Stability of phase-modulated quantum key distribution systems

Abstract

Phase drift and random fluctuation of interference visibility in double unbalanced M-Z QKD system are observed and distinguished. It has been found that the interference visibilities are influenced deeply by the disturbance of transmission fiber. Theory analysis shows that the fluctuation is derived from the envioronmental disturbance on polarization characteristic of fiber, especially including transmission fiber. Finally, stability conditions of one-way anti-disturbed M-Z QKD system are given out, which provides a theoretical guide in pragmatic anti-disturbed QKD. On the other hand, optical fibres are appropriate for communication on the ground without influence of weather and atmospheric pollution. However, it is a calamity to use polarization as basis for encoding quantum information due to significant birefringence. Hence, phase coding is widely used in fibre-optic QKD-prototype systems.The most typical fiber QKD prototype was designed in Ref. [11]. In that prototype, two photon pulses pass through the same transmission fiber (quantum channel), and the same disturbance, from transmission processes, is expected to be counteracted on Bob's side. But in fact, this prototype gives us a bad systemic stability. In order to improve the stability, two groups [12, 13] developed a new prototype independently, in which the two pulses transmit a round trip with a Faraday reflection in mid-course. Unfortunately, this prototype leaves a chance to Eavesdroppers in the key exchange. Eve can send Trojan-horse photons to tail signal photons thus pass in and out of Bob's secure office, collecting Bob's coded information without being discovered [14]. Furthermore, any go-and-return QKD protocol [15] is essentially unsecured, and true secure QKD should be based on photons propagating along one way. Therefore, for practical use of QKD, it is very important to investigate the stability of 2 double M-Z interferometers. This paper investigates the stability in experiments and establishes a theoretical model. Two kinds of perturbation motion, i.e., phase drift and random fluctuation of interference visibility, are observed and discussed. Formerly, it was ever believed that the dominant disturbance arises from different environments of interferometers on Alice and Bob's sides, and the disturbance in quantum channel is equivalent to both pulses (with ns-scale time interval) which can be counteracted in the final interference. But our experiments show that interference visibility depends intensively on length of transmission fiber (L) and systemic stability is also influenced by the way disturbance. Theoretical analysis, here, deduces the conditions for systemic stabilization of double unbalanced M-Z interferometers. It will offer important reference for stable one-way QKD systems. Typical QKD prototype is described in Fig. 1. Two uniform unbalanced M-Z interferometers were built with common single mode fiber (SMF-28), which is respectively the coder of Alice or Bob. Quantum channel between Alice and Bob and beamsplitters (50/...

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