Investigation Of Thermal Quantum Correlations For Two-spin System

The characterization and quantification of quantum correlations in composite systems is of primary importance in quantum information theory.Quantum entanglement and quantum discord are two different types of nonclassical correlations that have been regarded as a valuable resource for quantum information processing.The investigation of quantum discord has mostly focused on two-qubit systems due to the complicated minimization involved in quantum discord for high-dimensional states.In this work we study the thermal quantum correlation in two two-spin-1/2 and-1 coupling systems,where the correlation is described in terms of three modified geometric discords.The generalized negativity is calculated for reference.We analytically and numerically simulate three geometric discord of the thermal state in two systems.It is shown that three geometric discords are more robust against temperature and magnetic field than the generalized negativity in both spin-coupling systems.However,all four quantities exhibit a similar behavior at lower temperature and weak magnetic field.Meanwhile,the hierarchy among three discords is discussed.For weak coupling and magnetic field,three geometric discords have a hierarchy D1>D3>D2 in the regime of high temperature.Remarkably,two modified geometric discords at finite temperature reveal the phenomenon of sudden changes at different magnetic fields in two-spin-1/2 system while the 1-norm geometric discord and generalized negativity do not.For the two-spin-1 system,all three geometric discords at finite temperature reveal the phenomenon of double sudden changes at different magnetic fields,and the related fields for their occurrence are strongly dependent on temperature,which means that the sudden changes of the geometric discord is sensitive to the choice of the measure of quantum correlation and system parameters.The corresponding results have been published[Acta Phys.Sin.65(2016)220301;Int.J.Quantum Inform.14(2016)1650016].In summary,we have studied the thermal quantum correlations in two typical systems.We can adjust three geometric discords and quantum entanglement with the varied coupling,temperature,and magnetic field.Those are useful for the understanding of quantum correlations in high-dimensional states.