Quantum cryptography demonstrated over existing fiber lines

Posted by Kate Taylor

Unbreakable quantum cryptography could become a mainstream reality, following the discovery that it's possible to use it over today's fiber networks.

Toshiba's Cambridge Research Laboratory, together with the Cambridge University Engineering Department, say they have succeeded in extracting the very weak signals used for quantum cryptography from ordinary telecom fibres transmitting data traffic.

Quantum cryptography relies upon encoding each bit of the digital key upon a single photon - and quantum mechanics means that if a hacker intercepts a photon, they will inevitably disturb its encoding in a way that can be detected.

Until now, it's been necessary to send the single photons through a dedicated fibre quite separate from the fibres carrying the ordinary data signals in the network.  

This is because the data signals are much more intense than the single photon signals and would overwhelm them if sent down the same fibre.  

"The requirement of separate fibres has greatly restricted the applications of quantum cryptography in the past, as unused fibres are not always available for sending the single photons, and even when they are, can be prohibitively expensive," says Dr Andrew Shields, assistant managing director at Toshiba Research.

"Now we have shown that the single photon and data signals can be sent using different wavelengths on the same fibre."

The Cambridge team used a detector that is sensitive for only 100 millionths of a micro-second at the expected arrival time of the single photons. This means it's largely unaffected bythe scattered light caused by the data signals, allowing the weak single photon signals to be recovered from the fibre.

The team's put its ideas into practice, successfully implementing quantum cryptography on ordinary telecom fibres while simultaneously transmitting data at 1Gbps in both directions.  They managed a secure key rate of over 500Kbps over 50 km of fibre - about 50000 times higher than the previous best value for this fibre length.