Excitonic Energy Transfer and Multipartite Entanglement in Quantum Networks: with Applications to Cavity Quantum Electrodynamics

Abstract

Excitation transport is a fundamental mechanism underlying many processes and networks in science, engineering, and technology; it has a pivotal role in quantum information exchange, condensed matter systems, and in particular, the energy transfer in photosynthetic molecules. Photosynthetic complexes have evolved to harvest available light energy with remarkably high, nearly unitary quantum efficiency. While in most conventional quantum systems, especially in those used for quantum computation and engineering, uncontrollable environmental noise is perceived as a detriment for functionality and efficiency, multiple studies in light harvesting complexes have revealed that dephasing noise can constructively enhance the rate of energy transfer compared to purely coherent systems.

In this thesis, we develop a framework for studying the role of quantum coherence, noise, and multipartite entanglement in energy transfer dynamics of abstract quantum networks through analytical discussions and computational methods. Inspired by light harvesting systems such as the Fenna-Matthews-Olsen (FMO) complex, we model interacting two-level systems (TLS) in Markovian environments to evaluate transfer and entanglement dynamics in Fully Connected Networks (FCN), linear aggregates, and the 8-site model of the FMO complex.

In the first half, we study the population and multipartite entanglement dynamics of network models, verifying that dephasing noise can enhance excitation transfer by destroying localized excitations in an invariant subspace. By introducing an original method to measure multipartite entanglement using the quantum Fisher information (QFI) through Bayesian optimization, we are able to clearly see the generation and decay of global and partial entanglement during excitation transfer processes. We argue that multipartite entanglement is a reliable tool to characterize and detect state subspaces that inhibit efficient transport and delocalization. In the second half of the thesis, we extend our results of excitonic energy transfer to study similar dynamics of quantum networks coupled to a resonant optical cavity mode. By studying the polariton-exciton system and its dark state reservoir, especially in the strong coupling regime where unique entanglement and delocalization properties are expected to influence dynamics, we harvest the complementary effects of photon coupling with noise to control excitation transport: we demonstrate that polaritonic states can significantly enhance population transfer in very noisy systems by extending the regime of environment-assisted quantum transport (ENAQT), as well as facilitate long-range transfer through entanglement generation. We report evidence for entanglement retention in coupled light-matter systems during efficient energy transfer through strategic initial state preparation. We conclude with discussions of the role of multipartite entanglement, coherence, and dark states in excitation transport in general quantum networks, and propose opportunities for experimental realizations of optimized energy excitation transfer in cavity QED.