Information Transmission In Quantum Communication

Quantum communication is a new way of transmitting information. Based on quantum mechanics, it is possible for us to realize unconditionally secure commu-nication and teleportation. Since the publication of first quantum key distribution in 1984 and quantum teleportation in 1993, quantum communication has develope-d very quickly and obtained significant achievements in theory and experiments. Many scientists have focused on this field.The content of this dissertation includes quantum key distribution (QKD), bidirectional quantum secure communication (BQSC), quantum teleportation(QT) and remote state preparation (RSP).In the area of quantum key distribution, we proposed a new protocol based on four-particle cluster and the technique of quantum entanglement swapping, and its security is also analyzed.In the area of bidirectional quantum secure communication (BQSC), due to the drawback of "information leakage" existing in most of present BQSC, we devised three methods to overcome this shortcoming. First is about two BQSD schemes that based on the idea of QKD, in our schemes, private EPR pairs and single photons are respectively used as quantum channels. These two protocols can not only resist common attacks but also discard the drawback "information leakage". In the second protocol, we introduced the auxiliary particle and utilized the special character of EPR pair, "correlation extractability", and put forward a BQSC protocol that possessed security. This scheme is suitable when the secret message of Alice is twice as many as that of Bob. In the third protocol, we designed a BQSC protocol based on non-destructive measurement. In this scheme, by employing N-particle GHZ state quantum channel and non-destructive measurement based on one auxiliary particle, we can realize BQSC without information leakage. The advantage of this scheme is the communication efficiency can reach 100% if N is big enough.In the area of quantum teleportation, from the perspective of Shannon entropy we study the cost of classical communication in teleportation of Bell state in MM scheme, the result shows that two bits of classical information are enough for MM scheme. Consequently, by using the technique of tensor representation, we described the process of teleportation of N-particle GHZ state, from the tensor form of the collapsed state at receiver and the definition of Shannon entropy, we calculate the cost of classical information in teleportation of N-particle GHZ state. Our result shows that (N+1) bits of classical information are sufficient for teleportation of N-particle GHZ state if N EPR pairs act as quantum channels.In the area of remote state preparation, we put forward two schemes for remote preparation of arbitrary two-particle state. One scheme is realized by using four-particle cluster, and the other is achieved by two-step projective measurements, the latter protocol has high success probability.