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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