Processes in the environment around us seem to go in a certain temporal direction: dandelions ultimately transform into blowballs. The quantum world, however, operates differently. Researchers from the University of Vienna and IQOQI Vienna have recently shown that for certain quantum systems, it is possible to change the time-direction of processes. The so-called rewinding procedure is shown in this Optica article.
There are many changes in daily life that are clearly known yet very hard to reverse, such as the transformation of a dandelion into a blowball. If one understood exactly how each molecule in the plant moved in time, they may be able to conceive correcting this transition one step at a time. The difficulty increases when we consider quantum physics, where one of the fundamental ideas is that a system may change just by being seen.
This makes it hard to trace a system’s evolution over time and undo the process, even in theory. The rules of quantum physics, however, also provide novel opportunities, such as universal rewinding protocols. These enable the undoing of modifications in a quantum system without being aware of what those changes were.
A universal rewinding protocol created by theoretical physicists led by Miguel Navascués has been successfully implemented by experimental physicists under the direction of Philip Walther at the University of Vienna and IQOQI Vienna. The team demonstrated that it is feasible to undo modifications made to a quantum system by combining this innovative theoretical approach with a complex optical apparatus. They achieved this using free-space interferometers configured as a quantum switch and ultra-fast optical fibre components.
Without knowing how a single photon evolved through time or even what its beginning and end states were, they were nevertheless able to reverse it. Peter Schiansky, the paper’s first author, notes that “remarkably, this protocol does not even need the nature of the interactions with the quantum system to be understood.”
Their universal rewinding technique has the best runtime efficiency and may be expanded to have an arbitrary high chance of success. Rewinding protocols’ general existence and technological viability have been shown, which advances our knowledge of basic quantum physics. These protocols may one day be valuable in quantum information technology.
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