How to reverse unknown quantum processes

(a) In the classical world, there is an infallible direction to time, exemplified here through the process of aging, a process that cannot be reversed in practice despite its deterministic nature. In this paper, we show that the same constraints do not apply in the quantum field. (b) The unity of quantum mechanics guarantees that an inverse of a given time evolution  always exists, even though it may be unknown. (c) By letting a target quantum system pass through an interaction region, a perturbed time evolution 𝑉 can be realized. (d) A quantum switch causes the target system to evolve in a superposition of its free evolution 𝑈 and perturbed evolution 𝑉. This superposition of time evolutions can be used to “rewind” the system back in time, without requiring any knowledge of either 𝑈,𝑉, or the state | Ψ ⟩. Credit: Optica (2022). DOI: 10.1364/OPTICA.469109

In the world around us, processes seem to follow a certain time course: Dandelions eventually turn into inflatable balls. However, the quantum realm does not play by the same rules. Physicists from the University of Vienna and IQOQI Vienna have now shown that for some quantum systems, the direction of processes in time can be reversed. This demonstration of the so-called retrieval protocol is published in Optica.

Everyday life is full of changes that are well understood but virtually impossible to reverse; for example, the metamorphosis of a dandelion into a ball. However, one could imagine undoing this transformation, step by step, if one knew exactly how each molecule in the plant moved in time. In the quantum field the problem becomes even more complicated: One of the fundamental principles of quantum physics is that simply observing a system causes it to change.

This makes it impossible, even in principle, to track a system change over time and process change. However, at the same time, the laws of quantum mechanics also open up new possibilities such as universal recovery protocols. These allow reversing changes in a quantum system without knowing what they were.

In a collaboration between the University of Vienna and IQOQI Vienna, experimental physicists led by Philip Walther have successfully implemented such a universal retrieval protocol developed by theoretical physicists led by Miguel Navascués. By combining this new theoretical protocol with a complex optical setup, the group showed that it is indeed possible to reverse the changes of a quantum system. For this, they used ultrafast fiber-optic components and free-space interferometers arranged as a quantum switch.

They successfully inverted the time evolution of a single photon without knowing how it changed over time, or even what its initial and final states were. “Remarkably, this protocol does not even require knowing the nature of the interactions with the quantum system,” says Peter Schiansky, first author of the paper.

Their universal rewrapping protocol is optimally efficient at runtime and can be extended to succeed with arbitrarily high probability. The proof that rollback protocols exist in this general form and that they are technically feasible contributes to our understanding of fundamental quantum mechanics. In the future, these protocols may become a useful tool in quantum information technologies.

More information: P. Schiansky et al, Demonstration of universal time reversal for qubit processes, Optica (2022). DOI: 10.1364/OPTICA.469109

Journal information: Optica