Ultralow-loss integrated photonics enables bright, narrow-band, 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, 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

A chip-integrated comb-based microwave oscillator

Low-noise microwave oscillators are cornerstones for wireless communication, radar and clocks. Optical frequency combs have enabled photonic microwaves with unrivalled noise performance and bandwidth. Emerging interest is to generate microwaves using chip-based frequency combs, namely microcombs. Here, we demonstrate the first, fully integrated, microcomb-based, microwave oscillator chip. The chip, powered by a microelectronic circuit, leverages hybrid integration of a DFB laser, a nonlinear microresonator, and a high-speed photodetector. Each component represents the best of its own class, yet allows large-volume manufacturing with low cost in CMOS foundries. The hybrid chip outputs an ultralow-noise laser of 6.9 Hz linewidth, a microcomb of 10.7 GHz repetition rate, and a 10.7 GHz microwave of 6.3 mHz linewidth -- all three in one entity of 76 mm$^2$ size.The microwave phase noise reaches -75/-105/-130 dBc/Hz at 1/10/100 kHz Fourier offset frequency. Our results can reinvigorate our information society for communication, sensing, timing and precision measurement