Sequential Attack with Intensity Modulation on the Differential-Phase-Shift Quantum Key Distribution Protocol

In this paper, we discuss the security of the differential-phase-shift quantum key distribution (DPSQKD) protocol by introducing an improved version of the so-called sequential attack, which was originally discussed by Waks et al. Our attack differs from the original form of the sequential attack in that the attacker Eve modulates not only the phases but also the amplitude in the superposition of the single-photon states which she sends to the receiver. Concentrating especially on the "discretized gaussian" intensity modulation, we show that our attack is more effective than the individual attack, which had been the best attack up to present. As a result of this, the recent experiment with communication distance of 100km reported by Diamanti et al. turns out to be insecure. Moreover it can be shown that in a practical experimental setup which is commonly used today, the communication distance achievable by the DPSQKD protocol is less than 95km.