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 and 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, to our knowledge, a record-high ratio of 1.5% between the single-photon nonlinear coupling rate (g /2π = 11.2 MHz) and cavity-photon loss rate. We demonstrate second-harmonic generation with an efficiency of 71200 ± 10300%/W in the InGaP photonic circuit and photon-pair generation via degenerate spontaneous parametric downconversion with an ultrahigh rate exceeding 27.5 MHz/µW—an order of magnitude improvement of the state of the art—and a large coincidence-to-accidental ratio up to 1.4 × 10⁴. Our work shows InGaP as a potentially transcending platform for quantum nonlinear optics and quantum information applications.