Research On Quantum Repeater Based On Entanglement Purification And Exchange

In the actual quantum communication, quantum channels generally are noisy, so it will limit the transmission of fidelity. Unlike classical communication, quantum communication signals indicated by quantum states are in entangled quantum states in many cases. Those quantum states cannot be cloned under the principle of quantum mechanics otherwise the original quantum characteristics were destroyed. In current quantum communication protocols, quantum key distribution schemes based on entangled particles had more information and the research value than single particle, and their maximum entanglement entangled states were four bell states. Therefore, how to efficiently establish the EPR pairs between Alice and Bob is the precondition of quantum communication.Quantum communication channels are mainly optical fiber and free space at present. However, information transmission was influenced by channel noise, and photon losses (absorption) grown exponentially against the channel length whether in which channel. The safe distance of quantum key distribution system was about100km on the experiment at present, and it can meet the needs of local area network (LAN) basically. In the long-distance quantum key distribution systems, however, we will need quantum repeaters to implement quantum key distribution. The idea of quantum repeater is to extend communication distance by divided the whole quantum channel into several segments with the security of quantum key distribution distance; and then establish a longer EPR pair between the adjacent nodes using entanglement swapping and entanglement purification technology. Finally, we repeat these steps to obtain high fidelity quantum entangled states with long-distance. This paper does the following researches mainly:a) Study on nested Quantum Repeaters protocols and its total required time and physical resources that successfully established an EPR pair.b) To investigate purification efficiency on purification schemes by introducing a parameter, and then a fidelity curve converged faster is obtained. c) Study on the trade-off between information gain and disturbance of quantum repeaters, and find out the DFS under imperfect quantum gate operations.