  A key obstacle to the experimental realization of many photonic
quantum-enhanced technologies is the lack of low-loss sources of single photons
in pure quantum states. We demonstrate a promising solution: generation of
heralded single photons in a silica photonic chip by spontaneous four-wave
mixing. A heralding efficiency of 40%, corresponding to a preparation
efficiency of 80% accounting for detector performance, is achieved due to
efficient coupling of the low-loss source to optical fibers. A single photon
purity of 0.86 is measured from the source number statistics without filtering,
and confirmed by direct measurement of the joint spectral intensity. We
calculate that similar high-heralded-purity output can be obtained from visible
to telecom spectral regions using this approach. On-chip silica sources can
have immediate application in a wide range of single-photon quantum optics
applications which employ silica photonics.
