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Quantum Cryptography Hits the Fast Lane

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Quantum key distribution ensures that no spy intercepts the stream of zeros and ones used to encode secrets messages. Researchers can now distribute such secret bits fast enough to encode video.

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Toshiba researchers have developed a quantum cryptography system they say is fast enough to encrypt a video transmission. The system can send bits of key at a megabit per second across a 50-kilometer fiber, says Toshiba's Andrew Shields. The researchers also have demonstrated that the system can run continuously for 36 hours. The key to running faster is a better photon detector, Shields says.

The Toshiba system use devices called semiconductor avalanche photodiodes, in which a photon hits a bit piece of semiconductor to trigger an "avalanche" of electric charge. New photodioes can detect smaller avalanches and run faster, according to Shields. The researchers used a feedback system to stretch certain optical fibers by a few nanometers, which keeps the ratio of those lengths constant and enables the system to run for hours at a time. Without such stabilization, key distribution would have to stop every few minutes to allow the equipment to recalibrate itself.

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