NEC extends quantum cryptography range and speed

NEC Corp. researchers have developed a quantum cryptography system with sufficient speed and range to make it commercially viable. It could go on sale in the second half of 2005, the researchers said Thursday.

The system can generate quantum keys at a speed of 100Kbps and transmit them over distances of up to 40 kilometers along commercial fiber optic lines. This combination of speed and distance is a world record, and means the system is suitable for commercial use, according to Kazuo Nakamura, senior manager of NEC's quantum information technology group at the company's Fundamental and Environmental Research Laboratories.

The system, which was first tested successfully in April at the company's System Platforms Research Laboratories in Tamagawa, west of Tokyo, contains several breakthroughs from previous technologies used by the company, according to Akio Tajima, assistant manager at the laboratory.

Quantum cryptography systems allow users to exchange keys with the knowledge that they have not been tampered with during transmission. The systems work by embedding the encryption key on photons. As photons cannot be split, they can only end up in one place, either with the receiver or with an eavesdropper.

NEC has been developing a 'round-trip' quantum cryptography method that has a laser and receiver at one end, and a mirror at the other. Until this April, technical issues with the receiver and mirror meant that the system had not been able to work at high speed over long distances, Tajima said.

With prior NEC systems, the detector worked too slowly to cope with correctly registering the photons hitting it. When photons hit the detector, they are turned into electrons. Because the detector creates an avalanche of electrons for each photon strike, it was necessary to find a way to clear the swarm of electrons out of the device quickly so that it could accurately register the arrival of the next photon. The new detector developed by Tajima's team clears that delay faster, so that the system can work reliably at speeds of 100k bps. That's fast enough to be useful commercially, Tajima said.

NEC has improved the system's mirror. NEC's prior systems used a type of mirror called a Faraday Mirror, a device that reflects light in a 90 degree rotation from the input light. The performance of Faraday Mirrors changes with temperature, which affects efficiency. NEC has changed the technology with the mirror so that it works accurately at temperatures between minus five degrees and 70 degrees Celsius, Tajima said.

Scientists have struggled to develop quantum key systems that are fast enough to work through long enough distances in networks to be commercially viable. Photons tend to get scattered and lost in fiber optic cables. More powerful lasers that are needed to shunt more photons over longer distances tend to cause more noise, which degrades efficiency. NEC's new system includes a conventional laser whose power has been optimized so that it creates less noise, Tajima said.

The combination of technologies means the system is a world-first for speed and distance, said Nakamura. The University of Geneva has achieved quantum transmission over a distance of over 60 kilometers, but at a much lower speed, while a system developed by Japan's National Institute of Advanced Industrial Science and Technology, a major government laboratory, has achieved nearly the same speed as NEC's system, but only at about half the distance, he said.