  Quantum information processing holds great promise for communicating and
computing data efficiently. However, scaling current photonic implementation
approaches to larger system size remains an outstanding challenge for realizing
disruptive quantum technology. Two main ingredients of quantum information
processors are quantum interference and single-photon detectors. Here we
develop a hybrid superconducting-photonic circuit system to show how these
elements can be combined in a scalable fashion on a silicon chip. We
demonstrate the suitability of this approach for integrated quantum optics by
interfering and detecting photon pairs directly on the chip with
waveguide-coupled single-photon detectors. Using a directional coupler
implemented with silicon nitride nanophotonic waveguides, we observe 97%
interference visibility when measuring photon statistics with two
monolithically integrated superconducting single photon detectors. The photonic
circuit and detector fabrication processes are compatible with standard
semiconductor thin-film technology, making it possible to implement more
complex and larger scale quantum photonic circuits on silicon chips.
