The authors demonstrate on-chip diamond nanophotonics with a high-efficiency fiber-optic interface, achieving >90% power coupling at visible wavelengths. They use this approach to create a bright source of narrowband single photons, based on a silicon-vacancy color center embedded in a waveguide-coupled diamond photonic-crystal cavity. Their quantum nanophotonic interface yields a high flux of coherent single photons into a single-mode fiber, enabling possibilities for quantum networks that couple multiple emitters, either on the same chip or separated by long distances.
Diamond photonics has enabled efficient interfaces for quantum memories and is predicted to be a critical component of quantum networks. However, scalable network architectures require spatial, temporal, and spectral control of photons, which relies on nonlinear and electro-optic functionalities that diamond alone cannot provide. Here, we demonstrate heterogeneous integration of a thin-film lithium niobate (TFLN) platform, which has strong chi-2 nonlinearity and electro-optic effects, with thin diamond films. We demonstrate high-Q diamond photonic crystal cavities (Q factors exceeding 5x10^4 at 735 nm) that are lithographically aligned with TFLN photonic backbone and critically coupled to it. This allows us to realize low-loss diamond-TFLN "escalators" (loss ~1 dB/coupler) that support efficient light transfer between them. At cryogenic temperatures (5K), we can collect photons emitted from silicon vacancies (SiVs) embedded within the diamond structure via the TFLN photonic circuit. This approach establishes a scalable route toward integrated photonic circuits for practical quantum networking and other technologies.