Generation and protection of entanglement in the presence of local decay processes

Abstract

Entanglement, highlights all the counter-intuitive trait of the theory now employed for quantum information processing (QIP) tasks. However, entangled states are inherently fragile, prone to decoherence and decays through environmental interactions. Therefore the protection and preservation of entanglement becomes stringently important. The thesis describes the theoretical framework for the generation of various entangled states, their characterization and protection methodology involving both the discrete and continuous variables. First two proposals deal with engineering and protection of entanglement between quantum dots (QDs) placed in the vicinity of a nanoring having a surface plasmon (SP). Interaction of SP emanating from nanoring generates entanglement among the QDs and the counter propagating modes of SP. We also generalize it for multipartite case. QD interaction with SP is also utilized for protection of entanglement among QDs. The system with proven experimental feasibility open up a new vista for a sustainable entangled networks. Another way of protecting entanglement is to engineer environment. Here we consider an open system to be part of larger system-bath combination which is closed to the outside that is switching the system environment coupling from Markovian to non-Markovian. We investigate the dynamics and decoherence of entanglement in a system consisting of two atomic qubits trapped in two separate cavities containing structured reservoir in non-Markovian domain. Entanglement is quantified for various initial entangled states showing delay of sudden death of entanglement. For continuous variable regime, quantum beat laser (QBL) is considered. In order to preserve entanglement and to minimize entanglement degradation, structured environment is introduced by placing the system's doubly resonant cavity into two mirrors forming another large leaky cavity. Here, the entanglement is plotted verses time for various values of driving field strength, purity and thermal noise. The results demonstrate that the employed preserving schematic not only enhances the entanglement among two modes of the lasers but it preserve the entanglement for longer time than in Markovian approximation for its efficient applications in quantum information processing tasks.