U. of Tokyo, Fujitsu advance towards quantum cryptography

TOKYO -- A joint research project of Fujitsu Ltd. and The University of Tokyo has made progress towards realizing a viable quantum cryptography system. Such a system allows parties to share encryption keys via telecommunication networks with full confidence that they have not been compromised en route.

The team has succeeded in generating and detecting a single photon at wavelengths useful for telecommunications, said Yasuhiko Arakawa, director of the Nanoelectronics Collaborative Research Center at The University of Tokyo and leader of the research project, in an interview on Tuesday.

The reliable generation and detection of single photons is vital if quantum cryptography systems are to leave the laboratory and enter practical use and the team has managed this through the development of a new photon generator.

Quantum cryptography is based on the physical properties of photons.

If two parties want to exchange encrypted data they need to share the electronic key that will be used to encode the data. The data is encoded with a corresponding private key, so using the genuine public key is vital. Should a fake key be substituted for the real one the data could be read by a third party rather than the intended recipient. Sharing of keys across telecommunication networks can expose the key to tampering so many users exchange keys offline via physical media, such as a floppy disk or CD-ROM.

Under public key infrastructure (PKI) schemes, public keys are certified as being genuine by a certificate authority.

Quantum cryptography systems allow users to exchange keys across networks with the knowledge that they haven't been tampered with during transmission.

This is because each data bit of the key is encoded onto individual photons of light. A photon cannot be split so it can only end up in one place: with the intended receiver or with an eavesdropper. Should a key be completely received the recipient can be sure it hasn't been compromised and should it be incorrectly received there's a chance that it has been intercepted and so a new key can be issued.

Thus, for a viable quantum cryptography system it must be possible to reliably generate a single photon. If two or more photons are generated the key's security is gone.

"We have to avoid the key being received by other people," Arakawa said. "It's not easy to avoid but if we use single photons it's possible. So its very important to develop a single photon source."

Until now most experiments involving quantum cryptography have used lasers as their photon source and these haven't proven to be completely reliable generators of single photons.

"By reducing the output power of the laser we can create one photon sometimes, however it is impossible to control accurately the number of photons," Arakawa said. Reducing the laser power also means the overall transmission speed is slowed.

Arakawa's team has developed a new generator based on materials developed by Fujitsu and Japan's National Institute for Materials Science. The material is embedded with quantum dots, which are like tiny holes into which individual electrons can enter and a photon be produced.