NTT advances quantum crypto key distribution

NTT Labs has demonstrated experimentally that quantum cryptography keys can be distributed via all-optical network switches.

In what could be a step forward in the development of quantum cryptography systems, a Japanese laboratory has demonstrated that it can send quantum keys through an optical switch designed for normal optical communications.

The system the lab has developed allows the keys to be switched to different paths at data rates and at distances that could prove useful for companies wanting to send keys to different places, and the technology could be commercialised two years from now, the lab said Monday.

Quantum cryptography systems are being developed because they are supposed to be much more secure than today's data communications. They are seen as secure because users who exchange cryptographic keys can tell whether or not the key has been tampered with during transmission.

The systems use photons to carry the keys. The encryption key is encoded on individual photons by setting their polarisation. As photons cannot be split and the information they carry is destroyed when interfered with or measured, they can only end up in one place, with the receiver or the eavesdropper.

But such systems are also difficult to commercialise because they need specialist hardware, including special emitters to produce the streams of photons. The photons can be easily lost in fiber optic cables, and the systems require very sensitive receivers. Such issues limit the data rates and distances over which systems work.

Labs around the world are making progress on such problems and getting higher data rates along longer lengths of cables. But another major issue for quantum cryptography systems has been to develop switches that can route the keys to multiple destinations, so that, for example, banks can use such systems to distribute keys to different data centers without having to have a dedicated transmitter for each.

Conventional switches change the pulses of light in fiber optic cables into electrical signals and then back to light again. This process changes the photons used to carry the keys in quantum cryptography systems, destroying the key.

Switches have been developed that can, however, transfer keys. But the switches announced so far work at very low data rates and the maximum number of different paths has been limited to four, according to Yasuhiro Tokura, group leader of the Quantum Optical State Control Research Group at NTT Basic Research Laboratories.

NTT Basic Research Laboratories, based west of Tokyo, is the research arm of Nippon Telegraph and Telephone.

NTT has taken a switch that it developed to optically switch data to eight different paths for normal transmission and demonstrated that it can use the switch for quantum cryptography purposes so that keys can be switched from a single source to one of up to eight different paths, Tokura said.

In lab tests last year, NTT was able to send the key down a 10 kilometer optical fiber cable and put it through the switch, reliably routing the key through other separate 5 kilometer fiber optic cables, Tokura said.

At the moment the data rate of about 2K bps (bits per second) is not good enough for commercial use. By boosting the sensitivity of the detector used to receive the key, Tokura is confident that the system can work at 10 times the data rate over longer cables so that the system works to a distance of about 30 kilometers.

If the system can achieve these goals, it could be good enough to sell, Tokura said.

The technology could be ready in two years from now. The first target is Japanese customers, but NTT may sell the systems internationally, he said.

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