Study On Protocols For Quantum Communication

Because of the unconditional security ensured by the Heisenberg uncertainty principle and quantum no-cloning theory, quantum communication has progressed quickly. There are several remarkable branches of quantum communication, such as Quantum Key Distribution (QKD), Quantum Secret Sharing (QSS), Deterministic Secure Quantum Communication (DSQC), Quantum Authentication (QA) and so on. Since there is no practical perfect single photon source at present, the weak coherent pulses (WCP) are often used as a substitution in experimental and practical systems. The eavesdropper Eve may perform the photon-number-splitting (PNS) attack to get information, thus the idea of decoy-state QKD was born. With the development of quantum communication, Quantum Communication Network (QCN) will come into being. This dissertation is focused on decoy-state QKD, QSS, DSQC and QCN, the main contributions of this work can be summarized as follows:In the aspect of decoy-state QKD, firstly through simulation decoy states can be used to identify the PNS attack is proved; Secondly, the formulae for the quantum key generation rate of the decoy state QKD with a heralded single-photon source (HSPS) are deduced, the optimal intensities and the key generation rate of the decoy state QKD with WCP and HSPS are analyzed respectively. Analysis results show that the quantum key generation rate increases with the detection efficiency of the sender; the key generation rate and the security communication distance are increased for decoy state make the estimation of the yield of the single photon pulse more exact. Finally, a new method of decoy-state QKD with a HSPS is proposed. In this scheme, Alice uses the parametric down-conversion to generate entangled photon-pairs, one of the pair is used as heralding photon. By the results of the trigger detector the heralded photons are divided into trigger and non-trigger sets. The states of the photons in the trigger set are used as the signal states and the other ones in the non-trigger sets are used as the decoy states. The yield and error rate of the single-photon pulse are estimated through the yields and error rates of the two sets. The key generation rate is deduced and simulation results show that by this method the same security distance can be reached as with a perfect single photon source; although the key generation rate is approximate two thirds the rate of the three intensities decoy state QKD, but this method does not need change the intensity of the photon source, is easy to implement. In the aspect of QSS, firstly the protocol of QSS between multiparty and multiparty without entanglement is discussed, and it is easy to know that if the senders do not pay attention to the publishing orders of the basis and the information, the dishonest agent can get the information without the collaboration of all agents. The method by which the agents publish the information in order and the basis in reverse order is proposed. Secondly, the protocol of multiparty QSS of classical messages based on Entanglement Swapping is also discussed, and it is easy to know that if the order of the two photons sent to the same agent does not changed, the dishonest agent can attack the protocol and the honest agent will get wrong information. Thus a new method by which the two photons are sent to the same agent in random order is proposed. Finally, the protocol of QSS based on Entanglement Swapping is studied, and analysis results show that the protocol is not secure since the entanglement states are generated by each agent. Then a new method of QSS based on Entanglement Swapping is proposed, by which the entanglement states are all generated by the sender, and the order of two photons sent to the same agent are randomly changed. After the photons arrived at the receiver, in case of the detection mode, the order of the two photons is announced, the two parties detect the security of the quantum channel; in case of the information mode, the two receivers respectively do Bell measurement on the two photons they own. Then the receivers communicate through classical channel to share the secret key with the sender. Analysis result shows that this protocol can ensure the security and correctness of the shared information.In the aspect of DSQC, based on the analysis of the proposed protocols, One-way DSQC protocol based on single photons is proposed. In this protocol, the XOR operation by bits of the information sequence and random sequence is performed and a checking sequence is inserted before the sender's coding operation. When the photons arrive at the receiver, they are delayed at the receiver and the sender then publishes the coding basis, so the photons can be measured in the correct basis. Then the two parties estimate the security of the quantum channel by the checking bits. When the channel is secure the sender publishes the random bits where the receiver has results, and the information sequence can be recovered by the receiver. Even the channel is not secure, what the eavesdropper gets is the random sending sequence, the information sequence is still secure. This protocol has the advantages of higher transmission efficiency and easier implementation compared with the two-way communication.In the aspect of QCN, a scheme for QCN and quantum switch is suggested. Moreover, a scheme for wide-area Quantum Secure Direct Communication (QSDC) network based on decoy states is proposed. A Server used to prepare and measure photons is set in each local area network, so the communication distance is increased. In addition, the idea of decoy states is introduced to QSDC to ensure the security of the communication when WCP is used. Moreover, the transmission probability is estimated based on the channel parameter, which can be used as a reference for channel coding. Security analysis result shows that this protocol can be used to realize long distance QSDC.