  We theoretically investigate polarization-filtered two-photon correlations
for the light emitted by a multichromophoric system undergoing excitation
transport under realistic exciton-phonon interactions, and subject to
continuous incoherent illumination. We show that for a biomolecular aggregate,
such as the Fenna-Matthews Olson (FMO) photosynthetic complex, time-asymmetries
in the cross-correlations of photons corresponding to different polarizations
can be exploited to probe both quantum coherent transport mechanisms and
steady-state coherence properties, which are not witnessed by zero-delay
correlations. A classical bound on correlation asymmetry is obtained, which FMO
is shown to violate using exact numerical calculations. Our analysis indicates
that the dominant contributions to time-asymmetry in such photon
cross-correlations are population to coherence transfer for Frenkel-Exciton
models. Our results therefore put forward photon correlation asymmetry as a
promising approach to investigate coherent contributions to excited-stated
dynamics in molecular aggregates and other many-site quantum emitters.
