  We report the observation of an anomalous antibunching dip in intensity
autocorrelation function with photon correlation measurements on a
single-photon emitter (SPE). We show that the anomalous dip observed is a
manifestation of quantum nature of SPEs. Taking population dynamics in a
quantum two-level system into account correctly, we redefine intensity
autocorrelation function. This is of primary importance for precisely
evaluating the lowest-level probability of multiphoton generation in SPEs
toward realizing versatile pure SPEs for quantum information and communication.
