  A central requirement for photonic quantum information processing systems
lies in the combination of nonclassical light sources and low-loss,
phase-stable optical modes. While substantial progress has been made separately
towards ultra-low loss, $\leq1$ dB/m, chip-scale photonic circuits and high
brightness single-photon sources, integration of these technologies has
remained elusive. Here, we report a significant advance towards this goal, in
the hybrid integration of a quantum emitter single-photon source with a
wafer-scale, ultra-low loss silicon nitride photonic integrated circuit. We
demonstrate triggered and pure single-photon emission directly into a
Si$_3$N$_4$ photonic circuit with $\approx1$ dB/m propagation loss at a
wavelength of $\approx920$ nm. These losses are more than two orders of
magnitude lower than reported to date for any photonic circuit with on-chip
quantum emitter sources, and $>50$ % lower than for any prior
foundry-compatible integrated quantum photonic circuit, to the best of our
knowledge. Using these circuits we report the observation of resonance
fluorescence in the strong drive regime, a milestone towards integrated
coherent control of quantum emitters. These results constitute an important
step forward towards the creation of scaled chip-integrated photonic quantum
information systems.
