Effect of Non-interacting Quantum Background on Entanglement Dynamics

This thesis presents a new approach to the analysis of open system entanglement dynamics by taking into account explicitly the usually neglected non-interacting part of the quantum background. We call such a background as the "marginal" party. It is a surrogate for the combination of unknown fields, forces and objects that were only engaged prior to the time of interest. We follow quantitatively entanglement information flow between a system of interest and the conventionally traced interacting background (usually called "reservoir", or sometimes "bath"). Analytic three-party inequalities are observed showing that the entanglement information transfer between a generic qubit system and its interacting reservoir is restricted by the unspecified marginal quantum background. In particular, new forms of non-linear entanglement conservation relations are achieved for a large class of system states. Extended analyses for N-qubit and qutrit systems are also presented. Part of these results are verified by experiments using photon polarizations. We also investigate marginal effects on disentanglement processes of entangled two-party open systems. For two-qubit states under local reservoir interaction, the properties of complete disentanglement (entanglement decay to zero at a finite time) and threshold disentanglement (entanglement decay to a prescribed non-zero value at a finite time) are shown to be controlled by two parameters, i.e., the marginal entanglement and initial two-qubit entanglement. Various disentanglement phases are identified in terms of the two parameters. Our results suggest that, in addition to the interacting reservoir, the non-interacting marginal party as a part of the quantum background also plays an important role in an open system's entanglement dynamics. Our system-reservoir-marginal analysis provides a new perspective in the study of quantum background effects on open systems.