The Study Of Some Problems In Quantum Communication

Quantum information science is a new subject combined with quantum mechanics and information science, which has made a surprise progress both in theoretical and experimental field in the past two decades. It has created many miracles, such as absolute secure quantum key, quantum dense coding, quantum teleportation, especially quantum secure direct communication which is proposed recently. With the research of related theories and the development of hardware technology, improving the security and accurately during the information transmitting becomes an important aspect in communication technology. Quantum secure direct communication is an important part of quantum communication. Briefly, two parties (sender and receiver) share a pair of entangled qubits in the same state. According to the messages, the sender performs one of the unitary operations on the particles in her site. Because of the properties of the entanglement, the operation leads to a change of the particles in the receiver’s site. Then, the receiver can obtain information from the measurement results by measure his particles in proper measurement basis (MB).This is so called "Quantum Secure Direct Communication".Quantum communication is a new means of communication, which is combination of quantum mechanics and information science. Because of the special physical properties of quantum methods, quantum communication will help us to break through the bottleneck of exiting information system. At present, quantum communication mainly includes quantum teleportation, quantum dense coding, quantum key distribution, and so on.In this paper, we present schemes for remote state preparation of an arbitrary two or three-qubit state using Bell states, four-qubit cluster state, three-qubit GHZ states, six-qubit cluster state as the quantum channel. The probability of success regarding this preparation scheme is calculated in both general and some particular cases. Our results show that in general such remote state preparation can be realized with a probability of specific. But in several special cases, the probability of success can be improved to1/2or even1. In addition, a novel efficient deterministic secure quantum communication protocols with four-qubit cluster states or four-qubit W states is proposed. In our protocol, the secret messages can be encoded on the four-qubit cluster states or four-qubit W states by employing eight or four unitary operations. By making Bell measurement and two-particle joint measurement, the two legitimate users can transmit the secret messages after exchanging some additional classical information. Furthermore, this protocol is feasible with present-day technique.