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Thursday, 13 December, 2001, 19:59 GMT
Paving the way for 'uncrackable' codes
Optical fibres, EyeWire
Quantum cryptography will use optical fibres
Ivan Noble

The heart of a new light-emitting diode (LED) developed in Cambridge, UK, can be controlled so precisely that it emits just one single photon of light each time it is switched on.

Single photon emitting diode, Toshiba Research Europe Limited
The emitters are around five millionths of a millimetre high
The device could be a key component in quantum cryptography, a code-making technology which, it is hoped, will be uncrackable.

The ability to send single packets of light already exists but this is the first time a device has been built without using lasers.

Its designers say they are optimistic that the device could be produced cheaply and quickly in industrial quantities.

Cool operator

"Quantum cryptography provides a way for two people to form a secret key. The laws of quantum mechanics dictate that it provides a way to guarantee that no-one has intercepted that key," Andrew Shields of Toshiba Research Europe Limited (TREL) told BBC News Online.

Our device... could potentially be produced quite cheaply

Andrew Shields
Dr Shields and colleagues from TREL and the University of Cambridge built the single photon-emitting diode using standard semiconductor manufacturing techniques.

At the moment it only works at extremely low temperatures.

"That can be improved and we know how to do it," said Dr Shields. "Our device is made by photolithography... and could potentially be produced quite cheaply."

Eavesdropper's downfall

The reason security experts are interested in using single photons to carry encoded messages is that they appear to provide the guarantee of secrecy that other technologies lack.

The kind of encoding currently used to protect messages relies on the fact that, without the key, it would take an extremely long time to do all the calculations needed to unscramble a secret message.

But there is no guarantee the key has not been stolen, or that computers will not improve to a point where the time factor is less significant.

With quantum cryptography, the very act of intercepting a single photon on its way down an optical fibre would change the information it was carrying.

The interception would then be apparent to the sender and receiver.

Missing pieces

There are, however, obstacles to the development of this super-secure technology.

"We need the detection technology for single photons," said Dr Shields. "But most of the other elements are there. It uses standard telecoms cables.

"There is attenuation in the fibre so at present fibre lengths are limited to between 50 and 100 kilometres, but it would be useful in cities," he added.

The Cambridge team is proud of the fact that its device is so controllable.

Previous emitters have occasionally let out more than one identical photon at a time, a problem that could have opened the door to an eavesdropper.

"That's a security loophole and our device is going to plug that," said Dr Shields.

The device is described in the journal Science.

See also:

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