Dynamics Of Quantum Correlations In Two-atom-vacuum Field Systems

Quantum entanglement between quantum objects is a special category of quantum correlations. As a fundamental resource, it has been widely used in the quantum information processing for its outstanding non-locality in quantum mechanics. However, recent investigations have found that quantum entanglement does not account for all of the non-classical properties of quantum correlations, and it is not the only type of useful correlations for quantum technology. Quantum discord can capture quantum correlations in separable states in which non-entanglement quantum correlations have been found to be responsible for the achievement of quantum speed-up in certain computational tasks. There is also evidence that quantum discord is more robust against de-coherence and sudden death than quantum entanglement. It implies that quantum algorithms based only on quantum discord are more robust than those based on quantum entanglement. It stems from the fact that quantum discord includes the part of quantum correlations without entanglement. Moreover, quantum discord as a figure of merit characterized the resources. Therefore, the study of quantum correlations has become a hot issue in quantum computing and quantum information.In this thesis, dynamics of quantum correlations in two-atom- vacuum field systems are studied by using the quantum correlations theories and the Lindblad master equation of open quantum systems, and then some useful results are obtained. The main points of this thesis are as follows:Chapter1is an introduction of basic theory of quantum correlations, including the concept of entangled states, measurement of quantum entanglement of two-qubit system; and the concept of quantum discord along with an analytic expression of quantum discord for two-qubit system.Chapter2is a brief introduction of density matrix of temporal evolution in open quantum systems and the Lindblad quantum master equation is introduced.In chapter3, quantum correlation without entanglement in a two-atom-vacuum field system is investigated. The influence of the atomic dipole-dipole interaction on quantum correlation is also examined. It is found that, when both atoms are initially in separable excited states, the quantum discord of two atoms appears while their entanglement wholly vanishes throughout the evolution. We explain the results in detail. The results provide a novel method of achieving a quantum correlation resource without entanglement in the real system.In chapter4, the quantum correlations of two two-level atoms interacting with a single mode vacuum field are investigated. The relationship between the quantum discord and the entanglement of formation, the influence of the atomic dipole-dipole interaction along with two-atom initial states on quantum correlations of two atoms are discussed. The results indicate that when two-atom is initially in an entangled state, quantum discord is consistent with entanglement of formation. Meanwhile, there is no occurrence of sudden death phenomenon of quantum correlations throughout the temporal evolution, and then under strong dipole-dipole interaction, the relative stable quantum correlations resources can be achieved.In chapter5, the quantum correlations of two two-level atoms interacting with a vacuum reservoir are researched. The influences of non-Markovian effect of the vacuum reservoir and the atomic dipole-dipole interaction on the quantum correlations of two atoms are also considered. The results show that, the quantum correlations of two two-level atoms can be adjusted by the decay rate of a vacuum reservoir and the atomic dipole-dipole coupling constant.In chapter6, a summary and outlook is proposed.