  Photon-pair sources are critical building blocks for photonic quantum
systems. Leveraging Kerr nonlinearity and cavity-enhanced spontaneous four-wave
mixing, chip-scale photon-pair sources can be created using microresonators
built on photonic integrated circuit. For practical applications, a high
microresonator quality factor $Q$ is mandatory to magnify photon-pair sources'
brightness and reduce their linewidth. The former is proportional to $Q^4$,
while the latter is inversely proportional to $Q$. Here, we demonstrate an
integrated, microresonator-based, narrow-band photon-pair source. The
integrated microresonator, made of silicon nitride and fabricated using a
standard CMOS foundry process, features ultralow loss down to $3$ dB/m and
intrinsic $Q$ factor exceeding $10^7$. The photon-pair source has brightness of
$1.17\times10^9$ Hz/mW$^2$/GHz and linewidth of $25.9$ MHz, both of which are
record values for silicon-photonics-based quantum light source. It further
enables a heralded single-photon source with heralded second-order correlation
$g^{(2)}_\mathrm{h}(0)=0.0037(5)$, as well as a time-bin entanglement source
with a raw visibility of $0.973(9)$. Our work evidences the global potential of
ultralow-loss integrated photonics to create novel quantum light sources and
circuits, catalyzing efficient, compact and robust interfaces to quantum
communication and networks.
