InGaP quantum nanophotonic integrated circuits with 1.5% nonlinearity-to-loss ratio

Optical nonlinearity plays a pivotal role in quantum information processing using photons, from heralded single-photon sources, coherent wavelength conversion to long-sought quantum repeaters. Despite the availability of strong dipole coupling to quantum emitters, achieving strong bulk optical nonlinearity is highly desirable. Here, we realize quantum nanophotonic integrated circuits in thin-film InGaP with a record-high ratio of $1.5\%$ between the single-photon nonlinear coupling rate ($g/2\pi=11.2$ MHz) and cavity-photon loss rate . We demonstrate second-harmonic generation with an efficiency of $71200\pm10300\%$/W in the InGaP photonic circuit and photon-pair generation via degenerate spontaneous parametric down-conversion with an ultrahigh rate exceeding 27.5 MHz/$\mu$W -- an order of magnitude improvement of the state-of-the-art -- and a large coincidence-to-accidental ratio up to $1.4\times 10^4$. Our work shows InGaP as a potentially transcending platform for quantum nonlinear optics and quantum information applications.