In the world around us, processes appear to follow a certain time-direction: Dandelions eventually turn into blowballs. 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 certain quantum systems, the time-direction of processes can be reversed. This demonstration of a so-called rewinding protocol has been published in Optica.
Everyday life is full of changes that are well understood, yet practically impossible to reverse; for example, the metamorphosis of a dandelion into a blowball. However, one could imagine undoing this transformation, step by step, if one knew precisely how each molecule in the plant moved in time. In the quantum realm the problem gets even trickier: One of the core 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’s change in time and reverse the process. However, at the same time, the laws of quantum mechanics also open up new possibilities such as universal rewinding protocols. These allow for reversing changes in a quantum system without knowing what they were.
