A breakthrough from Toshiba's Cambridge Research Laboratory means it is now possible to secure ordinary telecom fibres, and existing telecoms networks to allow quantum cryptography to be used on a wider scale.

Working alongside Cambridge University's engineering department, Toshiba said that this is a breakthrough in protecting communication networks from unauthorised snooping.

According to the two teams, they have succeeded in extracting the very weak signals used for quantum cryptography from ordinary telecom fibres transmitting data traffic, meaning that existing telecom networks can now be secured with this ultimate form of encryption.

The Cambridge team achieved this using a detector that is sensitive only for a very brief window (100 millionths of a micro-second) at the expected arrival time of the single photons. The detector thereby responds largely to just the single photon signals and is insensitive to the scattered light caused by the data signals. This allows the weak single photon signals to be recovered from the fibre.

Using this technique the Cambridge team has successfully implemented quantum cryptography on ordinary telecom fibres, while simultaneously transmitting data at 1Gbps in both directions. A secure key rate over 500Kbps for 50km of fibre was demonstrated, this was around 50,000 times higher than the previous best value for this fibre length.

Its report claimed that until now, it has been necessary to send single photons through a dedicated fibre that is distinct from the fibres carrying the ordinary data signals in the network.

As the data signals are much more intense, one bit of data is carried by over one million photons, the disparity in the intensity of the signals means that scattered light caused by the data signals would contaminate and overwhelm the single photon signals if sent along the same fibre.

Dr Andrew Shields, assistant managing director of Toshiba Research Europe, said: “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. Now we have shown that the single photon and data signals can be sent using different wavelengths on the same fibre.”