Abstract

In this paper so-called 'large pulse attack' is investigated. This attack is one of the possible methods of conventional optical eavesdropping, a new strategy of eavesdropping on quantum cryptosystems, which eliminates the need of immediate interaction with transmitted quantum states. It allows the eavesdropper to avoid inducing transmission errors that disclose her presence to the legal users. As an object of the eavesdropping, phase-state fibre optic schemes are considered. With large pulse attack, settings of transmitting and/ or receiving apparatus are interrogated by external high-power light pulses. Applicability conditions of this method are given. Type and amount of information learned by the eavesdropper is estimated, depending on parameters of the interrogating pulse and apparatus. An experimental set-up for an eavesdropping experiment is proposed and results of successful preliminary measurements are presented. It is concluded that additional protection is necessary for currently implemented quantum key distribution systems. The paper suggests several security measures against this kind of attack.