  Resonance fluorescence (RF) of a two-level emitter displays persistently
anti-bunching irrespective of the excitation intensity, but inherits the
driving laser's linewidth under weak excitation. These properties are commonly
explained disjoinedly as the emitter's single photon saturation or passively
scattering light, until a recent theory attributes anti-bunching to the
laser-like spectrum's interference with the incoherently scattered light.
However, the theory implies higher-order scattering processes, and led to an
experiment purporting to validate an atom's simultaneous scattering of two
photons. If true, it could complicate RF's prospects in quantum information
applications. Here, we propose a unified model that treats all RF photons as
spontaneous emission, one at a time, and can explain simultaneously both the
RF's spectral and correlation properties. We theoretically derive the
excitation power dependencies, with the strongest effects measurable at the
single-photon incidence level, of the first-order coherence of the whole RF and
super-bunching of the spectrally filtered, followed by experimental
confirmation on a semiconductor quantum dot micro-pillar device. Furthermore,
our model explains peculiar coincidence bunching observed in phase-dependent
two-photon interference experiments. Our work provides novel understandings of
coherent light-matter interaction and may stimulate new applications.
