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 dB/cm and intrinsic Q factor exceeding 10^7. The photon-pair source has brightness of 1. 17×10^9 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 g₇^ (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.