Researchers use regular fibre for quantum cryptography
1 min read
Toshiba's Cambridge Research Laboratory, along with Cambridge University's Engineering Department, has managed to extract the very weak signals used for quantum cryptography from ordinary telecom fibres transmitting data traffic. The breakthrough means existing telecom networks can now be secured with this form of encryption.
Until now, it has been necessary to send single photons through a dedicated fibre because regular data signals are more intense than the single photon signals used for quantum cryptography: one bit of data is carried by more than 1million photons. This disparity in intensity means light scattered by the data signals would 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."
The Cambridge team used a detector that is sensitive only for a very brief window – 100fs – at the expected arrival time of the single photon. The detector thereby responds largely to single photon signals and is insensitive to the scattered light from data signals. This allows the weak single photon signals to be recovered.
Using this technique, the Cambridge team has successfully implemented quantum cryptography on ordinary telecom fibres while simultaneously transmitting data at 1Gbit/s in both directions. They demonstrated a secure key rate of more than 500kbit/s for 50km of fibre – about 50,000 times higher than the previous best value for this fibre length.
