Quantum Channel Noise In The Preparation And Purification Scheme For A Class Of Four-photon Entangled States

In quantum information and quantum computation, we use quantum mechanics to do the research of information processing and calculation program, The preparation an-d purification of special quantum entangled states is an important work. Quantum en-tanglement is a common phenomenon in quantum mechanics, the research on quantum entanglement propelling the development of quantum information and quantum comput ation. Preparation and purification schemes for multi-photon entangled states are impor-tant research directions. After the preparation program of a photon entangled state, the entangled state will suffer the interference of external environment, which is quantum channel noise. Those noises may be bit-flip and phase-flip errors. A correction scheme is need to correct the false results, and the fidelity of states should be increased after quantum correction scheme is carried out.This thesis focus on the quantum channel noises in a preparation and purification scheme for a class of four-photon entangled states and the fidelity of this quantum st-ates under quantum channel noise. The entangled state preparation method cited in this thesis is the spontaneous parametric down-conversion source, those entangled states su-ffer the evolution of a series of linear optical elements (phase shifter, beam splitters, and polarization beam splitters, etc.), so one can obtain the state which have entangle-ment in both polarization and spatial degrees of freedom. Then we calculated the fide-lity of the entangled states which suffered from quantum channel noises.In Chapter 1, we introduced some concepts and knowledge about quantum state, quantum operator, density operator and quantum computation.In Chapter 2, we introduced the knowledge about quantum entanglement, measu-re of entanglement, quantum coherence and fidelity.In Chapter 3, we worked on the problem about quantum channel noise in a pr-ocess of preparation and purification of a class of four-photon entangled states, we ca-lculated the fidelity of the quantum states. Then we give two detailed cases:single s-patial mode which is depolarizing quantum channel and single spatial mode which on- ly bit-flip occurs, and we computed their fidelity respectively.