  Recent evidence suggests that quantum effects may have functional importance
in biological light-harvesting systems. Along with delocalized electronic
excitations, it is now suspected that quantum coherent interactions with
certain near-resonant vibrations may contribute to light-harvesting
performance. However, the actual quantum advantage offered by such coherent
vibrational interactions has not yet been established. We investigate a quantum
design principle, whereby coherent exchange of single energy quanta between
electronic and vibrational degrees of freedom can enhance a light-harvesting
system's power above what is possible by thermal mechanisms alone. We present a
prototype quantum heat engine which cleanly illustrates this quantum design
principle, and quantify its quantum advantage using thermodynamic measures of
performance. We also demonstrate the principle's relevance in parameter regimes
connected to natural light-harvesting structures.
