Quantum Information Processing Based On The Cavity Quantum Electrodynamics And Quantum Correlation Under Decoherence

Quantum theory and relativity theory emerged at the beginning of the twentieth century in order to give answers to unexplained issues in physics:the blackbody spectrum, the structure of atoms and nuclei, the electrodynamics of moving bodies. Since the birth of quantum mechanics theory, it has been used in bio informatics, medical, chemical, biological, and physical and other fields and has obtained a very big success. The quantum information science with combination of quantum mechanics and information science is a relatively new branch of quantum theory.The physical implementation of quantum information processing and quantum computing is based on the diversified physical systems. Through the study we found that these physical systems must have four basic requirements:"represent the quantum bit and keep its quantum nature","perform the specific unitary transformation","prepare a specific set of quantum states", and "measure the output state of the system". There are recognition of the physical systems:solid-state quantum dots (it uses the electron spin imprisoned in nanocrystalline as a quantum bit); linear optical element; liquid nuclear magnetic resonance (NMR) scheme (it uses large molecular solution for quantum computing); cavity quantum electrodynamics (cavity QED); ion trap scheme. Cavity QED uses the atomic energy level and (or) the number of photons in the cavity modes to encode quantum information. By the interaction between the atoms and the microwave cavity (or optical cavity), we can implement the coupling between these quantum bits. The scheme has the advantage of short operation time, and it can realize the exchange of information between atoms and photons. It is also helpful to make the connection between the quantum computation and quantum communication. The most important value of this method is that it can relate photons and atoms,and it has been widely used in the field of quantum information and communication. With success of the atomic entanglement and quantum remote transmission in the lab, the development of the quantum communication has come to the point where close to practical. This thesis concentrates on the discussion how to use the cavity QED to realize the preparation and purification of the quantum entanglement. Furthermore, we also investigated the evolution of the quantum correlations under the quantum decoherence.1. The concentration protocol of the entangled state among three independent cavitiesWe propose a scheme for concentrating arbitrary two-particle non-maximally entangled state into a maximally entangled state assisted with three separate cavities. The scheme involves two interaction-detection cycles and resonant interaction between atom and cavity mode. With the help of the atom trapped in the cavity, the concentration of a two-particle non-maximally entangled state trapped in separate cavities can be realized according to the results of photon detectors. The important feature of our scheme is that we can realize the concentration of arbitrary two-atom non-maximally entangled state and do not perform Bell-state measurements.2. Generation of three-atom Greenberger-Horne-Zeilinger entangled states based on separate cavitiesAn efficient scheme is proposed for the generation of a three-atom Greenberger-Horne-Zeilinger (GHZ) entangled state via three separate cavities. The scheme involves interaction-detection cycles and uses resonantly coupled atoms with an additional ground state not coupled to the cavity field. The generation of the three-atom GHZ state can be realized with a certain probability according to the results of photon detectors.3. The evolution of quantum correlations of the three-particle W-class state under quantum decoherenceWe investigate the quantum characteristics of a three-particle W-class state and reveal the relationship between quantum discord and quantum entanglement under decoherence. We can also identify the state for which discord takes a maximal value for a given decoherence factor, and present a strong bound on quantum entanglement-quantum discord. In addition, a striking result has been obtained that the quantum discord is not always stronger than the entanglement of formation in the case of decoherence. Furthermore, we also theoretically study the variation trend of the monogamy of quantum correlations for the three-particle W-class state under the phase flip channel, and find that the three-particle W-class state could transform from polygamous into monogamous owing to the decoherence.