  We discuss two-photon correlations from the side peaks that are formed when a
two-level system emitter is driven coherently, with a detuning between the
driving source and the emitter (quasi-resonance fluorescence). We do so in the
context of the theories of frequency-resolved photon correlations and
homodyning, showing that their combination leads to a neat picture compatible
with perturbative two-photon scattering that was popular in the early days of
quantum electrodynamics. This should help to control, enhance and open new
regimes of multiphoton emission. We also propose a way to evidence the quantum
coherent nature of the process from photoluminescence only, through the
observation of a collapse of the symmetry of the lineshape accompanied by a
surge of its intensity of emission. We discuss several of our results in the
light of recent experimental works.

Quantum Physics