February 07, 2023
Processes in the environment around us seem to go in a specific time direction: dandelions eventually transform into blowballs. The quantum world, however, operates differently. Researchers from the University of Vienna and IQOQI Vienna have now demonstrated that for some quantum systems, it is possible to change the time orientation of processes. This "rewinding protocol" demonstration was documented and published in Optica.
There are many changes in daily life that are clearly known but very hard to reverse, such as the transformation of a dandelion into a blowball. If one knew precisely how each molecule in the plant moved in time, they might 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 can change just by being observed.
This renders it hard to track a system's change over time and undo the process, even in theory. The laws of quantum mechanics, however, also present fresh opportunities, such as universal rewinding procedures. 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 possible to undo modifications made to a quantum system by combining this innovative theoretical approach with a complex optical apparatus. For this, they used free-space interferometers set up as a quantum switch and ultra-fast optical fibre components.
Without knowing how a single photon evolved over time or even what its initial 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 require the nature of the interactions with the quantum system to be understood."
Their universal rewinding protocol has the best runtime efficiency and may be expanded to have an arbitrary high probability of success. Rewinding protocols' general existence and technical viability have been demonstrated, which advances our knowledge of basic quantum physics. These protocols may one day prove valuable in quantum information technologies.
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