Breakthrough: new record bit rate for quantum key distribution

Published 21 April 2010

Quantum encryption is the ultimate in unbreakable encrypted communication; it is based upon sending encoding single photons (particles of light) along the fiber; the laws of quantum physics dictate that any attempt by an eavesdropper to intercept and measure the photons alters their encoding, meaning that eavesdropping on quantum keys cannot not be detected; the major problem quantum encryption faces is the relatively short distance of encrypted transmissions

The Cambridge Lab of Toshiba Research Europe the other day announced a major breakthrough that will allow ultra-secure encryption of sensitive data sent by banks, hospitals, and government organizations. They have succeeded in demonstrating continuous operation of quantum key distribution (QKD) with a secure bit rate exceeding 1 Megabit/sec over fifty km of fiber for the first time. Averaged over a 24 hour period, this is 100-1000 times higher than anything reported previously for a 50 km link. It was achieved using two innovations developed by the Cambridge team: a novel light detector for high bit rates and a feedback system which maintains a high bit rate at all times and requires no manual set-up or adjustment. The results will be reported in the scientific journal, Applied Physics Letters.

Significantly, the breakthrough will enable the everyday use of “one-time pad” encryption, the only known method that is theoretically perfectly secret.

Although ultra-secure, the application of one-time pad encryption has been restricted in the past as it requires the transmission of very long secret keys — the same length as the data itself. For this reason it has only been used for short messages in situations requiring very high security, for example by the military and security services. Monday’s bit rate breakthrough will extend the application of this ultra-secure communication method for everyday use.

Dr. Andrew Shields, who directs this work at Toshiba Research Europe commented, “Although the feasibility of QKD with megabits per second has been shown in the lab, these experiments lasted only minutes or even seconds at a time and required manual adjustments. To the best of our knowledge this is the first time that continuous operation has been demonstrated at high bit rates. Although much development work remains this advance could allow unconditionally secure communication with significant bandwidths.”

As an example of the new capability afforded by this system, these higher bit rates would allow real time encryption of video using the one-time pad. This is now possible due to the much higher and continuous bit rates that can be delivered with the new technology. Previously it had been possible to encrypt continuous voice data, but not video images.


Toshiba now plans to install a QKD technology demonstrator at the National Institute of Information and Communications Technology (NICT) in Tokyo. Co-ordinator of the Tokyo QKD Network, Dr. Masahide Sasaki, commented: “The secure key rate of 1 Megabit/sec over 50 km has