Research On Security Of Key Distribution In Quantum Private Communication Systems

Quantum key distribution(QKD) is cruc ial part of quantum private communication system, which can make legal users share an identical secure key bit string. However, there are always some imperfections in practical QKD systems, especially the security loopholes induced by some important devices, which can be used by a potential eavesdropper to compromise the security. This article investigates the security of two kinds of QKD systems. Results obtained are as follows:1. One-way Faraday-Michelson quantum cryptography system is a very useful practical quantum cryptography system where Faraday mirrors(FMs) play an important role. In C hapter 3 the security of this system against imperfect FMs is analyzed. A passive Faraday Mirror attack is simulated in this system. By changing the values of angle errors of the sender?s FMs, the quantum bit error rate induced by the eavesdropper and the probability to obtain outcomes successfully are calculated. The simulation results show that the imperfection of one FM makes the system sensitive to this attack. Then the security analysis of the system against that the eavesdropper makes use of four FMs simultaneity is presented. Finally, a secure key rate as a function of the FM imperfections is given and the analysis indicates that imperfect FMs of the sender has a much more remarkable effect on the secure key rate than the receiver?s.2. Imperfect FMs in quantum cryptography systems can extend the states emitted by the sender from two-dimensional Hilbert space to three-dimensional Hilbert space, which may leak more information about the secure key to an eavesdropper. In Chapter 4, a simple experimental scheme to implement attacks against quantum cryptography systems that use imperfect FMs is proposed. As an example, the positive operator valued measure(POVM) in passive Faraday mirror attack on Faraday–Michelson quantum cryptography system is implemented with a linear optical setup. It becomes easier to implement when the states sent from the sender and the POVM operators of the eavesdropper are extended into four-dimensional Hilbert space, without changing the effect of the attack. Moreover, the method can also be applied to other quantum cryptography systems that use imperfect FMs by adjusting the parameters in the setup properly.3. Instead of actively modulating the intensity of the signals, the passive decoy-state method employs built- in decoy states in a parametric down-conversion photon source, which can decrease the side channel information leakage in decoy state preparation and hence increase the security of the QKD system. In Chapter 5, a comprehensive theoretical analysis of the passive decoy-state QKD experiment is presented. By modeling the experiment, the photon number distribution of the parametric down-conversion source is analyzed and tested. The simulated key generation rates are well consistent with the experiment results, which provides an important theoretical support for the experiment. It is expectable that long-distance passive decoy-state QKD can be realized by improving the experiment devices and techniques.