Quantum Discord And Its Evolution Of Two-Qubit System

Quantum information and quantum computation, an interdisciplinary field whichinvolves quantum mechanics, the theory of information and computer science, hasattracted more and more interests. Quantum entanglement is a key resource in realizingquantum information and quantum computing and also the ultimate factor for this fieldexhibiting great advantage and application foreground. Recently, further researches showthat quantum entanglement does not account for all the quantum correlations, and thequantum correlations apart from entanglement bring greater advantages for quantumcomputing and quantum information processing tasks. So quantum discord (QD) as adescription of the total information of quantum correlations is introduced. So in this thesis,we focus our research on the quantum discord and the quantum discord dynamics oftwo-qubit quantum systems. The main results are as follows：Firstly，the thermal quantum discord (QD) is investigated in the two-qubit anisotropicHeisenberg XXZ model under an external nonuniform magnetic field along the Z axis. Weobtain the analytical expressions of the thermal QD and thermal entanglement measured byconcurrence (C). It shows that for any temperature T, QD gradually decreases with theincrease of nonuniform magnetic field b, in some regions where C increases while QDdecreases. It is also found that thermal quantum discord does not vanish at finitetemperatures, but concurrence vanishes completely at a critical temperature. It is shownthat for a higher value ofJ Z, the system has a stronger QD. There is a critical magneticfieldB c, which increases with the increasing b. QD decay monotonically (for B<B c)when temperature T increases, or initially increases to some peaks and thendecrease (for B>B c). Secondly，the dynamical behaviors of quantum discord between two atoms coupledwith a vacuum cavity are investigated. If the two qubits are initially prepared in twoextended Werner-like states, the quantum discord and entanglement can be numericallycalculated. There are remarkable differences between the time evolutions of the quantumdiscord and entanglement under the same conditions. These results imply that quantumdiscord is not zero for some unentangled states and in some regions entanglement candisappear completely. A large amount of quantum discord exists between the two-qubit.Thus, the quantum discord is more robust than entanglement for the quantum systemexposed to the environment. The quantum discord shows sudden change and its existencedepends on the initial state of the system. This property of quantum discord may haveimportant implications for experimental characterization of quantum phase transitions.