Studies On Long Distance Quantum Communication

Information science played a critical role in the society development during the 20^th century. The increasing demand makes us improve the information technology continuously. However, the information systems are limited now. Quantum mechanics, which was developed at the beginning of last century, has explained many phenomena and has been applied into many fields till now. A combination of quantum mechanics and information science yields a new subject: quantum information science.As a subject of quantum information science, Quantum communication has been researched for many years. In general, quantum communication includes quantum cryptography, quantum teleportation, dense coding, long-distance quantum communication (LQC) and the complexity of quantum communication. The significant results of recent years have attracted the interests of many countries and groups all over the world. The goal of LQC is to establish quantum channel and realize quantum communication between distant sites. The bottleneck for communication between distant nodes is the scaling of the error probability with the length of the channel connecting the nodes. LQC is of importance in quantum information science due to the great future of application.In this Dissertation, we focus our research on the realization methods and the physical implementation of LQC. The main results are as follows:1. Remote Quantum Logic Operation Using Atomic EnsemblesWe present an experimentally feasible scheme to generate "polarization" entanglement between two atomic ensembles. Laser manipulation of atomic ensembles without addressing of individual atoms is normally easier than the coherent control of single particles. More importantly, it increases the signal-noise-ratio greatly compared with the case of interaction between laser and...