The Fidelity And The Violation Of Bell Inequality Of Quantum States After Going Through Decoherence Channel

Quantum entanglement is a very important physical resource in quantum information,and has drawn more and more attention.Some unachievable tasks in classical information can be realized by using the entanglement properties of two-partite or multipartite quantum states.These tasks include quantum dense coding,quantum teleportation,parallel quantum computing and so on.To prepare entangled states in optical systems is an important research field.Unfortunately,the entangled states are easily disturbed by the environment,and these disturbances are usually regarded as quantum channel noise.Therefore,it is also important to study the channel noise in the process of preparation of entangled states.We also need to investigate the type of noise and its effect,so as to design a quantum error correction scheme based on actual environment.In this paper,we mainly discuss the preparation and purification schemes for a class of four-photon entangled states based on parametric down-conversion sources,Kerr media and linear optics.Besides,we also discuss the quantum channel noise that may arise in the quantum information processing,and calculate the fidelity and violation of the Bell inequality of quantum states after going through decoherence channel.In chapter 1,we introduce some basic knowledge of quantum information,such as pure state,mixed state,fidelity and so on.In chapter 2,we introduce a scheme for preparing and purifying of the four-photon Greenberger-Horne-Zeilinger(GHZ)state.In chapter 3,we discuss in details the channel noise that may appear in the preparation scheme of four-photon entangled state.Four common decoherent channel noises are listed and we calculate the fidelity for each noisy channel.In chapter 4,we introduce two kinds of Bell inequalities,and perform detailed calculations and analysis on the violation of Bell inequality of quantum states after going through decoherence channel.