  Color centers in diamond provide a promising platform for quantum optics in
the solid state, with coherent optical transitions and long-lived electron and
nuclear spins. Building upon recent demonstrations of nanophotonic waveguides
and optical cavities in single-crystal diamond, we now demonstrate on-chip
diamond nanophotonics with a high efficiency fiber-optical interface, achieving
> 90% power coupling at visible wavelengths. We use this approach to
demonstrate a bright source of narrowband single photons, based on a
silicon-vacancy color center embedded within a waveguide-coupled diamond
photonic crystal cavity. Our fiber-coupled diamond quantum nanophotonic
interface results in a high flux of coherent single photons into a single mode
fiber, enabling new possibilities for realizing quantum networks that interface
multiple emitters, both on-chip and separated by long distances.
