Experimental Free-Space Quantum Key Distribution

With the rapid development of information technology, the securities of communication and information have become more and more important. Secret communication not only plays a very particular role in military science and defense technology ,but also becomes very significant in the field of bussiness and daily communication.Cryptography is one of the most important technique measures in vast ways of secret communication.The application of cryptography has infiltrated all the fields of social life,as in data encryption,web security, digital cash and so on.Classic secret communication system employed at present usually base the security of communication on the arithmetic complexity. However, this system is confronted with a great challenge because of the rapid development of quantum computer and quantum algorithm.Quantum cryptography is a newly science which is the combination of quantum mechanics and cryptography.It successfully resolved the problem can not be resolved only with mathematical methods. Any eavesdropper who tries to eavesdrop the information in the quantum communications can be detected surely,based on the Heisenberg uncertainty principle and no-cloning theorem. Many countries invest large amount of people and financial resources into quantum key distribution(QKD) experiments since the birth of quantum cryptography. QKD is the key part of the quantum cryptography. The experimental researches of QKD can deepen our comprehension of the theory and practice of quantum information science.It has can help us explore in quantum information measures and develop new technologies and can accelerate the practical application quantum information theory to our national defense.This article has several parts as follows :1. After chiefly presenting the classic cryptography , we particularly introduce QKD,and notify the differences of them. We prove the absolutely security of QKD using the basic laws of quantum mechanics.2. We explain the prevalent protocols for QKD,and analyse the security and efficiency of all the protocols.We also introduce the recent developments of QKD protocols,and review the fresh progress of QKD experiments.3. We study the process of QKD, and analyse the concrete technic issue of QKD, focusing on the material discussion of sigle-photon sources,quantum channels,single-photon detector,and choose the technical scheme based on our current experimental condition and subject objective.4. Using BB84 protocol as the example ,we elaborate on our free-space QKD experimental system,and contrast with B92 protocol 0 Depicit the integrity process of the experiment, discuss the experimental results at large, analyse the sourse of error ,and aiming at improve our system, we advance some notions for ameliorating. The raw key bit rate we got is about 60kbits/s, with a bit error rate less than 5%.