  Color centers in diamond are promising spin qubits for quantum computing and
quantum networking. In photon-mediated entanglement distribution schemes, the
efficiency of the optical interface ultimately determines the scalability of
such systems. Nano-scale optical cavities coupled to emitters constitute a
robust spin-photon interface that can increase spontaneous emission rates and
photon extraction efficiencies. In this work, we introduce the fabrication of
2D photonic crystal slab nanocavities with high quality factors and cubic
wavelength mode volumes -- directly in bulk diamond. This planar platform
offers scalability and considerably expands the toolkit for classical and
quantum nanophotonics in diamond.
