantage of this: One of the core principles of quantum physics is that systems change through observation alone. This makes it impossible, even in principle, to trace the change of a system over time and make the process reversible. Because just following along changes the conditions irreversibly. But this is different with the newly developed universal rewinding protocol: It can be applied without having to know the process in question in the quantum system and its initial and final state in detail.

High yield photon time reversal

The physicists demonstrated how this works in an experiment. To do this, they used a photonic platform in which individual photons were directed onto different paths using mirrors and beam splitters and polarized differently depending on the path. The team describes this as an “unknown but repeatable perturbation” that triggers a change in the particle over time. “In principle, this perturbation can be achieved through any physical interaction and therefore in any quantum system,” explain Schiansky and his team.

To return the photon to its original state, the physicists use a special quantum switch, which leads to an interferometric superimposition of the manipulated photon and its states generated in optical loops. Ultimately, this structure allows the development of the photon to be rewound. “With this photonic platform, we have achieved an average rewinding rate of over 95 percent,” the researchers report.

Wide practical use

With this, Schiansky and his team have developed a universal rewinding protocol and experimentally proven that this form of time reversal is feasible in quantum systems. “Our experimental quantum protocol significantly outperforms the optimal classical strategies in terms of the reliability of the resulting states,” states the team. “Thus, our protocol brings quantum time-reversal into a regime of practical relevance.”

According to the physicists, such protocols could become a useful tool in quantum information technologies in the future – also because of their broad applicability. “Our photonic implementation offers a particularly simple and robust approach based on a sophisticated platform,” they explain. At the same time, the principle works for photons as well as for other quantum particles. (Optica, 2023; doi:10.1364/OPTICA.469109 )