The Security Of Quantum Key Distribution For Practical Applications

From ancient times to Information Ages that we live now, secure communications are requiring everywhere. Not just for military and diplomacy of countries but also for personal privacy, it acts more and more important. Conventional cryptography serves well to achieve this goal for a long time. Nevertheless, the security of conventional cryptography is based on the assumption of computational complexity, such as finding the discrete log or factoring. As a result, it meets a huge challenge as the computer technology develops rapidly and the concept of quantum computer is presented. That is why people start seeking a better method to keep secrets.As the interdiscipline of quantum mechanics and cryptography, quantum cryptography can solve the problem well. Quantum cryptography, precisely named quantum key distribution, can supply two distant users, Alice and Bob, expanding the sharing secure key by transmitting quantum states through a quantum channel. Compared to the conventional one, the advantage of quantum cryptography is its absolute security, because it is based on the quantum principles:the no-clone theorem and the uncertainty principle. As long as quantum mechanics is correct, the secrecy of quantum cryptography can be guaranteed. Since the first quantum key distribution protocol is presented by Bennett and Brassard in 1984, the whole field is blooming in research and implementation in practical commercial communication systems.Considering its application in practice, system of quantum key distribution has to refined, in order to adapt to massive user services and tough field environments. This dissertation exactly focuses on applications of quantum key distribution technology and is arranged as follows:1. We built a practical quantum key distribution system with active monitoring. Commercial laser generators are used in the real communication, as a weak coherent state source. It is true that they are convenient and refined, but there are fluctuations of the output intensity. What is worse, the laser can be manipulated by eavesdroppers to foil the quantum cryptography process. In this improved system, a module of active monitoring is add to randomly inspect the pulse intensity, which will give us a real-time knowledge and an alert if someone try to crack the system. The final result shows that, the decrease of key generation rate is not significant when the whole fluctuation of the intensity is limited. So does the maximal secure distance.2. We did research about the influence of a photon-number-resolving detector to the quantum key distribution system. Combing the photon-number-resolving detector and a parameter down conversion source, we can build a heralded single photon source, which can use to generate different sub-Possionian distribution states as we choose different output of the detector. With such a virtual source, we analyze the performance of BB84 and SARG04 protocols. Meanwhile, two types of photon-number-resolving detectors are taken into consideration. One is a superconducting transition-edge sensor and the other is a time multiplexing detector. The simulation result indicates that both of them can boost the key generation rate and the secure distance at the same time. Quantum efficiency and dark count of the detectors affect the result a bit, which infers that an imperfect photon-number-resolving detector can also help a lot.3. We demonstrated the first hierarchical metropolitan quantum cryptography network with seven nodes on the inner-city commercial telecom fibers, realized by Faraday-Michelson Interferometer set-ups. For a high running efficiency, the whole network is divided into two parts:one is a full-mesh backbone net and a subnet based on the technique of the optical switch and the trusted rely which can well guarantee the feasibility and expandability of the quantum network. Meanwhile, we utilize the secure key distributed by the quantum network in a practical video conference for all the nodes including the transmittance of instant video, sound, text messages and confidential files. The whole implementation with hierarchical quantum key distribution network links and well-developed application software clearly shows a big step toward the practical user-oriented quantum network.