  Intensity squeezing, i.e., photon number fluctuations below the shot noise
limit, is a fundamental aspect of quantum optics and has wide applications in
quantum metrology. It was predicted in 1979 that the intensity squeezing could
be observed in resonance fluorescence from a two-level quantum system. Yet, its
experimental observation in solid states was hindered by inefficiencies in
generating, collecting and detecting resonance fluorescence. Here, we report
the intensity squeezing in a single-mode fibre-coupled resonance fluorescence
single-photon source based on a quantum dot-micropillar system. We detect
pulsed single-photon streams with 22.6% system efficiency, which show
subshot-noise intensity fluctuation with an intensity squeezing of 0.59 dB. We
estimate a corrected squeezing of 3.29 dB at the first lens. The observed
intensity squeezing provides the last piece of the fundamental picture of
resonance fluorescence; which can be used as a new standard for optical
radiation and in scalable quantum metrology with indistinguishable single
photons.
