Theoretical Studies On Quantum Correlations In Cavity QED System

Quantum information theory is an interdiscipline between quantum mechanics and infor-mation science, which consists of quantum computation, quantum communication and quan-tum cryptography. The quantum information processing is much more advanced than classi-cal information processing. For example, a quantum computer can exponentially speedup the algorithms that cannot be performed with a classical computer, and quantum communication enables us to transfer information in a definitely safe manner. The necessary requirement for performing quantum information processing is that the system is quantum-correlated. Therefore, studies on quantum information are essentially those on quantum correlations in some sense.Quantum correlation is not only the feature that distinguishing quantum mechanics from classical mechanics but also plays an essential role in quantum information theory. Recently, as the measures of quantum correlation, both quantum entanglement and quantum discord have attracted considerable attention. On the other hand seeking for a suitable quantum system as the ’hardware’ to perform quantum information processing is also an urgent requirement. Among the systems currently under study for quantum information, the cavity quantum electrodynamics (cavity QED) system is believed to be one of the candidates of high potential. As a consequence studies on quantum correlation in cavity QED systems are of great importance and are the main themes of this thesis. The thesis consists of seven chapters in which Chaps.3to7covers the main research work performed during my doctoral study.In Chap.1, the creation and development of quantum information theory are reviewed and the significance of quantum correlation in quantum information theory is discussed.In Chap.2, some fundamental concepts used in this thesis, such as qubit, density matrix, measures of quantum entanglement and quantum discord are introduced.In Chap.3, the positive effects of the scattering strength of a microtoroidal cavity on atomic entanglement evolution are studies. The so-called scattering strength is used to characterize the coupling between two whispering gallery modes caused by the roughness of inner surface of the cavity. Intuitively, in order to obtain a larger entanglement one should make the inner surface of the cavity as smooth as possible. However, we find that the rough surface can play a constructive role. In particular, the rough surface can also compensate for the loss of entanglement during the evolution through cavity leakage and atomic spontaneous emission.In Chap.4, the creation of quantum discord between two two-level atoms trapped in a Fabry-Perot cavity in a noisy environment is discussed. It is shown that nonzero steady-state quantum discord can be obtained when the white-noise field is separately imposed on atoms and cavity, while the steady-state quantum discord reaches zero if both cavity mode and atoms are driven simultaneously by white-noise fields. In particular, we demonstrate that white-noise field in different cases can play different "constructive roles" in the generation of quantum discord.In Chap.5. a scheme for long-distance teleportation of an unknown atomic state between two atoms that are trapped in separate optical cavities. In this scheme, the probability of success is independent of the state to be teleported. Moreover, in the Lamb-Dicke limit, the requirement of the simultaneous clicks of the detectors is not necessary.In Chap.6. the direct measurement of quantum discord is discussed. A scheme for directly measuring the exact value instead of a lower or upper bound of geometric quantum discord is proposed. It only requires the projectors in the all anti-symmetric subspace and is more efficient in contrast to the widely adopted quantum state tomography scheme in the sense that fewer parameters are needed to be measured. Moreover, the present scheme can be easily realized with the current experimental techniques.In Chap.7, nondestructive detection and identification for a special class of states are discussed. A scheme for nondemolition measurement of a Werner state is discussed and then generalized to the case in which two qubits are separately shared. In addition, a scheme for identifying the Bell diagonal state is discussed in this chapter. The distinct advantage of the present scheme is that the evolved joint quantum state will collapse onto the original Bell diagonal state ensemble after the measurement on the probe qubit. This means the scheme is nondestructive. The experimental realization of both schemes are also discussed in the framework of cavity QED.Finally, conclusions and prospects are given.