Title: Ultralow-loss integrated photonics enables bright, narrow-band, photon-pair sources

Abstract: 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.