Toshiba Research Europe Ltd., Cambridge Research Laboratory

Quantum Information Group, World Leading Performance

Toshiba has extended the range and bit rate of quantum key distribution systems. They were the first to report quantum key distribution over a single link of standard optical fibre exceeding 100 km in length. Subsequently they have extended the distance for full key distribution to 122km (see reference below), and the distance for single photon interference to 165km - both world records.

The records were achieved by using an architecture, in which the photons travel a one-way path along the fibre, thus avoiding problems associated with the backscattering of bright counter propagating pulses in send-and-return type set-ups. Also important for achieving key distribution over the longest possible distances was the development of single photon detectors with very low dark count rates based on InGaAs avalanche photodiodes.

These developments allow quantum keys to be distributed over a single link up to 120km in length, long enough for many applications in Metropolitan Area Networks. Longer distances can be served by a network of secure routers. Toshiba's system extends the maximum possible distance between such routers.

Further Reading

	Type	Title and author(s)	Source
1	Technical	Quantum key distribution over 122km of standard telecom fibre by C Gobby, Z Yuan and A J Shields	Applied Physics Letters 84, 3762 (2004)
2	Technical	Unconditionally secure key distribution over 50 km of standard telecom fibre by C Gobby, Z Yuan and A J Shields	Electronics Letters 40, 1603 (2004)