Research Of Entanglement Concentration And Quantum Router

Quantum information processing is an interdisciplinary of physics and information science. The theory of quantum information processing provides us with a new way to deal with information. Quantum entanglement plays a very important role in the field of current quantum communication and quantum computation, such as quantum teleportation, quantum key distribution, quantum secure direct communication and some other quantum protocols. However, owing to the interaction with the environment, the maximally entangled states will degrade to mixed states or pure less-entangled states which will seriously affect the quality and efficiency of quantum communication and quantum computation. Entanglement concentration is a powerful approach to extract the maximally entangled state from the pure partially entangled state with local operation and classical communication(LOCC). Cascade Greenberger-Horne-Zeilinger(C-GHZ) state based on the Greenberger-Horne-Zeilinger(GHZ) state, is a state with fault-tolerance. Actually,C-GHZ state is a entanglement of logical qubit, which can be widely used in quantum high-precision metrology, fault-tolerant quantum computing and distributed quantum networks.The conventional entanglement concentration protocols(ECPs) are analyzed in this thesis. The works of this thesis are as follows: Firstly, the ECPs based on the controlled-NOT(CNOT) gate for the C-GHZ state are proposed. These ECPs are firstly discussed in two simple cases and then extended to the entanglement concentration of C-GHZ states in any forms. The circulating concentration can be realized in these protocols to increase the success probability of the ECPs. Secondly, the simplified ECPs which are based on the polarization beam splitters(PBSs) and photon detectors are proposed. All of the devices used in these protocols are all linear optical devices which can be verified by experiments under realistic conditions. These protocols can also be applied to the C-GHZ states of any forms. Thirdly, for the entangled coherent states(ECSs), the ECPs are proposed. These ECSs can be applied to the multimode entangled coherent states.Another research focus of this thesis is the quantum router. Quantum router is a very important device in quantum communication. It is necessary to construct quantum communication networks, which is the key node in the quantum network. The classical models of quantum router are analyzed in this paper. Then, the cascaded multi-level linear-optical quantum routers are proposed.