Photosynthetic organisms use networks of chromophores to absorb and deliver solar energy to reaction centres. We present a detailed model of the light-harvesting complexes in purple bacteria, including explicit interaction with sunlight; radiative and non-radiative energy loss; and dephasing and thermalizing effects of coupling to a vibrational bath. We capture the effect of slow vibrations by introducing time-dependent disorder. Our model describes the experimentally observed high efficiency of light harvesting, despite the absence of long-range quantum coherence. The one-exciton part of the quantum state fluctuates continuously, but remains highly mixed at all times.These results suggest a relatively minor role for structure in determining efficiency. We build hypothetical models with randomly arranged chromophores, but still observe high efficiency when nearest-neighbour distances are comparable to those in nature. This helps explain the high transport efficiency in organisms with widely differing antenna structures, and suggests new design criteria for artificial light-harvesting devices.

Anisotropy pump-probe experiments have provided insights into the character of excitons formed in photosynthetic complexes. Rapid decay in the observed anisotropy is cited as evidence of the strength of coupling of the excitonic degrees of freedom to their environment. Here we show that ensemble averaging over realistic model Hamiltonians leads to a rapid decay of anisotropy to a value close to the observed asymptote, and at a rate comparable to observed decay rates, even in the absence of coupling to the environment. While coupling to the environment will clearly play a role in the dynamics of such systems, our calculations suggest that caution is needed in deducing the strength of this coupling from anisotropy experiments. We also set out to clarify the nature of the quantum states and processes involved in the dynamics of such systems and the associated terminology.

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