October 3, 2002	Thursday	Rajab 25, 1423

PARIS, Oct 2: Quantum cryptography, a technique of producing secret messages that are reputedly uncrackable, may soon be used by orbiting communications satellites thanks to experiments by British and German researchers.

The traditional weakness of sending encoded messages is eavesdropping. Quantum cryptography gets around this by sending an encoded message and, separately, a key to decode it, which are transmitted in pulses of individual light particles called photons.

By the nature of quantum mechanics, if a single photon is intercepted en route, that changes the state of the information package as it arrives at the other end.

That is a telltale for the legitimate recipient that his message has been tampered with — the same as if someone received a letter that had been clumsily opened and then resealed, leaving traces of glue and fingerprints on the envelope.

The problem with quantum codes, though, has been how to send messages over long distances.

Data is of course already sent by laser light down fibre-optic networks. But this technique is unsuitable for quantum cryptography, for the laser signal has to be boosted every 10 kilometres, which causes the quantum state of the key to be rearranged.

Researchers from QinetiQ, the commercial arm of the British military research agency, and from Munich’s Ludwig-Maximilian University say they have now demonstrated that it is possible to send a quantum-encoded message through the air.

Reporting in Thursday’s issue of Nature, the British science weekly, they say they successfully transmitted packages across 23.4 kilometers between mountains in the German Alps.

A laser transmitter was set up at the top of the 2,950-metre Zugspitze, and sent out pulses to a receiver, a 25-centimetre shop-bought telescope, positioned on line of sight on another peak, the 2,244-metre Westlichekarwendespitze.

With some adjustments to amplify the signal, it should be possible to send keys to satellites in near-Earth orbit, at an altitude of 500-1,000 kilometres, the scientists say.

Quantum codes have obvious uses for military and government communications.

The big question, though, is whether they should be allowed to enter the commercial domain, where they could be used by organised crime and terrorism to thwart eavesdropping by police.—AFP