  We demonstrate photonic quantum circuits that operate at the stringent levels
that will be required for future quantum information science and technology.
These circuits are fabricated from silica-on-silicon waveguides forming
directional couplers and interferometers. While our focus is on the operation
of quantum circuits, to test this operation required construction of a
spectrally tuned photon source to produce near-identical pairs of photons. We
show non-classical interference with two photons and a two-photon entangling
logic gate that operate with near-unit fidelity. These results are a
significant step towards large-scale operation of photonic quantum circuits.
