Theoretical Study Of Two-qubit Quantum Correlation In Coupled Cavity QED Systems

Quantum correlation is a basic concept in quantum mechanics,the difference between quantum physics and classical physics,it plays an important role in many-body physics,quantum statistics and quantum information.Quantum entanglement is able describe the non-locality in quantum mechanics,however,it can not reflect all the characteristics of the quantum correlation.Compared with quantum entanglement,quantum discord as a measure of quantum correlation,is a more general form of "quantumness" of correlations that includes entanglement but goes beyond it.Since both entangled states and separable states can have non-zero quantum discord,which then not only displays potential applications in the field of quantum information,but also play an important role in studying quantum phenomenon,such as quantum phase transition and the maxwell's demon.In addition,we consider the vanishing of quantum discord as a criterion for the preferred effectively classical states of a system,i.e.,the pointer states.Quantum discord is more robust than the entanglement against decoherence and dissipation,so that quantum algorithms based only on quantum discord correlations may be more robust than those based on entanglement.Hence,we will utilize quantum discord as the measure of the two-particle quantum correlation in a system where two entangled qubits interacts with thermal fields.In our thesis,based on the theory of cavity quantum electrodynamics,we investigate the time-evolutional dynamics of the quantum discord for a system where two atoms trapped in the coupled cavities are initially prepared in a maximally entangled state,and each atom individually interacts with a single-mode thermal field.The main contents are as follows.1.We calculate the quantum discord versus rescaled time for the two atoms initially prepared in the maximally entangled state ?a(0)= 1/(?)(|eg>+|ge>)and the fields initially in the vacuum state.We give a basic analysis for the causes resulting in variation of the quantum discord of the two-particle system.2.We investigate the time-evolutional dynamics of the quantum discord with the two atoms initially in the maximally entangled state interacting with single-mode thermal fields.We analyze the influence of the detuning between the cavity frequency and the atomic transition frequency,the asymmetry of the atom-cavity coupling strengths,the photon hopping rate between the two cavities,and the average photon number of the thermal fields on the two-particle quantum discord.In such a system,while the temporal evolution of the two-atom concurrence presents sudden death,the quantum discord almost disappear in only some discrete time.Therefore,the robustness of quantum discord to the thermal environment is inspiring to realize quantum computation.Next,we study a system where three atoms separately trapped in three directly coupled cavities.We found that a three-qubit XY Heisenberg interacting model can be built and then a distributed three-qubit Toffoli gate can be implemented with the help of the Hadamard gate.In this scheme,the cavity modes are virtually excited and the excited states of the atoms are adiabatically eliminated.We obtain a resonably high fidelity of the gate by appropriately selecting parameters.In the future,we will study the quantum correlations for arbitrary pairs of the atoms in this system.