Cryptology

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 fastest known cryptographic system based on transmission of single photons---the smallest pulses of light---has been demonstrated by a team at the Commerce Department's National Institute of Standards and Technology (NIST). The transmissions cannot be intercepted without detection, so that messages encrypted with the system can be kept secret.

The NIST "quantum key distribution" (QKD) system transmits a stream of individual photons to generate a verifiably secret key--a random series of digital bits, each representing 0 or 1, used to encrypt messages--at a rate of 1 million bits per second (bps). This rate is about 100 times faster than previously reported systems of this type.

The demonstration, described in the May 3 issue of Optics Express,* is the first major reported result from a new NIST testbed built to demonstrate quantum communications technologies and cryptographic key distribution.

It has been well-known since the pioneering work of Claude Shannon in the 1940s that a message transmitted with optimal efficiency over a channel of limited bandwidth is indistinguishable from random noise to a receiver who is unfamiliar with the language in which the message is written. In this letter we demonstrate an equivalent result about electromagnetic transmissions. We show that when electromagnetic radiation is used as the transmission medium, the most information-efficient format for a given message is indistinguishable from black-body radiation to a receiver who is unfamiliar with that format. The characteristic temperature of the radiation is set by the amount of energy used to make the transmission. If information is not encoded in the direction of the radiation, but only its timing, energy or polorization, then the most efficient format has the form of a one-dimensional black-body spectrum which is easily distinguished from the three-dimensional case.