Departmental Oral Examination (Thesis Title: “Modelling exciton dynamics in light-harvesting molecules”)

I investigate the dynamics of multi-state central systems coupled bilinearly to an external oscillator bath within the noninteracting-blip approximation. I focus on both a 3-site configuration, as well as a 2-site model for the central systems of interest.
Both diagonal and non-diagonal system-bath couplings are considered, and for the case of the 2-site central system, a dual coupling approach is taken. The bath spectral densities considered in this work include both Ohmic and super-Ohmic forms as well as single optical phonon peaks. This work is motivated by the recent observance of long-lived quantum coherence effects in the photosynthetic organism known as the Fenna-Matthews-Olson complex.
The models investigated in this thesis are applied to this system in an attempt to reproduce the experimentally observed coherence times, and potentially explain the underlying physical mechanisms responsible for these observations. The dual-coupling-2-site model is shown to reproduce the relatively long coherence times observed in the Fenna-Matthews-Olson complex remarkably well. The non-diagonal system-bath coupling is shown to play a crucial role in this process, not only increasing the decoherence times in the system, but also increasing the dimer oscillation frequency in accordance with the well-known phonon-assisted transfer mechanism. These findings suggest that the physical mechanism responsible for the observed quantum-coherence effects in photosynthesis might be the presence of non-diagonal system-bath couplings.