  Entangled-photon pairs are an essential resource for quantum information
technologies. Chip-scale sources of entangled pairs have been integrated with
various photonic platforms, including silicon, nitrides, indium phosphide, and
lithium niobate, but each has fundamental limitations that restrict the
photon-pair brightness and quality, including weak optical nonlinearity or high
waveguide loss. Here, we demonstrate a novel, ultra-low-loss
AlGaAs-on-insulator platform capable of generating time-energy entangled
photons in a $Q$ $>1$ million microring resonator with nearly 1,000-fold
improvement in brightness compared to existing sources. The
waveguide-integrated source exhibits an internal generation rate greater than
$20\times 10^9$ pairs sec$^{-1}$ mW$^{-2}$, emits near 1550 nm, produces
heralded single photons with $>99\%$ purity, and violates Bell's inequality by
more than 40 standard deviations with visibility $>97\%$. Combined with the
high optical nonlinearity and optical gain of AlGaAs for active component
integration, these are all essential features for a scalable quantum photonic
platform.
