Entanglement: Engineering and its Dynamical Studies in Dissipative Environments

Abstract

This thesis covers the two inter-related topics of recent research in the field of quantum information theory. One topic is about the robust generation of entanglement among Gaussian states in quantum optical passive and active devices like beam splitters and quantum beat lasers and the other topic is about the degradation of entanglement due to decoherence when entangled states interact with the surrounding environments. The subject of entanglement generation is studied in continuous variable systems for two initial separable single-mode Gaussian states in beam splitters and in quantum beat lasers. As a general treatment, one single-mode Gaussian state is defined in terms of arbitrary values of nonclassicality and purity while the other single-mode Gaussian state is considered as the thermal field. The role of different parameters in the presence of thermal noise on the entanglement generation is explored, for example, the nonclassicality and purity of single-mode Gaussian state; and angle of beam splitter or driving field strength in quantum beat laser are important for robust entanglement generation. For entanglement analysis, logarithmic negativity is implied as a measure of entanglement for two-mode Gaussian states. Second topic investigates the dynamics of a class of initial entangled states in dissipative environments. It is the study of decoherence mechanism in different systems and is crucial as real systems always interact with the surrounding environments. The dynamics of the two-qubit atomic systems and high-dimensional bipartite field states inside the two high-Q cavities surrounded by thermal environment are investigated. The two-qubit atomic systems are explored as both interacting (close) and non-interacting (distant) systems. In the interacting systems, atoms are considered close together so that the atoms may exchange energy, thus the role of collective damping and dipole-dipole interaction becomes important. It is also noted that entanglement may be generated for initial separable states in thermal environment. Wootters concurrence is used as a quantitative measure of entanglement for two-qubit atomic systems. For the entangled bipartite field states in thermal environment, the high dimensional states as non-interacting systems are studied. It is concluded that sudden death of entanglement (SDE) always occurs in non-interacting systems in thermal environments. The increase in the temperature of the environment results in earlier disappearance of entanglement. For entanglement analysis, negativity is used for high-dimensional entangled field states in high-Q cavities.