  One of the important nonclassical effects in quantum optics is the
anti-bunching, which has been observed in a large class of physical systems -
including light-harvesting antennas with cyclic structures. The units of the
ring couple with adjacent ones through dipole-dipole interactions. We show how
this strong dipole-dipole interaction leads to photon-blockade resulting in the
suppression of double excitation pathway and anti-bunching in photosynthesis
systems. The robustness of the photon blockade is demonstrated against the
disorder in the ring structures. We hypothesis that the effect may be utilized
by light-harvesting systems to avoid damages from excess energy.
