Free-space Quantum Key Distribution

Cryptography has a long history from the beginning of people's civilization. Secret communication is always important to military and national defense. With the development of computer and Internet, the security of information becomes more and more important to economic and privacy. Modern cryptography tells us that secret communication depends on the security of the key. But the security of the key is still an unsolved problem in the classic field. For the public key, for example RSA, the security has not been proved until now. With the development of computer science, the security based on the public key is very dangerous. The private key can guarantee the security of communication, while the distribution is still a problem. Quantum key distribution (QKD) can distribute the key without being eavesdropped by the third person. The security of the quantum key is based on the quantum physics, and never changed with the development of the science and technology.From the first QKD protocol BB84 to now, QKD becomes mature through 20 years development. In the fiber, QKD has been realized in the real fiber cable exceeding 100 km, and some commercial products have emerged. Recently, free space QKD of 144km has been reported in the Europe. Globe QKD network is now the next stage of the QKD. But this network cannot be established only through point-to-point fiber QKD. It is very difficult to distribute the key exceeding 200km. As we know, 90% atmosphere is around 10 km above the ground. The transparency of the atmosphere can be above 60% in the atmosphere window. If the local area QKD network based on fiber can be connected by satellite, a globe secure communication network can be realized. In this dissertation, we firstly talk about the free space QKD experiments we have finished, and then we have an introduction to the satellite to ground QKD. The background light of the satellite-to-ground is especially investigated. The main content are listed below.1 Firstly, we give an introduction to the classic cryptography and the theory of QKD. The transmission medium of QKD can be classified as fiber and free space. We compare them and get the advantage of free space QKD. At last, satellite to ground...