  We demonstrate a quantum nanophotonics platform based on germanium-vacancy
(GeV) color centers in fiber-coupled diamond nanophotonic waveguides. We show
that GeV optical transitions have a high quantum efficiency and are nearly
lifetime-broadened in such nanophotonic structures. These properties yield an
efficient interface between waveguide photons and a single GeV without the use
of a cavity or slow-light waveguide. As a result, a single GeV center reduces
waveguide transmission by $18 \pm 1\%$ on resonance in a single pass. We use a
nanophotonic interferometer to perform homodyne detection of GeV resonance
fluorescence. By probing the photon statistics of the output field, we
demonstrate that the GeV-waveguide system is nonlinear at the single-photon
level.
