A Number Of Studies Of The Quantum Cryptography

People have been desired to communicate secretly for a long time. Communication becomes more and more important nowadays, consequently people pay more and more attention to the security of the communication.In classical communication, the security is mainly based on hard mathematical problems such that enhancing the difficulty of solving them increases the security of the communication.However, an eavesdropper can get a copy of the classical information transmitted in the classical channel, so he or she can make an attempt to decrypt this information without the risk of being detected. If the eavesdropper gets a machine which is powerful enough, then he or she can calculate the key and as such, the classical cryptography based on hard mathematical problems can in principle be insecure.Quantum mechanics provides us a fundamentally new way of encrypting information based on quantum features of microscopic particles. Such a quantum cryptography has been developed for many years. As there are many essential differences between classical electrodynamics and quantum theory, quantum cryptography differs from its classical counterpart dramatically and insures that the eavesdropper cannot effectively acquire information without being detected.The main content of this thesis is about quantum key distribution (QKD). As compared with classical key distribution, QKD has some novel features which stem from its quantum foundations.In this thesis I introduce some physical foundations of QKD first. Scientists have achived many fruitful results on QKD, whose history will be summarized in the second part of the thesis. As the research on quantum mechanics deepens, many new facts have been discovered recently. In the third part of the thesis I will give several QKD protocols based on these new findings, among which I describe a new QKD scheme using hyper-entanglement swapping.