Quantum key travels record distance
Oct 3, 2002
A team of researchers from Germany and the UK has transmitted a key for quantum cryptography 23.4 km through the atmosphere, more than twice as far as the previous best distance. The experiment, performed by researchers from the Ludwig-Maximilian University in Munich and QinetiQ in the UK, took place at night between two mountains in the South German Alps. The result suggests that quantum-encoded transmissions could soon be established to and from low-orbiting satellites, enabling completely secure communications between any two points on Earth.
Quantum encryption keys are based on the laws of quantum mechanics and unlike other encryption codes based on mathematical number theory, they are impossible to intercept or break. As a result, two parties can use a successfully transmitted key to encode and decode secure messages with complete confidence.
Earlier this year a team of scientists from the 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 latest trial, single infrared photons at a wavelength of 850 nm were sent between a group of lasers located on the summit of Zugspitze (at an altitude of 2950 m) and a telescope on the summit of Karwendespitze (at an altitude of 2244 m). The experiment was carried out at high altitude to avoid problems with air turbulence and took place at night in order to minimise the effects of background light.
“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,” says John Rarity of QinetiQ. “The main problem now is not loss but pointing and tracking from the ground and from the satellite with sufficient accuracy.”
About the author
Oliver Graydon is editor of Optics.org and Opto & Laser Europe magazine