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UK-based boffins have set a new record for quantum key distribution in a move that paves the way towards faster high security communication networks for banks and governments.

Toshiba Research Europe Cambridge hit a sustained secure bit rate exceeding 1 Mbps over 50 km of fibre for the first time, around 100-1000 times faster than anything previously demonstrated for this length of cable. The boffins were able to up the speed of the systems through the introduction of two key innovations: a light detector for high bit rates and a control system that monitored performance and automatically tweaked settings thus avoiding the need for adjustments.  The light detector was the key innovation in getting the speed of transmissions up, as the boffins explain.

The Toshiba QKD system is based on one-way optical propagation and the BB84 protocol using decoy pulses. This protocol has been proven to be unconditionally secure, ie satisfying the most stringent security criterion.

Current QKD systems are limited by the semiconductor devices (avalanche photodiodes) used to detect the single photons. One photon triggers an avalanche of millions of electrons in this semiconductor device which can be sensed by electrical circuitry in the QKD system. The problem in present systems is that some of these avalanche electrons can be trapped in the device and later stimulate a second spurious detection count. As these noise counts cause errors in the key, current detectors must be operated with long dead times to allow the decay of any trapped electrons.

This has limited the clock rate of current QKD systems to around 10 MHz and thus the average secure key bit rate to a few kbit/sec for a 50 km fibre. The Toshiba team has devised a method to detect much weaker avalanches. This strongly reduces the chance for an electron to be trapped, allowing the detector to be operated at much faster rates beyond 2 GHz. As the detector is based on a compact and rugged semiconductor device, it is suitable for real-world applications.

Quantum key exchange allows the everyday use of “one-time pad” encryption which, when properly implemented, offers absolute security. Assumptions about the security of the technique are not dependant on the computing power of an adversary because security of the system relies on quantum effects that mean it is not possible to eavesdrop of conversations without altering the data streams. Such alterations would be detected as errors, immediately alerting the recipient who can then take action to ‘lock down’ the data stream.

One-time pad algorithms works with secret keys of the same length as ciphertext, limiting its use to the military and spies. Higher bit rate technology of the type demonstrated by the Toshiba team would open up the use of the technology to far wider application, including the transmission of video images.

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,” he added.

The results of the demonstration are due to be reported in a forthcoming issue of Applied Physics Letters. The Toshiba team hope to develop their work in order to build metropolitan area networks based on quantum cryptography as part of a Japan-EU collaboration involving the National Institute of Information and Communications Technology (NICT) in Tokyo over the next few years. ®

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