  The idea that excitonic state (electronic) coherences are of fundamental
importance to natural photosynthesis gained popularity when, a decade ago,
slowly dephasing quantum beats were observed in the two-dimensional electronic
spectra of the Fenna-Matthews-Olson complex at 77 K. These were assigned to
quantum superpositions of excitonic states; a controversial interpretation, as
the spectral linewidths suggested fast dephasing arising from strong
interactions with the environment. While it has been pointed out that
vibrational motion produces similar spectral signatures, concrete assignment of
these coherences to distinct physical processes is still lacking. Here we
revisit the coherence dynamics of the Fenna-Matthews-Olson complex using
polarization-controlled two-dimensional electronic spectroscopy, supported by
theoretical modelling. We show that the long-lived quantum beats originate
exclusively from vibrational coherences, whereas electronic coherences dephase
entirely within 240 fs even at 77 K - a timescale too short to play a
significant role in light harvesting. Additionally, we demonstrate that
specific vibrational coherences are excited via vibronically coupled states.
The detection of vibronic coupling indicates the relevance of this phenomenon
for photosynthetic energy transfer.
