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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).

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.

“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.

Encryption keys used today rely on the belief that it takes huge computing resources to break them, says Jeffrey Shapiro, of MIT’s Optical and Quantum Communications Group. But if someone invents a vastly more powerful quantum computer, that advantage would be lost. In addition, the random sequences of numbers generated to make today’s encryption keys aren’t truly random. They’re generated by mathematical operations, and a smart code breaker might be able to figure out the algorithm being used to generate them. Quantum bits, on the other hand, are completely unpredictable, so the keys based on them should be unbreakable. That’s appealing to businesses that want to send financial data securely, as well as to governments, which have all sorts of sensitive communications. “We all know that the security of data is one of the essential issues these days,” says Zeilinger.

“I think it’s wonderful work,” Shapiro says of the European group’s paper. “The impressive thing about this is they’ve done it over such a long distance.”

The best that researchers had previously done was to detect entangled photons across distances of about 10 kilometers. To improve on that, Zeilinger’s team switched to a laser that emits light in pulses instead of a continuous beam. The pulsed laser only has a repetition rate of 249 megahertz–far slower than the 10 gigahertz lasers commonly used in optical communications networks, which limits how much of a signal can be sent in a given time period. The pulsed laser is also not quite as good as the continuous one at producing entanglement. But it’s close, and it gave the team members much more control over when they were producing photons, which helped them separate the photons they wanted from stray light at the detector, so they could read the signal more reliably. The researchers also had to deal with atmospheric turbulence that distorts the photons’ path. They used an automated system that continually adjusted the alignment of the telescope to take care of that, although the light beam still wandered over the detector somewhat.

The hope, Zeilinger says, is to improve the lasers and detectors enough that such free-space links work between ground stations and satellites, so that encryption keys can be sent from anyplace on Earth to any other. As most communications satellites orbit at heights of 300 to 500 kilometers, “with our 144 kilometers, we are getting there,” he says.

The fact that the team covered that distance in free space “certainly is very significant,” says Prem Kumar, director of the Center for Photonic Communication and Computing at Northwestern University. He has sent entangled photons over optical fiber, which is fine for short distances, he says. But because fiber absorbs photons, it’s not practical for more than 100 to 200 kilometers, which wouldn’t allow for worldwide distribution.

The researchers are part of a European consortium of about 20 groups, called SECOQC, working on secure communications based on quantum cryptography. The consortium aims to test a secure system in Europe sometime next year.

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