Vibrationally-assisted collective quantum optical effects

Abstract

This thesis considers a variety of systems which are designed to take advantage of collective optical effects in the presence of a vibrational environment; a necessary condition for such systems to exist on a quantum platform outside of superconducting qubits. The first research chapter considers the effect of superabsorption, the time-reversed process of Dicke superradiance. A series of conditions for a guide-slide superabsorber are proposed, which allow a system to sacrifice some optical coupling for the benefit of still operating when coupled to a phonon bath. We suggest a system geometry which meets these properties, and then use this as a case study to show such a system does indeed display the hallmarks of superabsorbing behaviour. This remains when disorder is introduced to the system, as well as with a strongly-coupled vibrational environment in the polaron frame, amongst several other model extensions. The second research chapter looks at optical ratcheting, a process whereby an artificial light-harvester with an extraction bottleneck can improve performance by relaxing to a state which is dark with respect to optical relaxation, while still being able to absorb more photons. We examine how the performance scales with system size before looking at how the phenomena performs in the polaron frame. While strong vibrational coupling can be the undoing of collective optical effects, we find that it is still possible to observe ratcheting in the strong-coupling regime. We use the model as a platform to investigate more accurate means of incorporating extraction via a trap, by treating it as an additional dipole in the system. This change means one needs to consider both the optical and geometric properties of the trap to achieve optimal performance. The third research chapter stems from collaborative work with quantum biologists. We study the recently resolved structure of a photosynthetic complex, iron stress-induced protein A (IsiA), to establish if the structure allows the complex to utilise vibrational relaxation into a collectively dark delocalised state to reduce the likelihood of loss. By distorting different components, we establish which parts of the structure this effect is susceptible to changes in, providing insight into the function of the complex. Finally, the fourth research chapter investigates the feasibility of using a molecular aggregate of two identical absorbers as a gain medium in a laser. With adequate control of the geometry, we show that the combination of collective optical coupling and rapid vibrational relaxation make population inversion possible. By coupling a disordered collection of such systems to a resonant cavity, we demonstrate that a stable laser field can be generated. The results of this work support an approach derived by our collaborators, allowing larger, more complex aggregates to be used instead, which require less fine control over the molecular geometry to achieve lasing behaviour.