Research On Quantum Correlation Dynamics In Atom-cavity System

Quantum entanglement, which is kind of quantum correlation, is an important resource in quantum information processing. However, entanglement is not unique the correlation that is useful for quantum information processing. Recently, it has been found that mixed separable (unentangled) states can also have nonclassical correlation. The quantum correlation of quantum states includes entanglement and other kinds of nonclassical correlation. Moreover, such kinds of non-entangled quantum correlation is more general and robust than entanglement, it may also play important roles in certain quantum information processes. Therefore, the study of quantum correlation dynamics in various quantum systems, particularly the quantum dynamics of multipartite system, not only have great theoretical interests in understanding the concept of quantum correlation in quantum system but also have potential application values in quantum information processing and quantum computing in the future.The main results of the thesis are as the following:1. The dynamics of quantum discord and entanglement in cavity QED is studied. Assume two identical two-level atoms A and B coupled with two spatially separate single-mode of two high-Q cavities a and b respectively. Making use of geometrical of depiction quantum discord and entanglement of formation (EoF), we show that there exists quantum discord sudden death (DSD) as well as entanglement sudden death (ESD) and the time interval of DSD is shorter than that of ESD in the presented quantum system. In addition, the rate of decreasing of quantum discord is slower than that of entanglement. We have shown that the amount of quantum discord and entanglement and the relative ordering between quantum discord and quantum entanglement depend on the purity p in the presented quantum system. And we have also shown that the discord and entanglement of the qubits AB can transfer to the cavities ab and vice versa. Surprisingly, the sudden changes of discord of qubits and cavities are observed when the initial states of qubits are mixed. 2. The dynamics of quantum discord and entanglement of two two-level atoms interacting with two coherent fields of two spatially separated and dissipative cavities is investigated. It is shown that the behavior of quantum discord and entanglement depend on both the initial state of the field|α) and the purity p of the initial state. We also show that the relative ordering of the long-term behavior of quantum discord and entanglement depends on the initial state of the field|α) and the purity p of the initial state. Furthermore, we find that there exhibited long-lived quantum discord and entanglement (or stationary-state quantum discord and entanglement) in the present system. In a short term regime, we observe that there exists the DSD and ESD phenomenon under some certain conditions. However, in the long term, the quantum discord of the two atoms tends to a fixed value. This value depends only on the initial state of the two atoms, the atom-field coupling constant, and the decay rate of fields.3. The dynamics of quantum correlation (QC) in terms of geometric discord and its transfer in dissipative reservoirs is studied. Firstly, we study the dynamics of geometric discord contained in a system consisting of two qubits A, B being initially in an extended Werner-like state with each qubit coupled to an independent multimode reservoir. Further, we investigate the dynamics of geometric discord contained in a three-qubit system that are initially in an extended W-like state. In the both systems, the short-term and long-term dynamics of geometric discord for qubits AB and reservoirs ab and its transfer from qubits AB to reservoirs ab are investigated. It is shown that the geometric discord of qubits AB decays asymptotically to zero and that of the reservoirs ab increases from time t=0, which exhibits a clear transfer of discord from the qubits AB to the corresponding reservoirs ab. Furthermore, in both systems, the results show that the geometric discord of qubits AB decays asymptotically to zero and that of the reservoirs ab can increase from zero to steady value in the long-time evolution.4. The dynamics of geometric discord and its transfer in a dissipative system consisting of two independent atom-cavity-reservoir subsystems under the strong coupling and the weak coupling regimes is investigated. It is shown that the geometric discord of the atoms and the cavities oscillatorily decays to zero while the reservoirs begin to present nonzero geometric quantum discord already immediately after t=0in the strong coupling regime. However, in the weak coupling regime, the geometric discord between the atoms progressively decays becoming zero and the discord between the reservoirs arises from zero to a stable value, while the cavities remain almost uncorrelated during the evolution. We also show that the amount of geometric discord contained in atoms and reservoirs depends on the purity p and it is proportional top, the smaller the value of p, the smaller the amount of geometric discord. It is worth noting that, in both strong coupling and the weak coupling regimes, the results show that geometric discord initially stored in the atoms will eventually be completely transferred to the reservoirs, independent of the parameters, but the transfer is mediated via the cavities in the strong coupling regime, while it is almost directly in the weak coupling regime.