Nanocavity Design for Reduced Spectral Diffusion of Solid-State Defects

Thenegativelychargednitrogen-vacancy(NV) center in diamond has an electronic spin state that can be optically initialized, manipulated, and measured. Entanglement generation between two spatially separated quantum memories can be generated by coupling them to optical modes. Coupling NV centers to nanophotonic devices such as waveguides and cavities will boost the NV-NV entanglement rate by increasing the emission and collection rate of photons entangled with the spin resonators.
We can fabricate 1D photonic crystal nanobeam cavities in diamond with quality factors larger than 16,000. Unfortunately, an optimally coupled NV center in such a cavity will be only 30 nm from surfaces, and the linewidths of NV centers in such cavities is increased to 10s of GHz (1000x the natural lifetime limited linewidth) due to spectral diffusion.

To obtain NV centers with GHz linewidths in a cavity with a high-quality factor, we design and fabricate novel “Alligator” cavities. A bandgap is created via a sinusoidal width modulation. A high-Q mode is trapped in a defect created by reducing the amplitude of the modulation. The optimized mode has a Q > 100,000 in simulation. We fabricate these cavities from single crystal bulk diamond. A scanning electron micrograph of one. In experiment, we measure cavities with a mean Q value of ~7000. We show the spectrum of such a cavity. These structures should allow coupling between single NV centers with limited spectral diffusion and high-quality factor cavity modes.