Toshiba shows practical quantum cryptography

Toshiba Research Europe demonstrated last week what it claims is the world's first reliable automated quantum cryptography system and run it continuously for over a week.

The system, which relies on single photons to transmit an untappable key over standard optical fibres, is capable of delivering thousands of keys a second and can be effective over distances of more than 100km.

Although no price or launch date has been set yet, Toshiba is already in talks with a number of telcos and end users in preparation for commercialisation of the technology -- which offers the possibility of significantly more secure networking.

"We're talking to a number of potential end users at the minute," Dr Andrew Shields, group leader of Toshiba's Cambridge-based Quantum Information Group told ZDNet UK. "We're planning to do some trials in the City of London next year, and are targeting users in the financial sector. We’ve also had some interest from telcos, including MCI with whom we've been running the installed fibre tests."

The system works by transmitting a long stream of photons modulated to represent ones and zeros, most of which are lost along the way. These photons can be modulated in one of two ways through two different kinds of polarisation, but according to Heisenberg's Uncertainty Principle it is impossible to know both the kind of polarisation and the data represented by the photon. The receiver has to assume one to get the other, which it will frequently get wrong.

The receiver picks up and attempts to decode a few out of those that make it, and reports back to the sender which ones it received and decoded thus making up a key that both ends know. Any interceptor can't know what the value of those photons is, because by reading them in transit it will destroy them, and it can't replace them after reading them because it can never know their exact details.

Although Toshiba has been developing special hardware to create and analyse single photon transactions by quantum dots -- effectively artificial atoms integrated with control circuitry -- the current cryptographic equipment uses standard parts, including Peltier-effect cooled detectors operating at very low noise levels. The next generation of equipment is expected to use this new technology.

Toshiba is also looking at ways to increase the range of the systems beyond the limitations of a single fibre -- because a photon can't be intercepted and retransmitted, it's not possible for the technology to incorporate repeaters to overcome the losses in multiple segments. However, says Shields, there is a possibility that repeaters may be created using quantum teleportation -- a new and still experimental effect where the quantum state of a particle can be transmitted across distances without it needing to be fully measured.

Toshiba Research Europe Ltd is part of the European SECOQC project, which is working towards the development of a global network for secure communication using quantum technology.