  Several recent studies of energy transfer in photosynthetic light harvesting
complexes have revealed a subtle interplay between coherent and decoherent
dynamic contributions to the overall transfer efficiency in these open quantum
systems. In this work we systematically investigate the impact of temporal and
spatial correlations in environmental fluctuations on excitation transport in
the Fenna-Matthews-Olson photosynthetic complex. We demonstrate that the exact
nature of the correlations can have a large impact on the efficiency of light
harvesting. In particular, we find that (i) spatial correlations can enhance
coherences in the site basis while at the same time slowing transport, and (ii)
the overall efficiency of transport is optimized at a finite temporal
correlation that produces maximum overlap between the environmental power
spectrum and the excitonic energy differences, which in turn results in
enhanced driving of transitions between excitonic states.
