  The interaction of light with a single two-level emitter is the most
fundamental process in quantum optics, and is key to many quantum applications.
As a distinctive feature, two photons are never detected simultaneously in the
light scattered by the emitter. This is commonly interpreted by saying that a
single two-level quantum emitter can only absorb and emit single photons.
However, it has been theoretically proposed that the photon anti-correlations
can be thought to arise from quantum interference between two possible
two-photon scattering amplitudes, which one refers to as coherent and
incoherent. This picture is in stark contrast to the aforementioned one, in
that it assumes that the atom even has two different mechanisms at its disposal
to scatter two photons at the same time. Here, we validate the interference
picture by experimentally verifying the 40-year-old conjecture that, by
spectrally rejecting only the coherent component of the fluorescence light of a
single two-level atom, the remaining light consists of photon pairs that have
been simultaneously scattered by the atom. Our results offer fundamental
insights into the quantum-mechanical interaction between light and matter and
open up novel approaches for the generation of highly non-classical light
fields.
