Experimental Study Of Security In Practical Quantum Key Distribution System

Quantum cryptography is an interdisciplinary field of quantum mechanics and information theory, which offers information-theoretical security for communication. Since the first appearance of the Bennett-Brassard-1984quantum key distribution (QKD) protocol, quantum cryptography has been developed into a very practical and active re-search field. The QKD systems have experienced from laboratory demonstrations in short distance with low key generation rate to robust and practical field test implemen-tation within a multi-user network through long distance with much higher key gener-ation rate. Up to now, quite a few companies have marketed QKD systems, and QKD networks are under construction throughout the world.In spite of the undergoing progress, various security issues, stemmed from loop-holes in practical systems, become a serious problem that draws a lot of attention in academia. Quite a few quantum hacking strategies have been proposed, some of which have been experimentally demonstrated to show how the security of practical QKD systems can be compromised. These loopholes are rooted in the deviation of practical QKD systems from the theoretical assumptions. For example, the non-cloning theorem of quantum theory requires the source to be a single-photon source. This cannot be sat-isfied in reality. Instead, weak coherent sources are often used. Thus, the multi-photon components which contain multiple copies of a qubit would give full information to an eavesdropper.This thesis focuses on the researches in the security aspects of practical QKD sys-tems, including the source security (analysis of the source loophole, the attack strategy and its experimental demonstration) and the detector security (measurement-device-independent QKD, MDIQKD, system design for integration, long distance implemen-tation, and networking). In the source part, we analyze the security loophole of the decoy-state source without phase randomization, design an attack strategy based on this loophole, and then make an experimental demonstration to show how the security can be compromised. We conclude that implementing phase randomization is essen-tial to the security of decoy-state QKD systems under current security analysis. On the detector side, we develop an efficient and stable MDIQKD system with the help of superconducting nanowire single photon detector and several feedback systems. This is the first time we have demonstrated the MDIQKD experiment over a200-km spool fiber, which is comparable to the longest achievable distance of standard QKD proto-cols. In addition, we deploy the MDIQKD protocol in a realistic environment to verify its practicability and stability. In these works, a new world record is set in the secure transmission distance. Effective techniques for high-quality Bell-state measurement developed in our experiment pave the way for long-distance quantum communication. Based on these development results, the network of MDIQKD systems are under devel-opment. Using the optical switches to connect all the users and to share the detection system, we implement a three-user star-type MDIQKD network. Since the MDIQKD protocol has an intrinsic property desirable for constructing a quantum network with the star-type structure, it is very convenient to extend this network to include more users with only sources and modulators added. The key advantage of MDIQKD is that the users do not need to trust the detection systems. Finally, another interesting QKD protocol, namely detector-device-independent QKD (DDIQKD), is presented, which is related to MDIQKD protocol. We have designed and developed a phase-encoding DDIQKD system that is much simpler than the polarization-encoding system. Based on the high-frequency system, we are able to realize a key rate much higher than that of MDIQKD system and easier to implement with current techniques.