  We demonstrate quasi-phase matched, triply-resonant sum frequency conversion
in 10.6-um-diameter integrated gallium phosphide ring resonators. A
small-signal, waveguide-to-waveguide power conversion efficiency of 8%/mW is
measured for conversion from telecom (1536 nm) and near infrared (1117 nm) to
visible (647 nm) wavelengths with an absolute power conversion efficiency of
6.3% measured at saturation pump power. For the complementary difference
frequency generation process, a single photon conversion efficiency of 7.2%/mW
from visible to telecom is projected for resonators with optimized coupling.
Efficient conversion from visible to telecom will facilitate long-distance
transmission of spin-entangled photons from solid-state emitters such as the
diamond NV center, allowing long-distance entanglement for quantum networks.
