Physicists from Vienna make time run backwards

In our everyday world, time is always moving forward. A glass that falls on the floor and breaks does not reassemble and bounces on the table. It is different in the quantum world: physicists have now succeeded in running back time in a quantum system and returning its development to its initial state.

As they report in the journal “Optics”, it is not even necessary to know the initial state.

Theoretical physicist Miguel Navascues of the Institute for Quantum Optics and Quantum Information (IQOQI) of the Austrian Academy of Sciences (ÖAW) in Vienna came up with a so-called “rewind protocol”. It allows one to use an elegant mathematical trick to undo changes in a quantum system over time. Simply put, the development of the system is accompanied by another development.

The team led by Philip Walther from the University of Vienna and the IQOQI has now succeeded in experimentally realizing this theoretical recipe. The quantum system is a single photon whose polarization is changed several times. “It was one of the most difficult experiments we’ve ever set up for a single photon,” Walther told APA.

To implement the theoretical recipe, the physicists superimposed this development of the photon on a second operation — also a change in polarization — “so that we no longer know which of the two processes comes first,” says Walther. The dual use of this so-called “quantum switch” allows them to reverse time and return the light particle to the state it was in at the beginning of its journey. It is remarkable that one does not need to know how the photon changed over time, what caused this change and what the initial and final states of the light particle were.

“So we’ve built a machine that can reverse a development we don’t know about — using a general recipe that’s generally valid for systems of this size, such as the change in polarization of photons in our case,” Walther said. On the one hand, this is “fundamentally incredibly interesting” because you can return to a state you don’t even know. “But we are also convinced that this also has technological applications,” says the physicist. If you install such a “rewind protocol” in quantum processors, for example, you can use it to reverse errors or developments that you do not want.

(aeg/sda/apa)