  Epitaxial quantum dots have emerged as one of the best single-photon sources,
not only for applications in photonic quantum technologies but also for testing
fundamental properties of quantum optics. One intriguing observation in this
area is the scattering of photons with subnatural linewidth from a two-level
system under resonant continuous wave excitation. In particular, an open
question is whether these subnatural linewidth photons exhibit simultaneously
antibunching as an evidence of single-photon emission. Here, we demonstrate
that this simultaneous observation of subnatural linewidth and antibunching is
not possible with simple resonant excitation. First, we independently confirm
single-photon character and subnatural linewidth by demonstrating antibunching
in a Hanbury Brown and Twiss type setup and using high-resolution spectroscopy,
respectively. However, when filtering the coherently scattered photons with
filter bandwidths on the order of the homogeneous linewidth of the excited
state of the two-level system, the antibunching dip vanishes in the correlation
measurement. Our experimental work is consistent with recent theoretical
findings, that explain antibunching from photon-interferences between the
coherent scattering and a weak incoherent signal in a skewed squeezed state.

Mesoscale and Nanoscale Physics;Quantum Physics