The Study Of Efficiency In Quantum Repeater

In quantum information transmission process,the most important thing is to generate an entanglement state between two nodes.Such entanglement state is quantum channel which can be implement to quantum teleportation.We use photonic entanglement state,the only one we can use,during quantum communication.However,a single photon goes through optical fiber with a given absorption and depolarization rate nearly 0.2db/km,which means photon can only transmit about 150km in optical fiber.The idea of overcoming the difficulty associated with absorption and depolarization is to divide a long quantum channel into shorter segments,each segment length is comparable to depolarization length 150km.In the middle of two segments,we place a quantum repeater containing two quantum memories,which can store the state of photon to the state of quantum memories.At each of quantum repeater we implement entanglement swapping to extend the length of quantum teleportation channel,step by step,finally we could build up a quantum teleportation channel between Alice and Bob.Quantum repeater increase the photon fidelity through the fiber,but it will bring another problem:entanglement swapping could decrease the fidelity of states in quantum memories.Several times of entanglement swapping fidelity may drop out of range of any precise detector ability indicating we fail to generate entanglement state at two sites.To figure out how many quantum repeaters is better in a quantum channel,we introduce the concept of "entanglement distribution efficiency".Then we obtain the equation of entanglement distribution efficiency about number of quantum repeaters.From this equation,we estimate the optimal number of quantum repeaters at different distance of quantum channel and evaluate the efficiency of a certain quantum channel.In addition,we also know how to select certain entanglement states and Bell states in order to improve efficiency of quantum repeaters.