How One Quantum Particle Can Send Messages To Itself

Credit:  By Sophie Weiner  Feb 23, 2018

          

Quantum computing promises a future where technology is exponentially faster, more efficient, and more secure. But the mechanics of how it will work are still being figured out. Now, scientists from the University of Vienna and the Australian Institute for Quantum Optics and Quantum Information have demonstrated one possibility for quantum communication that was previously unknown: using one photon can act as both a sender and receiver of information.

A paper published this month by Flavio Del Santo, Borivoje Dakić, and Philip Walther in Physical Review Letters, and a follow up demonstration posted on arXiv.org explains how. The technique relies on quantum superposition—the idea that unobserved quantum particles can be in more than one place at once.

The researchers demonstrate that if two people, Alice and Bob (the usual names for imaginary subjects of quantum computation experiments), are at a distance from each other, they can use just one photon to communicate. This is how it works: Alice and Bob are in control of a photon which is in a superposition. Either Alice or Bob can manipulate the photon to send a 0 or a 1 to the other. If they both put either a 0 or a 1, Alice gets the photon. If they put in different bits, then Bob gets it. Since Alice knows whether she put in a 0 or 1, she can deduce that Bob input the opposite bit.

Science News describes how this theory was tested:

To show that such communication is possible, Walther and colleagues sent single photons through an arrangement of mirrors and other optical devices. The setup put the photon in a superposition, sending it simultaneously to two stations that represented Alice and Bob.

By changing the phase of the light’s electromagnetic wave — shifting where the troughs and peaks of the wave fell — the researchers encoded the photon with a 0 or 1 at each station. Then, at each station, the photon — still in limbo between Alice and Bob — was sent to the opposite station. Along the way, the photon interacted with itself, interfering like water ripples combining to amplify their strength or cancel out. That interference determined whether the final photon was detected at Alice’s station or Bob’s.

There’s no reason why this technique wasn’t discovered earlier—scientists just hadn’t thought of it yet. “Sometimes you overlook a cool idea, and then it’s just literally right in front of your nose,” Walther told Science News. It’s just another demonstration of the magic of quantum physics.

Source: Physical Review Letters, arXiv.org via Science News