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Quantum keys sent over ordinary fibre connection

Mainstream use within reach, says Cambridge team

Another month, yet another quantum key distribution (QKD) "breakthrough" but perhaps the innovation announced by Toshiba's Cambridge Research Laboratory this week will turn out to be highly significant.

The team headed by Dr Andrew Shields and the University's Engineering Department has found a way to send the incredibly weak light signals used in QKD over conventional fibre optic cables while they are being used for other types of data transmission.

A major problem with QKD - a technology for distributing encryption keys with the certainty that any tampering with these will be detected - is that until now it implied a dedicated end-to-end fibre network of its own.

To all intents and purposes, this made the technology impractical beyond a minuscule number of military applications. Dedicated QKD networks are simply vastly expensive and so commercial use of QKD is non-existent.

Sending a signal comprised of single photons over a fibre connection represented a huge technical challenge. The breakthrough was that the team was able to build a system "sensitive" to this stream without picking up the huge volume of noise created by the other wavelengths of light sharing the connection.

This allowed the scientists to send a QKD signal at a 'secure key rate' of 500kbit/s over a distance of 50km.

"The requirement of separate fibres has greatly restricted the applications of quantum cryptography in the past, as unused fibres are not always available for sending the single photons, and even when they are, can be prohibitively expensive," said Dr Andrew Shields.

"Now we have shown that the single photon and data signals can be sent using different wavelengths on the same fibre."

Shield's team is considered a world leader among a small, select group of researchers looking at the field form around the world. Breakthroughs happen from time to time, usually around the amount of data that can be reliably exchanged over a given distance.

Perhaps the most important breakthrough to date was the announcement in 2010 that his team had found a way to generate the photons that are the information 'ammunition' of a QKD system using simple LEDs.

Although surrounded by the terminology of quantum physics, the invention of the Entangled Light Emitting Diode was really a way to allow QKD systems to function without the need for a lab full of physicists using esoteric equipment.

Along with the latest development, the hope is that by turning QKD into a technology that can be implemented using more conventional equipment it will become affordable for businesses and government.

And why is QKD worth using in the first place? In essence because it adds certainty to the distribution of the encryption keys used to scramble and unscramble secure communications.

These can still be intercepted but the fact of that attack is immediately revealed according to the principles of quantum physics, namely that an observed (i.e intercepted) photon is changed. QKD systems simply detect that this event has taken place.

This certainty hasn't stopped researchers trying to find flaws in the way the equipment used in QKD is set up - the physics is bullet proof but the engineering, as ever, not necessarily as secure.


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