Theoretical Study On Quantum Correlation Of Two Systems With Different Correlation Measures

As a valuable resource in quantum information and quantumcomputation, quantum correlation has attracted numerousattention over past decade years that provides the possibility ofquantum teleportation and quantum dense coding, etc. Somerecent results suggest that quantum correlation captures thenonlocal correlation more general than entanglement, for mixedstates, entanglement are not identical with quantum correlations.Actually, entanglement is a special kind of quantum correlations,but not the only kind. It is valuable to study the quantumcorrelations of two systems with different correlation measures, itis helpful to the unity of quantum correlation measures.Based on the above reasons, in this thesis, quantum correlationproperties of two-qubit model system with different correlationmeasures is discussed. In Chapter1, the general situation ofquantum information and the theories of quantum correlations havebeen summarized, the major research subjects and the organizationof the dissertation are given at the end of this chapter. In Chapter2, the importance of quantum correlation and the differentcorrelation measures are introduced. Firstly, the basic conception of quantum entanglement and its measures are given. Then othertwo measures of quantum correlation are introduced, it includeQuantum Discord (QD) and Measurement induced disturbance (MID).In Chapter3，quantum correlations in an anisotropic HeisenbergXYZ chain is investigated with use of concurrence C andmeasurement-induced disturbance (MID). It shows the propertiesof the ground state correlation, and it explore the effects about theexternal and self parameters on entanglement and MID behaviors inthis spin system. It is found that there is a revival phenomenon inthe concurrence but not in the MID, and the reason why the groundstate MID does not exist a revival phenomenon is given. Another，the region of MID is always larger than the concurrence. That is tosay，the quantum correlation based only on MID is expected to bemore robust than based on the concurrence. In Chapter4, quantumcorrelation dynamics between two identical and spatiallyseparated atoms in free space is investigated. It is found that thebehaviors of QD are remarkably different from the entanglement inthis system. It is shown that the entanglement suffers from suddenbirth with time and the interqubit distance, while QD is onlyoscillating decays; And QD is always greater than the concurrence.That is to say, the quantum correlation based only on QD isexpected to be more robust than based on the concurrence. Withthe help of the incoherent pump, it also shows incoherent pumpingcan overcome the decay of the atoms and the life of thesteady-state QD is evidently larger than the steady-stateentanglement. In Chapter5, Quantum correlation dynamics in ananisotropic Heisenberg XYZ model under decoherence is investigated with use of concurrence C and quantum discord (QD).It shows that both the concurrence and QD are oscillation with timewhereas there is a "entanglement sudden death’9phenomenon inthe concurrence but not in the QD, Also, it shows the cases whichthe interval time of entanglement sudden death is found to bestrongly dependent on the initial states and the parameters b andA. Then，it implies that the steady entanglement and QD can beobtained in the long-time limit which means that the environmentaldecoherence cannot entirely destroy the quantum correlation. Inaddition, based on the analysis of the steady concurrence and QDwith the limit case of t=0, it give the reason why the magnitude ofthe steady concurrence and QO is so complicated with the changeof the parameters B and A and the parameter b is independent ofthe steady concurrence and QD. In Chapter6, the quantumcorreKation dynamics of two-qubit system interacted with thethermal reservoir is explored. It discussed the correlationproperties under the different initial states with time，and theinfluences of the mean photon numbers and the spontaneousemission rate on the quantum correlations. The result shows thatsmaller value of the mean photon numbers and the spontaneousemission rate can make the decays of quantum correlation slower,and it can obtain a stronger life of the quantum correlation.