  Photon pair sources are fundamental building blocks for quantum entanglement
and quantum communication. Recent studies in silicon photonics have documented
promising characteristics for photon pair sources within the telecommunications
band, including sub-milliwatt optical pump power, high spectral brightness, and
high photon purity. However, most quantum systems suitable for local
operations, such as storage and computation, support optical transitions in the
visible or short near-infrared bands. In comparison to telecommunications
wavelengths, the significantly higher optical attenuation in silica at such
wavelengths limits the length scale over which optical-fiber-based quantum
communication between such local nodes can take place. One approach to connect
such systems over fiber is through a photon pair source that can bridge the
visible and telecom bands, but an appropriate source, which should produce
narrow-band photon pairs with a high signal-to-noise ratio, has not yet been
developed in an integrated platform. Here, we demonstrate a nanophotonic
visible-telecom photon pair source for the first time, using high quality
factor silicon nitride resonators to generate bright photon pairs with an
unprecedented coincidence-to-accidental ratio (CAR) up to (3800 +/- 200). We
further demonstrate dispersion engineering of the microresonators to enable the
connection of different species of trapped atoms/ions, defect centers, and
quantum dots to the telecommunications bands for future quantum communication
systems.
