New Record for Quantum Cryptography

Researchers take a big step toward their goal of spy-proof communications via satellites.

European scientists have broken a distance record for sending quantum information from one place to another, paving the way for a system that relies on the laws of physics to provide communications that can't be tapped. If they can extend the reach of their signal a little further, they'll be able to use satellites to send perfectly secure data around the world.

Long-distance call: Entangled photons were sent 144 kilometers from a light source on La Palma to a receiver on Tenerife (top) housed in a local observatory (bottom). Credit: Institute for Experimental Physics, University of Vienna

The team used principles of quantum mechanics to create an encryption key in two locations simultaneously: one in a lab on La Palma, in the Canary Islands, and the second in an observatory on the neighboring island of Tenerife, 144 kilometers away. Such an encryption key can be used to encode data that only the sender and the receiver can decode.

"We want to see whether it is possible at all to establish worldwide quantum communication, worldwide quantum cryptography," says Anton Zeilinger, a professor of physics at the Institute for Experimental Physics at the University of Vienna, Austria. His team, along with a team led by Harald Weinfurter of the Max Planck Institute of Quantum Optics, in Garching, Germany, published its results online on June 3 in the journal Nature Physics.

To create the key, the team first had to create pairs of entangled photons. Entanglement, which Albert Einstein called "spooky action at a distance," means that the fate of one photon is tied up with the fate of the other. Measuring any quantum mechanical property of one photon automatically changes that same property in its entangled partner, no matter the distance between them.

In this case, the team measured polarization. Light can be polarized in any direction; it's a measure of which direction the light waves are fluctuating in--horizontal or vertical, for example. Researchers created entangled pairs of photons by firing a powerful laser beam through a crystal. For each photon that went in, two weaker, entangled photons came out. The researchers bounced one half of each pair off a mirror to a local light detector on La Palma. They sent the other photon through a lens and out across the water, where a telescope on Tenerife caught it and sent it to a second light detector.

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"I have these two photons, and if I measure them on both ends and I ask them, 'Are you horizontally or vertically polarized?'--a binary choice--they will give a random answer," says Zeilinger. "But because of the entanglement, both will give the same answer. On both sides you get a zero or on both sides you get a one."

Every time the detectors registered a photon and measured its polarization, that counted as a bit. A photon polarized in one direction was a one, and a photon polarized in the opposite direction was a zero. Add enough bits together, and you get an encryption key. And it's impossible to steal that key without the users' knowing about it. If someone were to intercept the flying photons , he could measure them himself, then send them on to the receiver. But the act of measuring them would change their quantum mechanical properties, so he'd be immediately exposed.