Quantum Entanglement And Its Application In Quantum Communication

Since the concept of quantum entanglement was proposed by Schrddinger in 1935, people have paid more and more attention to entanglement. With the recent development of Quantum Information, many physicists have done much work on quantum entanglement. In this thesis, the non-local property and general theory of quantum entanglement are discussed. Besides, the author has researched the entanglement in the identical Bose system and gives the criterion of the entanglement. Recently, quantum entanglement has many applications in quantum communication. In this thesis, the author proposes a theoretically efficient high-capacity quantum key distribution scheme and generalizes the quantum dense coding to super dense coding between multiparty.People have studied the entanglement of pure states, especially the 2-body and 3-body systems, from the point of view of measurement, criterion and physical properties of quantum entanglement. Recently, the entanglement in identical particles becomes a popular focus. Schliemann has discussed it in two-Feimi system and finds that the entanglement of in Feimi system is similar to that of distinguishable particles. But duo to the basic difference between Feimi- and Bose- system, it is important to find out the entanglement in Bose system. In this thesis, entanglement in Bose system is discussed.In addition, the application of entanglement in quantum communication will be investigated in this thesis. First, quantum key distribution (QKD) is introduced. QKD is borne from the development of quantum computer and quantum arithmetic of factoring a big number, which will threaten the security of cryptography in being. The security of QKD is guaranteed by the quantum measurement theory and quantum no cloning theorem. So far, there are 3 types of QKD schemes: BB84-type, Ekert-type and Goldenberg/Vaidman-type, etc.Using orthogonal states for QKD is not safe in some extend due to orthogonal states may be cloned; however, orthogonal states can be distinguished by project measurement, which means high efficiency if they can be used for secure QKD. In the thesis, the author together with Prof. Long gives a theoretically efficient high-capacity quantum key distribution scheme.Another aspect of quantum communication is quantum dense coding. In 1994, C.Bennet supposed to convey 2 bits information by 1 photon based on the no locality of quantum-entangled states. With the development of quantum information, it's reasonable and necessary to research high dimensional case of the applications of quantum entanglement, such as high dimensional dense coding between multi-party. In this thesis, the author generalizes the dense coding scheme. The most difficulty of this task is to find a set of orthogonal states, a corresponding set unitary operators and regulars to operate. In this thesis, they are given. And the author finds out that the information received by one measurement is log ?"+' bits, where d is dimension and N is the number of information senders.