Fundamentals and applications of quantum light-matter interactions

Abstract

Quantum optics provides a deep understanding of light-matter interactions, revealing answers to fundamental questions while enabling the development of cutting-edge quantum technologies. This thesis explores both foundational and applied aspects of quantum optics, with a focus on structured photonic environments and the resulting collective radiative effects and entanglement generation. We aim to provide an interdisciplinary framework that unites the fields of quantum optics and nanophotonics in earlier chapters and quantum optics and quantum field theory in later chapters. Initially, we develop a quantum formalism to describe a pair of two-level dipoles near to the planar surface of a dielectric medium, extending the formalism originally developed in Ref.\cite{Knoll2003}. Using a Born-Markov master equation, we analyse the populations and coherences of the composite system in the Dicke basis so as to elicit hallmark quantum signatures: superradiance and subradiance. We motivate this formalism in the context of quantum technologies by studying a pair of two-level quantum emitters coupled to a dielectric metasurface via modes supported by the metasurface, known as bound states in the continuum (BICs). We compare two flavours of BIC, namely the electric dipole and the magnetic dipole BIC, in order to choose the optimal candidate for enhancement of collective radiative effects. The delocalised nature of such modes allows them to mediate long-range interactions between emitters, significantly enhancing collective effects and entanglement generation. By analysing the concurrence, an entanglement monotone, as a function of time and emitter separation, we demonstrate that BICs provide a robust mechanism for enhancement of collective effects and entanglement. Pertaining to the delocalised nature of the modes, the results suggest that platforms harnessing coupling to such modes are not bound by restrictions of deterministic emitter placement and hence are compatible with near-term devices that use random emitter growth. We explore how gauge choice affects quantum subsystem definitions and study the ensuing implications for subsystem predictions. We use toy models of a single and a two-dipole system interacting with a single photonic mode to investigate how different gauge choices influence energy exchange, and atom-atom and atom-photon entanglement. We do so by employing an arbitrary gauge formalism which encodes the inherent gauge freedom of the description. We find that gauge choice results in a mixing of light and matter degrees of freedom which, for weak coupling, has negligible effects on subsystem predictions, however for stronger couplings, choice of gauge becomes significant. Finally, we examine the Unruh effect from a quantum optics perspective by extending the the standard light-matter Hamiltonian prescription for the Unruh-DeWitt detector model to an arbitrary gauge description. We calculate single-mode transition probabilities and analyse how the Unruh effect and spontaneous emission manifests in different gauges. Subsequently, we derive an arbitrary gauge Born-Markov master equation to describe the Unruh effect and arrive at multi-mode transition amplitudes. We find that the standard prescription leads to gauge non-invariant results, and as such, it insufficient to fully capture the physics and we motivate the need for a more rigorous treatment.

Date of Award	29 May 2025
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Alessandro Principi (Co Supervisor) & Ahsan Nazir (Main Supervisor)