Physicists run time backwards

evolution of a photon

Scientists have reversed time in a quantum system and returned a photon to its original state. The system can thus also reverse developments that are unknown.


Vienna, Austria). In physics , processes are subject to a specific time direction. A glass can fall to the ground and break there, but not reassemble and return to its place of origin. According to the second law of thermodynamics, according to which the entropy (degree of disorder) increases in every closed system, it is not possible for such a process or time to run backwards.

Other rules apply in quantum theory, according to which a process reversal also does not appear possible A core principle of quantum physics is crucial here, which postulates that changes occur in systems solely through observation. In principle, it is therefore impossible to observe changes over time in a quantum system and to reverse the process.


Reversal of time in the quantum system

Scientists at the University of Vienna have now succeeded in using the laws of quantum mechanics to restore a quantum system to its initial state while running time backwards at the same time. As the physicists explain in the specialist magazine Optica , they did not need to know the initial state of the quantum system.

Rewinding protocol for quantum systems

The experiment is based on a so-called rewinding protocol, which was devised by the theoretical physicist Miguel Navascues from the Institute for Quantum Optics and Quantum Information (IQOQI) of the Austrian Academy of Sciences (ÖAW). Put simply, the rewinding protocol allows a quantum system to reverse changes over time by coupling the evolution of the system to another evolution.

Physicists from the University of Vienna and the IQOOI led by Philip Walther have now experimentally realized the theory of navascues for the first time. As a quantum system, they used a single photon whose polarization was changed several times.


“It was one of the most difficult experiments we have ever set up for a single photon.”

Evolution of the photon superimposed

In their experiment, the researchers overlaid the development of the photon with another polarization. As a result, they could no longer know which of the two processes came first. The double use of the quantum switch ensured that time ran backwards within the quantum system and that the photon assumed its initial state. What is amazing is that to reverse the process, the physicists did not need to know what the initial and final state of the light particle was and how the photon changed over time.

“We have thus built a machine that can reverse a development that we do not know – through a general recipe that is generally valid for this size of system, as in our case the change in polarization of photons.”

The authors describe their discovery as “fundamentally incredibly interesting”. But they are also convinced that there are technological use cases. It is conceivable, for example, that a rewinding protocol will be integrated into a quantum processor so that errors or undesired developments can be reversed.

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