Researchers chart the 'secret' movement of quantum particles

Researchers from the University of Cambridge have taken a peek into the secretive domain of quantum mechanics. In a theoretical paper published in the journal Physical Review A, they have shown that the way that particles interact with their environment can be used to track quantum particles when they're not being observed, which had been thought to be impossible.

One of the fundamental ideas of quantum theory is that quantum objects can exist both as a wave and as aparticle, and that they don't exist as one or the other until they are measured. This is the premise that Erwin Schrödinger was illustrating with his famous thought experiment involving a dead-or-maybe-not-dead cat in a box.

"This premise, commonly referred to as the wave function, has been used more as a mathematical tool than a representation of actual quantum particles," said David Arvidsson-Shukur, a Ph.D. student at Cambridge's Cavendish Laboratory, and the paper's first author. "That's why we took on the challenge of creating a way to track the secret movements of quantum particles."

Any particle will always interact with its environment, 'tagging' it along the way. Arvidsson-Shukur, working with his co-authors Professor Crispin Barnes from the Cavendish Laboratory and AxelGottfries, a Ph.D. student from the Faculty of Economics, outlined a way for scientists to map these 'tagging' interactions without looking at them. The technique would be useful to scientists who make measurements at the end of an experiment but want to follow the movements of particles during the full experiment.

Some quantum scientists have suggested that information can be transmitted between two people – usually referred to as Alice and Bob – without any particlestravelling between them. In a sense, Alice gets the message telepathically. This has been termed counterfactual communication because it goes against the accepted 'fact' that for information to be carried between sources, particles must move between them.

"To measure this phenomenon of counterfactual communication, we need a way to pin down where the particles between Alice and Bob are when we're not looking," said Arvidsson-Shukur. "Our 'tagging' method can do just that. Additionally, we can verify old predictions of quantum mechanics, forexample that particles can exist in different locations at the same time."