One Quantum Particle Can Facilitate Two-Way Communications

Researchers out of the Austrian Academy of Science have developed a theory that it is possible to create a device that could both send and receive messages at the same time, making two-way communications through a single particle a theoretical possibility.  That particle would be a quantum particle.


Two-way communication is possible with a single quantum particle

Communication is a two-way street. Thanks to quantum mechanics, that adage applies even if you’ve got only one particle to transmit messages with.

Using a single photon, or particle of light, two people can simultaneously send information to one another, scientists report in a new pair of papers. The feat relies on a quirk of quantum mechanics — superposition, the phenomenon through which particles can effectively occupy two places at once.

Sending information via quantum particles is a popular research subject, thanks to the promise of unhackable quantum communication (SN: 12/23/17, p. 27). The new studies specify a previously unidentified twist on that type of technique. “Sometimes you overlook a cool idea, and then it’s just literally right in front of your nose,” says University of Vienna experimental physicist Philip Walther.

Theoretical physicists Flavio Del Santo of the University of Vienna and Borivoje Dakić from the Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences describe the theory behind the procedure in the Feb. 9 Physical Review Letters. Walther, Del Santo, Dakić and colleagues follow up with a demonstration of the technique in a paper posted at on February 14.

Imagine that two people, Alice and Bob, are stationed some distance apart. In standard classical physics, Alice and Bob would each require their own photon to send each other messages simultaneously, with each light particle transmitting a single bit, 0 or 1.

But if Alice and Bob possess a photon that is in a superposition — simultaneously located near Alice and near Bob — both of them can manipulate that photon to encode a 0 or 1, and then send it back to the other. How each manipulates the photon determines which of the two receives the photon in the end. If Alice and Bob put in the same bit — both 0s or both 1s — Alice receives the photon. If their bits don’t match, Bob gets it. Since Alice knows whether she sent a 0 or a 1, she immediately knows whether Bob encoded a 0 or 1, and vice versa.


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