Quantum Cryptography Hits the Fast Lane

19 April 2010 6:16 pm

Whether for online bills or military secrets, encryption schemes help keep digital communication secure. In recent years, physicists and engineers have been developing methods that transmit uncrackable encode messages in individual particles of light, or photons. Now, one team has taken such quantum cryptography a long step forward by demonstrating a system that’s fast enough to encrypt a video transmission. “From the applications point of view, it’s very important,” says Hoi-Kwong Lo, a physicist at the University of Toronto in Canada.

A digital message consists of a long string of zeros and ones and can be encrypted in many ways. For example, each bit can be added with one from a stream of random zeros and ones called the key. Adding the key once scrambles the message; adding it a second time unscrambles it. So long as two the people sharing the secret, say, "Alice" and "Bob," do not reuse the key, this “one-time pad” method is uncrackable. However, Alice must somehow pass the key to Bob without anybody intercepting it.

Messages on the Internet are encoded in another way, using so-called public key encryption. In a nutshell, a message is scrambled by running it through a mathematical function that's easy to run forward but very difficult to run backward. However, there’s no guarantee that, given enough computing power, a hacker won’t find a way to crack such a scheme.

So-called quantum key distribution (QKD) would ensure absolute security essentially by letting Alice and Bob pass the key for one-time pad encryption right under the nose of an eavesdropper, "Eve." Researchers have developed several protocols, all of which exploit a crucial feature of quantum mechanics: It’s generally not possible to measure the state of a particle like a photon without altering it. That means that if Alice encodes the key in the photons in the right way, Eve won’t be able to intercept and measure the photons without revealing her presence to Alice and Bob.

Researchers have been developing such systems for more than a decade, and in 2008 they connected six of them together to form a rudimentary quantum network in Vienna. Now, the electronics manufacturer Toshiba, one of the participants in that event, has boosted the rate a which its system can distribute bits of key across a 50-kilometer fiber to a megabit per second, up from a few kilobits per second, says Andrew Shields, an applied physicist and assistant managing director at Toshiba Research Europe Ltd. in Cambridge, United Kingdom. The team has also shown that the system can run continuously for 36 hours, much longer than the few minutes previously achieved at a megabit-per-second rate, it reports today in Applied Physics Letters.

Key to running faster is a better photon detector, Shields says. The Toshiba systems uses devices known as semiconductor avalanche photodiodes, in which a photon hits a bit of semiconductor to trigger an “avalanche” of electric charge. It takes time for that avalanche to build and pass, which limits the detector’s rate. New photodiodes can sense smaller avalanches and, hence, run faster, Shields says. To keep the system running for hours at a time, the Toshiba team also implemented a feedback system to stretch certain meters-long optical fibers within Alice's and Bob’s detectors by a few nanometers, thus keeping the ratio of those lengths constant to a few parts in a billion. Without such stabilization, key distribution would have to stop every few minutes to allow the equipment to recalibrate itself.

The new system will get a tryout in October as part of the Tokyo QKD Network, in which researchers will use various systems to connect two buildings belonging to Japan’s National Institute of Information and Communications Technology and three other buildings. Masahide Sasaki, a physicist at the institute, says that the ability to handle video conferencing is key to high-end applications such as governmental communications. Previous systems could handle only voice conferencing, he says.

Researchers still have a way to go to reach the ultimate goal of a completely quantum-mechanical network, however. In the current systems, a secret message must essentially be unscrambled and rescrambled at each node on a network, which are then potentially vulnerable to attack. Ultimately, developers hope to be able to relay a subtle quantum-mechanical connection called entanglement from Alice to Bob across intermediate nodes so that only Bob would ever be able to decode Alice’s message. Such a network requires devices called quantum repeaters, which have yet to be developed.

Without quantum repeaters the current system protects only the optical fibers between nodes. Still, those are the most exposed part of any network and are increasingly vulnerable to attack. “Ironically,” Sasaki says, “advances [made for] QKD, such as new photon detectors with very low noise, low-loss optical circuits, et cetera., could make [such attacks] possible.”

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