Ultralow-Loss Integrated Photonics Enables Bright, Narrowband, Photon-Pair Sources

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, narrowband photon-pair source. The integrated microresonator, made of silicon nitride and fabricated using a standard CMOS foundry process, features ultralow loss down to $0.03\text{ }\text{ }\mathrm{dB}/\mathrm{cm}$ and intrinsic $Q$ factor exceeding ${10}^{7}$. The photon-pair source has brightness of $1.17\ifmmode\times\else\texttimes\fi{}{10}^{9}\text{ }\text{ }\mathrm{Hz}/{\mathrm{mW}}^{2}/\mathrm{GHz}$ 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 ${g}_{\mathrm{h}}^{(2)}(0)=0.0037(5)$, as well as an energy-time 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.