  Excitonic transport in photosynthesis exhibits a wide range of time scales.
Absorption and initial relaxation takes place over tens of femtoseconds.
Excitonic lifetimes are on the order of a nanosecond. Hopping rates, energy
differences between chromophores, reorganization energies, and decoherence
rates correspond to time scales on the order of picoseconds. The functional
nature of the divergence of time scales is easily understood: strong coupling
to the electromagnetic field over a broad band of frequencies yields rapid
absorption, while long excitonic lifetimes increase the amount of energy that
makes its way to the reaction center to be converted to chemical energy. The
convergence of the remaining time scales to the centerpoint of the overall
temporal range is harder to understand. In this paper we argue that the
convergence of timescales in photosynthesis can be understood as an example of
the `quantum Goldilocks effect': natural selection tends to drive quantum
systems to the degree of quantum coherence that is `just right' for attaining
maximum efficiency. We provide a general theory of optimal and robust,
efficient transport in quantum systems, and show that it is governed by a
single parameter.
