  Primitive photosynthetic cells appear over three billion years prior to any
other more complex life-forms, thus it is reasonable to assume that Nature has
designed a photosynthetic mechanism using minimal resources but honed to
perfection under the action of evolution. A number of different quantum models
have been proposed to understand the high degree of efficient energy transport,
most of them are limited to the scenario of single-exciton. Here we present a
study on the dynamics in light-harvesting complexes beyond the single exciton
limit, and show how this model describes the energy transfer in the
Fenna-Matthew-Olson (FMO) complex. We find that the energy transfer efficiency
above 90% under realistic conditions is achievable.
