Transition from antibunching to bunching for two dipole-interacting   atoms
  It is known that there is a transition from photon antibunching to bunching in the resonance fluorescence of a driven system of two two-level atoms with dipole-dipole interaction when the atomic distance decreases and the other parameters are kept fixed. We give a simple explanation for the underlying mechanism which in principle can also be applied to other systems. PACS numbers 42.50.Ar, 42.50Fx 
Almut Beige and Gerhard C. Hegerfeldt