Breakthrough in 30 years of quantum cryptography ー From ‘detecting’ to ‘preventing’ the eavesdropping-

Quantum cryptography exploits the properties of quantum mechanics to achieve highly secure communication without making any assumption on the computational power and technology used by an eavesdropper. The conventional methods of quantum cryptography are all based on the uncertainty principle, which dictates that any eavesdropping attempt causes disturbance on the signal. Hence, through monitoring of the amount of noises in the communication channel, one is able to estimate the amount of the information leak. Professor Masato Koashi and Project Researcher Toshihiko Sasaki from the Photon Science Center in the University of Tokyo, in collaboration with Professor Yoshihisa Yamamoto from National Institute of Informatics, have proposed a method of quantum cryptography working on an entirely different principle. Their method was proved to achieve the secure communication without monitoring the amount of noises in the communication channel. It can be basically implemented by a combination of a laser source and an interferometer, and there is no way for an eavesdropper to acquire information beyond a fixed small amount. Their discovery is a paradigm shift from 'detecting the eavesdropping' to 'preventing the eavesdropping.' In comparison to the conventional methods of quantum cryptography, the new method is free from the cost of monitoring the channels, and it enables us to achieve secret communication when a communication channel suffers from a much higher noise. Their results have revealed an essentially new way of exploiting the properties of quantum mechanics, which had been unnoticed over the 30 years’ history of quantum cryptography, and hence the concept is expected to have a broad application beyond cryptography.