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UK crypto boffins turbo-charge quantum cryptography

Photodiodes get go-faster stripes

By John Leyden, 9 Oct 2008

Boffins at Toshiba’s Cambridge Research Labs have developed a technique for turbo-charging the speed of quantum cryptography transmissions.

Quantum key distribution (QKD) systems currently operate at dial-up rates of less than 10 kbps for 20km, making them suitable only for point-to-point links. The Cambridge researchers have developed a technique for boosting these speeds by a factor of a hundred, increasing rates of transmission to above 1Mbps.

Crypto researchers have been able to boost transmission rates from the dial-up to broadband range. They used a single photon detector developed by Toshiba, which can work 100-times faster than earlier devices. Toshiba explains:

Current QKD systems are limited by the semiconductor devices (avalanche photodiodes) used to detect the single photons. One photon triggers an avalanche of millions of electrons in this semiconductor device which can be sensed by electrical circuitry in the QKD system. The problem in present systems is that some of these avalanche electrons can be trapped in the device and later stimulate a second spurious detection count. As these noise counts cause errors in the key, current detectors must be operated with long dead times to allow the decay of any trapped electrons. This limits the clock rate of current QKD systems to around 10 MHz and thus the secure key bit rate to under 10kbps for a 20 km fibre.

The Toshiba team has devised a method to detect much weaker electron avalanches. This strongly reduces the chance for an electron to be trapped, allowing the detector to be operated at much faster rates. Using this method Toshiba has increased the clock rate of their QKD system to 1.036 GHz, approaching the value used in conventional optical communications. This allows a raw bit rate of 9Mbps over 20 km of fibre, which in turn provides 1.02 Mbps of secure key. This is the first time that a secure bit rate exceeding 1Mbps has been achieved.

Although Toshiba's advance offers the possibility of creating kickass quantum crypto networks that support multiple users, the technology is unlikely to be commercially available for some time. Toshiba researchers have only demonstrated the advance in the lab and are yet to build even a fully functional prototype.

Nonetheless Toshiba said its technique allows a form of encryption that is theoretically unhackable for optical fibre networks connecting far more users than was previously possible. More information about Toshiba's research in quantum crypto can be found here.

News of Toshiba's research comes alongside the launch of a quantum cryptography network in Vienna, Austria, as part of a collaboration between crypto boffins across Europe on the SECOQC project. This network is based in part on older, lower-speed technology from Toshiba, which makes it unable to support more than eight users on a quantum cryptography network at any time.

Quantum cryptography systems are designed to allow the exchange of secret keys which can then be used in one-time pads, creating a theoretically unhackable system for the secure exchange and authentication of messages. The polarisation of individual light photons is used to signify each bit of a key.

The rules of quantum mechanics mean that any eavesdropper who intercepts this stream of photons will be unable to simply relay the data stream intact. Because of this effect, any attempt to eavesdrop on a quantum cryptography key would come across as a corrupt transmission across a unacceptable noisy link. Such keys would be dropped and transmission repeated until exchanges without interception take place. ®

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