Quantum emitters can be near-ideal single-photon sources. However, a single-photon source is only as good as its output efficiency, and collection of single photons can be as challenging as their generation. Optical cavities are a frequently used solution: by exploiting the Purcell effect, single photons can be efficiently collected from quantum emitters. Owing to their high quality factor and small mode volume, suspended photonic crystal cavities are particularly suitable for this task and are compatible with a wide range of quantum emitters, e.g. diamond color centers. Alajlan et al. demonstrate Si₃N₄ photonic crystal cavities with quality factors exceeding 10⁴ and highly efficient (96%) output coupling. The authors show two different photonic crystal cavity designs, one using holes to create refractive index contrast and one using an ‘alligator’ design combining holes and extrusions. Both designs have quality factors exceeding 10⁴. To attain high coupling efficiency from the cavity to a travelling optical mode, the authors use a tapered, single-ended optical fiber to adiabatically couple to a tapered waveguide. These single-ended optical fibers can be reproducibly fabricated by etching optical fibers in hydrofluoric acid. The demonstrated devices can be coupled to diamond color centers, quantum dots and 2D materials.

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