  We describe some of the main external mechanisms that lead to a loss of
antibunching, i.e., that spoil the character of a given quantum light to
deliver its photons separated the ones from the others. Namely, we consider
contamination by noise, a time jitter in the photon detection and the effect of
frequency filtering (or detection with finite bandwidth). The emission from a
two-level system under both incoherent and coherent driving is taken as a
particular case of special interest. The coherent case is further separated
into its vanishing (Heitler) and high (Mollow) driving regimes. We provide
analytical solutions which, in the case of filtering, unveil an unsuspected
structure in the transitions from perfect antibunching to thermal (incoherent
case) or uncorrelated (coherent case) emission. The experimental observations
of these basic and fundamental transitions would provide another compelling
evidence of the correctness and importance of the theory of frequency-resolved
photon correlations.
