Dynamics Of Quantum Entanglement And Quantum Discord Of Two Qubits Under Dissipation

Quantum entanglement has been generally recognized as a crucial resource in quantum information processing and quantum computation. It plays key roles in many quantum tasks such as quantum key distribution, quantum dense coding, quantum secret sharing, quantum secure direct communication, factorization of large numbers, quantum search algorithm and etc. Quantum discord can capture quantum correlation in separate states. It may be the reason why there exist some quantum tasks which display quantum advantage without entanglement. It is known that the inevitable interaction between an actual quantum system and its surrounding environment will lead to decoherence of the quantum system, and different kinds of environment and different ways of interaction make decoherence mechanism and dynamical behavior of the quantum correlation different. There-fore, it is especially important to investigate the dynamics of quantum entangle-ment and quantum discord in various environmental models for effective methods to control quantum entanglement and quantum discord of open quantum systems and construction of useful quantum information processing systems. In this the-sis, dynamics of quantum entanglement and quantum discord of two qubits under dissipation are studied by using the theories of open quantum systems, quantum entanglement and quantum discord, and some useful results are obtained. The main points of this thesis are as follows:Chapter1is a brief introduction of basic theory of quantum entanglement and quantum discord, including definition of entangled states, measurement of quantum entanglement of two-body system, definition of quantum discord and analytic expression of quantum discord for two-qubit system. In chapter2, an introduction of operator-sum representation of evolution of open quantum system is firstly given, then operator-sum representation of am-plitude damping channel, phase damping channel and depolarizing channel are presented, finally, the Markovian quantum master equation is introduced.In chapter3, under amplitude damping channels, entanglement dynamics of a two-qubit system which is initially prepared in two kinds of extended Werner-like (EWL) states is studied in terms of initial purity and initial concurrence. It is found that there exist both ESD states and ESD-free states for these two kinds of EWL states and the boundaries between ESD states and ESD-free states are given in terms of initial purity and initial concurrence. Furthermore, by local unitary operations, ESD states can be transformed into ESD-free states.In chapter4, the exact dynamics of quantum discord of two strongly driven qubits, which are inserted in two independent dissipative cavities or in a common dissipative cavity, are studied. The results indicate that both in the two cases, the evolution of quantum discord is independent of the initial cavity state. For the two independent dissipative cavities, it is found that the phenomenon of sudden transition between classical and quantum decoherence exists and the transition time can be greatly delayed by suitably choosing the initial state parameter of the two qubits, the cavity mode-driving field detunning and the decay rate of the cavity. For the common dissipative cavity, it is shown that for some initial states of the two qubits, the quantum discord can increase for a finite time at first, and then it decreases to a steady value, while for some other initial states, the quantum discord can increase monotonously or with oscillation till a stable value is reached. Moreover, the creation of quantum discord for the two qubits in a common cavity is found. chapter5are a summary and an outlook of the thesis.