  Large scale simulations of light-matter interaction in natural photosynthetic
antenna complexes containing more than one hundred thousands chlorophyll
molecules, comparable with natural size, have been performed. Photosynthetic
antenna complexes present in Green sulfur bacteria and Purple bacteria have
been analyzed using a radiative non-Hermitian Hamiltonian, well known in the
field of quantum optics, instead of the widely used dipole-dipole Frenkel
Hamiltonian. This approach allows to study ensembles of emitters beyond the
small volume limit (system size much smaller than the absorbed wavelength),
where the Frenkel Hamiltonian fails. When analyzed on a large scale, such
structures display superradiant states much brighter then their single
components. An analysis of the robustness to static disorder and dynamical
(thermal) noise, shows that exciton coherence in the whole photosynthetic
complex is larger than the coherence found in its parts. This provides evidence
that the photosynthetic complex as a whole has a predominant role in sustaining
coherences in the system even at room temperature. Our results allow a better
understanding of natural photosynthetic antennae and could drive experiments to
verify how the response to the electromagnetic radiation depends on the size of
the photosynthetic antenna.
