  In this work, we demonstrate the on-chip two-photon interference between
single photons emitted by a single self-assembled InGaAs quantum dot and an
external laser. The quantum dot is embedded within one arm of an air-clad
directional coupler which acts as a beam-splitter for incoming light. Photons
originating from an attenuated external laser are coupled to the second arm of
the beam-splitter and then combined with the quantum dot photons, giving rise
to two-photon quantum interference between dissimilar sources. We verify the
occurrence of on-chip Hong-Ou-Mandel interference by cross-correlating the
optical signal from the separate output ports of the directional coupler. This
experimental approach allows us to use classical light source (laser) to assess
in a single step the overall device performance in the quantum regime and probe
quantum dot photon indistinguishability on application realistic time scales.
