Quantum Cryptography And Quantum Repeaters

With the development of human society, the importance of confidential information transmission in the commercial and defense fields is becoming more and more prominent. Cryptography is the study of how to transmit information secretly. Since Shannon proposed the information theory, modern cryptography refer in particular to transmission of information and its mathematical research, which is often considered as a branch of mathematics, computer science and information theory. With the rapid development of computer and Internet technology, cryptography has not only applied to national defense, large commercial companies, etc. but also to our daily lives, including ATM chip card, personal computer communication and e-commerce, etc.Cryptography can be divided into symmetric key cryptography and asymmetric key (public key). The security of public key cryptography has not proved by mathematical laws, but can only rely on unreliable complexity of calculation. With the other hand, the rapid development of computer science and technology, the security of public key cryptography is increasingly being challenged. The other seems to have a security advantage:the symmetric key with one-time pad method is the only unconditional secure way which is proved mathematically. However, this method also has its shortcomings:one-time pad requires a lot of security key and how to distribute secret key betweent the two parties!?Fortunately, quantum cryptography can overcome this problem. In essence, quantum cryptography is to solve the obstacle:how to distribute keys secretly. Specifically speaking, quantum cryptography refers to the means of distributing keys in an insecure channel with help of quantum mechanics, and any eavesdropping or theft of keys can be found by legitimate communication parties. According to the basic principles of quantum mechanics including quantum no-cloning theorem, the Heisenberg uncertainty principle and measurement-collapse theory, any eavesdropping on the key can be found by legitimate communication parties. This ensures that communicating parties can share secure key. After that, through one-time pad method, communicating parties can carry out unconditionally secure communications. Therefore, the essence of quantum cryptography is quantum key distribution process.The current quantum cryptography research is towards industrialization. Security and usability have been the core of its research. While the security of ideal quantum cryptography had been proved, the real-life setups may be vulnerable to some attcks due to some imperfections. The real-life quantum cryptography systems are often using weak coherent state light sources insdead of single-photon sources, which will degrade the security of quantum cryptography. Decoy states method is put forward to overcome this security loophole. During my Ph.D period, I study the decoy states method in the following three ways:1. Based on the Faraday-Michelson interferometer, which is invented by ourselves, we have first completed the 123 kilometers decoy-state quantum key distribution experiment. Two-intensity modulated laser is used and the pulse repetition frequency is 1 MHz.2.1 discussed in theory, how to implement the decoy states quantum key distribution based on the modified coherent states (Modified-Coherent-States, MCS) source. And we prove the secret bit rate of the quantum key distribution with this source will be higher than quantum key distribution with commonly used coherent states source. And the secure distance of the MCS scheme is also longer than old one.3. I studied the decoherence-free subspace (Decoherence-Free-Subspace, DFS) encoding quantum key distribution scheme. I proved the security vulnerabilities of this scheme, and proposed the decoy states method of this scheme.How to achieve long-distance quantum key distribution is also an attractive research field. Due to optical absorption and channel noises, the efficiency of transmitting a photon in the channel will scale exponentially with the length of channel. The current quantum key distribution systems are able to only distribute keys with distance of about 100km. In order to achieve long-distance (1000 kilometers or more) quantum key distribution, quantum repeaters must be used. I carried out the research of quantum repeaters in the following study:In theory, I first proposed a new quantum repeater architecture which is immune to phase noise, and the polarization disturbances. Our scheme is a very robust one.