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Quantum key travels record distance

02 Oct 2002

Researchers describe how they smashed the free-space quantum cryptography record in today's issue of Nature.

A German-UK team has set a new world record for free-space quantum cryptography (QC) by transmitting a stream of single photons a distance of 23.4 km. The line-of-sight experiment, performed by researchers from Ludwig Maximillians University, Munich and QinetiQ, UK, took place at night between two mountains in the South German Alps. (Nature 419 450)

The development suggests that optimized optical QC systems may soon have the range to reach a satellite in a near-earth orbit. This would potentially enable a completely secure communications link to be established between any two points on the globe.

"Using slightly bigger telescopes, optimized filters and anti-reflection coatings we expect to be able to build a system which is stable up to 34 dB of loss and capable of maximum ranges exceeding 1600 km, suitable for satellite key upload," John Rarity from QinetiQ told Optics.org. "The main problem now is not loss but pointing and tracking from the ground and from the satellite with sufficient accuracy."

The new distance record is more than double that of previous efforts. Earlier this year a team of scientists from Los Alamos National Laboratory in the US reported that they had transmitted a quantum key over 10 km in the mountains of New Mexico.

In the UK-German trial, single infrared photons at a wavelength of 850 nm were sent between a transmitter (Alice) located on the summit of Zugspitze (altitude 2950 km) and a receiver (Bob) on the summit of Karwendespitze (2244 m). The experiment took place at high altitude and during the night to avoid problems with air turbulence and background light.

•  What is quantum cryptography? QC systems are used to transfer a cryptographic key between two parties (Alice and Bob) with guaranteed security. As single photons are used to transmit the key, it is impossible for a snooper (Eve) intercept the key without being detected. As a result, the parties can then use a successfully transmitted key to encode and decode secure messages with complete confidence.

Oliver Graydon is editor of Optics.org and Opto & Laser Europe magazine.

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