Quantum Nonlinear Optics with a Germanium-Vacancy Color Center in a   Nanoscale Diamond Waveguide
  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. 
Mihir K. Bhaskar, Denis D. Sukachev, Alp Sipahigil, Ruffin E. Evans,   Michael J. Burek, Christian T. Nguyen, Lachlan J. Rog...
quant-ph cond-mat.mes-hall physics.optics