Quantum Cryptography Goes Mainstream: CDT Research featured in NY Times

A breakthrough achieved by Cambridge CDT student Ketaki Patel and researchers including Professor Richard Penty in collaboration with Toshiba Laboratories Europe allows ordinary telecom networks to be secured from eavesdropping. It has received widespread news coverage including in the New York Times.


The Cambridge Research Laboratory of Toshiba Research Europe Ltd, working in collaboration with academic staff and students from the Cambridge University Engineering Department linked to the Centre for Photonic Systems Development have announced a breakthrough in protecting communication networks from unauthorized snooping.  They have succeeded in extracting the very weak signals used for quantum cryptography from ordinary telecom fibres transmitting data traffic.  It means that existing telecom networks can now be secured with this ultimate form of encryption.

Quantum cryptography can be used to distribute the secret digital keys important for protecting our personal data, such as bank statements, health records, and digital identity.  Its security relies upon encoding each bit of the digital key upon a single photon (particle of light).  If a hacker intercepts the single photons, they will unavoidably disturb their encoding in a way that can be detected.  This allows eavesdropping on the network to be directly monitored.

Up until now it has been necessary to send the single photons through a dedicated fibre that is distinct from the fibres carrying the ordinary data signals in the network.  The data signals are much more intense than the single photon signals used for quantum cryptography: in fact one bit of data is carried by over 1 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, Toshiba Research Europe Ltd says: “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 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 1 Gbit/s in both directions.  They demonstrated a secure key rate over 500 kbit/sec for 50 km of fibre, about 50000 times higher than the previous best value for this fibre length.  The breakthrough will be reported [1] in the scientific journal, Physical Review X. You can view the paper here.

You can view coverage of the breakthrough in the New York Times here.