Research On Some Basic Problems In Quantum Communications

In this thesis, we first concisely introduce some basic concepts in quantum communications, such as qubits and quantum entanglement and so on, then we systematically summarize the principle of producing entangled states as well as some related optical techniques, including the required knowledge of physical optics concerning with operating and measuring entangled states. We also give the definitions of several multiphoton entangled states, such as GHZ state, Cluster state, W state, WGHZ state and so on. We then review the research history of photonic entanglement generation and the latest development, pointing out some existing problems, for example, there has always been a challenge of proposing a scheme to generate Cluster states of any number photons. On the other hand, we also introduce an important part in quantum communications---quantum teleportation. We describe concretely the principles of the first, second and third eras of quantum teleportation as well as their experimental development. Finally, we summarize some concepts related to quantum network, such as quantum repeater and quantum memorizer. We also review the developing history of Classical Complex Network Theory and some of its important concepts.Based on the existed work, we put forward some comprehensive schemes which can be used to generate GHZ and Cluster states containing arbitrary number of photons. For GHZ states, we propose a comprehensive scheme to generate as well as transmit GHZ states of any number photons. At the same time, this scheme can accomplish the open-destination teleportation of single photon state. This scheme can be considered as the development of five-photon's case. Later, we make an extension, analyze the open-destination teleportation of M-photon general GHZ state and give its experimental diagram.Based on the work of Briegel, Zou and Mathis, we propose a scheme to generate Cluster states of arbitrary number photons by combining two-photon entangled states. We also show how to use it to teleport single photon state for open-destination. Additionally, especially for Cluster states of even number photons, we find a new general way of production different from the former one. Beginning with the cases of eight- and ten-photon, we deduct a decomposition rule of even-number photon Cluster state. Not only do we give concrete theoretical analysis, we also give the corresponding schemes. The required optical facilities and techniques are all feasible under current technology.At the final part, we give summary, conclustion and outlook, extending the research objectives from unit techniques to the research of network scale, and point out some valuable topics that deserve further investigation.