Communications protected with the complete security of quantum cryptography are now possible over an ordinary 100-kilometre fibre optic cable, thanks to sophisticated photon detection equipment developed by UK researchers.

A team from Toshiba Research Europe, based in Cambridge, UK, has developed a quantum photon detector capable of significantly reducing the amount of random noise picked up as the super-secure cryptographic keys are generated. This boosts the fibre optic distance over which quantum cryptography is feasible.

Quantum cryptography guarantees secure communications by harnessing the quantum quirks of photons sent between users. Any attempt to intercept the photons will disturb their quantum state and, because this quantum quality is an integral part of key generation, the disturbance with immediately raise the alarm.

But reducing the amount of random noise picked up by a detector at either end of a fibre link is crucial. If the noise level is too high, the key generation process will fail too many times to be practical. Until now, no device has been able to make it work across such a long optical link.

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• Article: Quantum Cryptography (4 April 2002)