  Energy transfer between chromophores in photosynthesis proceeds with near
unity quantum efficiency. Understanding the precise mechanisms of these
processes is made difficult by the complexity of the electronic structure and
interactions with different vibrational modes. Two-dimensional spectroscopy has
helped resolve some of the ambiguities and identified quantum effects that may
be important for highly efficient energy transfer. Many questions remain,
however, including whether the coherences observed are electronic and/or
vibrational in nature and what role they play. We utilise a two-colour
four-wave mixing experiment with control of the wavelength and polarization to
selectively excite specific coherence pathways. For the light-harvesting
complex PC645, from cryptophyte algae, we reveal and identify specific
contributions from both electronic and vibrational coherences and determine an
excited state structure based on two strongly-coupled electronic states and two
vibrational modes. Separation of the coherence pathways also uncovers the
complex evolution of these coherences and the states involved.
