  Resonance fluorescence---the light emitted when exciting resonantly a
two-level system---is a popular quantum source as it seems to inherit its
spectral properties from the driving laser and its statistical properties from
the two-level system, thus providing a subnatural-linewidth single-photon
source. However, these two qualities do not actually coexist in resonance
fluorescence, since an optical target detecting these antibunched photons will
either be spectrally broad itself and not benefit from the spectrally narrow
source, or match spectrally with the source but in this case the antibunching
will be spoiled. We first explain this failure through a decomposition of the
field-emission and how this gets affected by frequency resolution. We then show
how to restore the sought joint subnatural linewidth and antibunched
properties, by interfering the resonance fluorescence output with a coherent
beam. We finally discuss how the signal that is eventually generated in this
way features a new type of quantum correlations, with a plateau of antibunching
which suppresses much more strongly close photon pairs. This introduces a new
concept of perfect single-photon source.
