The Prepartion Of Quantum Entanglement And Its Application In Quantum Communication

Nowadays, Quantum information science has recently attracted a great deal of attentionowing to its wide applications in quantum computation and quantum communication, such asquantum teleportation, quantum cryptography,quantum dense coding and so on. As we know,the generation and measurement of entanglement has been widely researched from discretevariable entanglement to continuous variable entanglement, and the focus of research is toproduce a long entangled state. There are two important points in quantum entanglement statetransmitting: one is how to prepare entangled quantum states since quantum entanglement isan absolutely necessary resource in quantum teleportation; the other is how to more efficientlytransport quantum entanglement in quantum communication.In this thesis, we mainly discuss the preparation of quantum entanglement and itsapplication in quantum communication. We firstly discuss the generation of two-modeentanglement in above and below threshold respectively, then we provide transmittingquantum entanglement state without bell-state measurement. The specific innovative contentsinvolve the following two aspects:Firstly, we propose a scheme to generate two-mode entanglement by using two lasersresonantly driving a single four-level atom embedded inside a two-mode optical cavity. In ourscheme, the responsible mechanism is atomic coherence, which is induced by the classicalfields. The strong pump field leads to the existence of direct two-photon transition and theprobe field induces the parametric interactions between two photons, which make thementangle with each other. so the generation of two mode entanglement in the above thresholdcan be achieved.We can find that the degree of entanglement and the number photons are allrelated to the Rabi frequencies of the classical laser fields, the cavity losses and the atomicdecays. Then we mainly discuss the entanglement mechanism in the below threshold, weutilize the method of the logarithmic negativity to analyze the entanglement degree. Ournumerical results reveal that the degree of entanglement between the two cavity modesdepends on the Rabi frequencies of the classical laser fields and the cavity losses.Then, we present a scheme in which the N-atom W state is teleported by employing theselective interaction of a cavity field with a driven three-level atom in the Λ configuration anddetecting a single atom in one of the ground states. The long-lived W state is teleported fromatom A to atom B when the atoms B and A are sent through a cavity successively and atom Ais then detected. The advantage is that the present one does not involve the Bell-state measurement and is robust against the atomic spontaneous emission.