Photon anti-bunching is the process in Resonance Fluorescence through which rate at which photons are emitted by a two-level atom is limited. A two-level atom is only capable of absorbing a photon from the driving electromagnetic field after a certain period of time has passed. This time period is modeled as a probability distribution  where  as . As the atom cannot absorb a photon, it is unable to emit one and thus there is a restriction on the spectral density. This is illustrated by the second order correlation function 
From the above equation it is clear that  and thus  resulting in the relation that describes photon antibunching
This shows that the power cannot be anything other than zero for . In the weak field approximation  can only increase monotonically as  increases, however in the strong field approximation  oscillates as it increases. These oscillations die off as .
The physical idea behind photon anti-bunching is that while the atom itself is ready to be excited as soon as it releases its previous photon, the electromagnetic field created by the laser takes time to excite the atom.