  Photonic integrated circuits (PICs) are attractive platforms to perform
large-scale quantum information processing. While highly-functional PICs (e.g.
silicon based photonic-circuits) and high-performance single photon sources
(SPSs, e.g. compound-semiconductor quantum dots (QDs)) have been independently
demonstrated, their combination for single-photon-based applications has still
been limited. This is largely due to the complexities of introducing SPSs into
existing PIC platforms, which are generally realized with different materials
and using distinct fabrication protocols. Here, we report a novel approach to
combine SPSs and PICs prepared independently. We employ transfer printing, by
which multiple desired SPSs can be integrated in a simple pick-and-place manner
with a theoretical waveguide coupling efficiency >99%, fulfilling the demanding
requirements of large-scale quantum applications. Experimentally, we
demonstrated QD-based SPSs with high waveguide coupling efficiencies, together
with the integration of two SPSs into a waveguide. Our approach will accelerate
scalable fusion between modern PICs and cutting-edge quantum technologies.
