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Towards secure three-party quantum communications

by Kurt Kleiner Apr 4 / 14

One of the most puzzling results of quantum mechanics is that two particles can be “entangled” with one another no matter how far apart they are. A new experiment now strongly establishes that the same is true for a system of three entangled particles.

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A new experiment demonstrated three-way quantum entanglement. The image above shows the physical location of the experiment, and the image below is a schematic representation. Image courtesy of Nature Photonics.

Although not unexpected – quantum theory holds that any number of particles can be entangled – the work by CIFAR researchers and their colleagues does close a loophole and provide the most solid evidence yet of three-way quantum nonlocality.

“For people who believe in quantum mechanics and the mysteries it imposes on us, this isn’t surprising,” says CIFAR Fellow Thomas Jennewein (Institute for Quantum Computing, University of Waterloo) of the program in Quantum Information Science. “But it pushes the boundaries further, letting us see how dramatic the quantum world actually is.”

Jennewein was a co-author on the paper, which appeared in Nature Photonics. Others who worked on the experiment included QIS Program Director Raymond Laflamme (Institute for Quantum Computing, University of Waterloo), QIS Fellow Gregor Weihs (University of Innsbruck) and CIFAR Global Scholar Krister Shalm (National Institutes of Standards and Technology).

Previous experiments have shown that pairs of photons, for instance, can be entangled so that their polarizations are correlated. In other words, if you measure one photon, a related measurement on the entangled photon will always be correlated with the first one.

This is true even if the measurement settings are chosen randomly, and so rapidly that any signal going at the speed of light would not have time to travel between the two photons. Somehow, each particle automatically “knows” about the state of the other.

This result is so counter-intuitive that Albert Einstein didn’t believe it, and coined the term “spooky action at a distance” to describe it. But it’s been shown many times to happen between entangled photon pairs.

However, this is the first experiment to show the effect on three entangled photons and at the same time to close the so-called “locality loophole.”

The experiment required three entangled particles to be generated and sent to three different locations about 700 meters away, and then measured within three nanoseconds of one another.

The distance and the timing meant that no signal could have traveled among the particles between the time of one measurement and the time of another.

In addition to proving one of the fundamentals of quantum mechanics, the result could be useful in the future for secure quantum communications, Jennewein says. Researchers are developing systems in which pairs of entangled photons can be used to ensure secure communications between two parties. The experiment could extend that work into multi-party communications.