  Photosynthetic organisms use networks of chromophores to absorb sunlight and
deliver the energy to reaction centres, where charge separation triggers a
cascade of chemical steps to store the energy. We present a detailed model of
the light-harvesting complexes in purple bacteria, including explicit
interaction with sunlight; energy loss through radiative and non-radiative
processes; and dephasing and thermalizing effects of coupling to a vibrational
bath. An important feature of the model is that we capture the effect of slow
vibrational modes 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 due to slow vibrational changes, but remains highly mixed at
all times. This lack of long-range coherence suggests a relatively minor role
for structure in determining the efficiency of bacterial light harvesting. To
investigate this we built hypothetical models with randomly arranged
chromophores, but still observed high efficiency when typical nearest-neighbour
distances are comparable with those found in nature. This helps to explain the
efficiency of energy transport in organisms whose chromophore networks differ
widely in structure, while also suggesting new design criteria for efficient
artificial light-harvesting devices.
