Quantum cryptography gets closer to reality

A joint research project between Fujitsu and The University of Tokyo has made progress towards realising a viable quantum...

A joint research project between Fujitsu and The University of Tokyo has made progress towards realising a viable quantum cryptography system. Such a system allows parties to share encryption keys via telecommunication networks confident 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 Centre at The University of Tokyo and leader of the research project.

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 disc 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 have not 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 has not been compromised and should it be incorrectly received there is 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 is not easy to avoid but if we use single photons it is possible. So its very important to develop a single photon source."

At present the team has succeeded in generating photons at both 1.3 micron and 1.55 micron wavelengths and verified single photon transmission at the former wavelength. Verification of the latter is one of the upcoming goals for the team. The project hopes to develop a practical single photon generator by 2007 and Arakawa predicts commercial systems based on the technology could be available in five years.

Details of the research are scheduled to be presented at the 27th International Conference on the Physics of Semiconductors, which will begin on 26 July.

Martyn Williams writes for IDG News Service

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