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[] Quantenverschlüsselung erstmals drahtlos versendet,

Quantum cryptography takes to the skies

Nature (vol 419, p 450) 2 October 02

Duncan Graham-Rowe    

Quantum cryptography keys encoded in photons of light have been
transmitted more than 23 kilometres through air, British researchers
have announced. They say the breakthrough is an important step towards a
global communications system that is completely secure.

Earlier in 2002 a Swiss company managed to send quantum keys over 60
kilometres. But this was through optical fibres, which limits the
technology to ground-based transmission. 

"Our experiment paves the way for the development of a secure global
key-distribution network based on optical links to low-Earth-orbit
satellites," says John Rarity, at QinetiQ, the public arm of the UK's
defence research agency. 

Rarity acknowledges that the 23 kilometres is still a long way short of
the 1000 kilometres needed to reach all LEO satellites, but he believes
that improvements in filtering out ambient light could make this
feasible soon. "I think I will have a system design to do that by March
2003," he says.

Another obstacle to be overcome is to engineer signals that, unlike the
visible light used, will not be absorbed by clouds.  

Interception detection 

Keys are cryptographic tools that allow one person to encode a message
and send it securely to another person, who then decodes it with a copy
of the key.

Historically, the challenge has been to find a way for one person to
send the decryption key securely to the other person, i.e. ensuring that
it is not intercepted by a third party. 

One cryptographic technique, called public key cryptography, gets round
this by using mathematical functions that are extremely difficult to
reverse. But public keys are theoretically breakable and Rarity argues
this could happen in the future with the advent of more powerful
computers or new mathematical techniques.

Quantum cryptography guarantees that keys cannot be intercepted without
the sender and receiver knowing by using the quantum properties of
individual photons (or particles) to encode the key - any measurement of
a photon will alter its quantum properties, betraying an interceptor. 

Peak to peak 

Rarity and his colleagues sent their message at night between two
mountain tops in Germany. In their experiments, they polarise individual
photons in opposite orientations to represent the zeros and ones of a
digital number. 

The sender transmits a telescopic, pulsed beam of light to the receiver,
who then uses a photon counter to record the arriving photons and their

However, most of the photons sent are lost, so the receiver must contact
the sender to let them know which bits they actually picked up. An
eavesdropper could tap this communication and discover which bits of the
pulse were to be used for the key, opening a vulnerability in the

But Rarity points out the interceptor would have no way of knowing
whether these bits were zeros or ones. The keys are comprised of
thousands of digits, meaning that cracking the key using brute force
computing would take many times the age of the Universe.

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