Photosynthetic complexes are exquisitely tuned to capture solar light efficiently,
and then transmit the excitation energy to reaction centres, where long term energy
storage is initiated. The energy transfer mechanism is often described by semiclassical
models that invoke 'hopping' of excited-state populations along discrete energy levels.
Two-dimensional Fourier transform electronic spectroscopy has mapped these energy
levels and their coupling in the Fenna-Matthews-Olson (FMO) bacteriochlorophyll complex,
which is found in green sulphur bacteria and acts as an energy 'wire' connecting a
large peripheral light-harvesting antenna, the chlorosome, to the reaction centre.
The spectroscopic data clearly document the dependence of the dominant energy transport
pathways on the spatial properties of the excited-state wavefunctions of the whole
bacteriochlorophyll complex. But the intricate dynamics of quantum coherence, which
has no classical analogue, was largely neglected in the analyses-even though electronic
energy transfer involving oscillatory populations of donors and acceptors was first
discussed more than 70 years ago, and electronic quantum beats arising from quantum
coherence in photosynthetic complexes have been predicted and indirectly observed.
Here we extend previous two-dimensional electronic spectroscopy investigations of
the FMO bacteriochlorophyll complex, and obtain direct evidence for remarkably long-lived
electronic quantum coherence playing an important part in energy transfer processes
within this system. The quantum coherence manifests itself in characteristic, directly
observable quantum beating signals among the excitons within the Chlorobium tepidum
FMO complex at 77 K. This wavelike characteristic of the energy transfer within the
photosynthetic complex can explain its extreme efficiency, in that it allows the complexes
to sample vast areas of phase space to find the most efficient path.