  Scaling beyond two-node quantum networks using nitrogen vacancy (NV) centers
in diamond is limited by the low probability of collecting zero phonon line
(ZPL) photons from single centers. Here, we demonstrate GaP-on-diamond disk
resonators which resonantly couple ZPL photons from single NV centers to
single-mode waveguides. In these devices, the probability of a single NV center
emitting a ZPL photon into the guided waveguide mode after optical excitation
can reach 9%, due to a combination of resonant enhancement of the ZPL emission
and efficient coupling between the resonator and waveguide. We verify the
single-photon nature of the emission and experimentally demonstrate both high
in-waveguide photon numbers and substantial Purcell enhancement for a set of
devices. These devices may enable scalable integrated quantum networks based on
NV centers.
