The Research On Quantum Secure Communication In Quantum Noisy Channel

Noise that inevitably existed in quantum channel is a hard nut to crack for the extensive use of quantum communication techniques and must be overcome well. Since there have been many theories for dealing with quantum noise, the seldom application of which to the practical schemes. The efficiency and security of the existed off-noise schemes in quantum key distribution should be improved. Besides, there is a supposition for almost all the quantum secure direct communication protocols that the quantum channel is noiseless which made these protocols insecure in practically noisy channel, for the legitimate correspondents can't distinguish natural noise and eavesdropping.To seize the original essence and development situation of quantum communication, a general model of quantum communication system is proposed first which includes quantum source, quantum channel and quantum sink.Then the quantum channel noise in the model is focused, with the channel noise in quantum key distribution and quantum secure direct communication being mainly researched. The six-photon error-avoiding code is constituted and a robust quantum key distribution scheme based on it is proposed. In order to improve the qubit efficiency and the security of quantum key distribution, all the possible quantum states in the six-photon error-avoiding code are assembled, as a result, an optimal scheme developed, by which two bits can be encoded in each state and Bob can decode the information successfully with an average probability of 7/16. Compared with the quantum key distribution scheme based on four-photon error-avoiding code, the qubit efficiency of the former is increased by 16.67% , and the security of the former is 3.5 times of the latter. In addition, a novel quantum secure direct communication scheme over some important quantum noise is presented by introducing the decoherence-free subspace into a typical quantum secure direct communication protocol and adding some proper unitary operations, whose communication is relatively simple and could be easily realized.In a word, the quantum channel noise which is the key and difficult point in quantum secure communication has been deeply researched in this paper, with two quantum secure protocols of high efficiency and security in noisy channel proposed, which not only provide the further research of quantum secure communication over noise with the significant reference, but also contribute to the practical use of quantum communication.