1. Department of Chemistry & QB3 Institute, University of California, Berkeley
   
2. Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
   
3. Department of Biology,
   
4. Department of Chemistry, Washington University, St Louis, Missouri 63130, USA
   
5. Present address: Institute of Physics of Charles University, 12116 Prague 2, Czech Republic.

Correspondence to: Graham R. Fleming^1,2 Correspondence and requests for materials should be addressed to G.R.F. (Email: grfleming@lbl.gov).

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^{1, 2}. Two-dimensional Fourier transform electronic spectroscopy^{3, 4, 5} has mapped⁶ 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^{7, 8, 9}. 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^{6, 10}. 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 ago¹¹, and electronic quantum beats arising from quantum coherence in photosynthetic complexes have been predicted^{12, 13} and indirectly observed¹⁴. 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.