  Noise-assisted transport in quantum systems occurs when quantum
time-evolution and decoherence conspire to produce a transport efficiency that
is higher than what would be seen in either the purely quantum or purely
classical cases. In disordered systems, it has been understood as the
suppression of coherent quantum localisation through noise, which brings
detuned quantum levels into resonance and thus facilitates transport. We report
several new mechanisms of environment-assisted transport in ordered systems, in
which there is no localisation to overcome and where one would naively expect
that coherent transport is the fastest possible. Although we are particularly
motivated by the need to understand excitonic energy transfer in photosynthetic
light-harvesting complexes, our model is general---transport in a tight-binding
system with dephasing, a source, and a trap---and can be expected to have wider
application.
