Single-crystal diamond, with its unique optical, mechanical and thermal properties, has emerged as a promising material with applications in classical and quantum optics. However, the lack of heteroepitaxial growth and scalable fabrication techniques remains the major limiting factors preventing more wide-spread development and application of diamond photonics. In this work, we overcome this difficulty by adapting angled-etching techniques, previously developed for realization of diamond nanomechanical resonators, to fabricate racetrack resonators and photonic crystal cavities in bulk single-crystal diamond. Our devices feature large optical quality factors, in excess of 10 5 , and operate over a wide wavelength range, spanning visible and telecom. These newly developed high-Q diamond optical nanocavities open the door for a wealth of applications, ranging from nonlinear optics and chemical sensing, to quantum information processing and cavity optomechanics.

We report on the fabrication and characterization of a Fabry-Perot microcavity enclosing a thin diamond membrane at cryogenic temperatures. The cavity is designed to enhance resonant emission of single nitrogen-vacancy centers by allowing spectral and spatial tuning while preserving the optical properties observed in bulk diamond. We demonstrate cavity finesse at cryogenic temperatures within the range of F=4000–12 000 and find a sub-nanometer cavity stability. Modeling shows that coupling nitrogen-vacancy centers to these cavities could lead to an increase in remote entanglement success rates by three orders of magnitude.

Coupling nitrogen-vacancy centers in diamond to optical cavities is a promising way to enhance the efficiency of diamond based quantum networks. An essential aspect of the full toolbox required for the operation of these networks is the ability to achieve microwave control of the electron spin associated with this defect within the cavity framework. Here, we report on the fabrication of an integrated platform for microwave control of an NV center electron spin in an open, tunable Fabry-Pérot microcavity. A critical aspect of the measurements of the cavity's finesse reveals that the presented fabrication process does not compromise its optical properties. We provide a method to incorporate a thin diamond slab into the cavity architecture and demonstrate control of the NV center spin. These results show the promise of this design for future cavity-enhanced NV center spin-photon entanglement experiments.Nitrogen-vacancy (NV) colour centers in diamond have emerged as attractive candidates for quantum photonic applications. Their electronic spin can be optically initialized, read out in a single shot 1 , and coherently manipulated with the use of microwave signals 2 . This spin-photon interface provides a platform for distant entanglement generation 3 , while additional coupling to nearby carbon-13 nuclear spins forms a multi-qubit quantum node 4-7 . These aspects make the NV center a good candidate for quantum network protocols 8-10 . The efficiency of entanglement generation between network nodes is currently limited by the NV center's low (≈ 3%) emission rate of the resonant zero-phonon line (ZPL) photons. This problem can be addressed by coupling NV centers to optical microcavities 11-23 , enhancing the ZPL emission rate and providing efficient photon extraction by means of the Purcell effect 24 . An appealing cavity design consists of an open, tunable Fabry-Pérot microcavity housing a large area diamond membrane [25][26][27] in which emitters retain their bulk-like properties 28 . The tunability of this design enables both spectral positioning of the cavity to be resonant with the emitter as well as selective lateral placement of the emitter within the center of the cavity mode. However, in order to use these emitters in quantum information protocols, microwave control must be integrated into the cavity architecture. Here, we present fabrication methods used to create a platform that integrates microwave control of an NV center spin within an optical cavity while maintaining the cavity's high finesse properties. While microwave addressing of a single NV center spin has already been realized in thin diamond slabs 29 and photonic crystal cavities 21 , this is the first demonstration of NV center spin addressing within a framework tailored to the implementation of a tunable microcavity.The cavity consists of a dimpled fiber tip and polished fused silica plate, both coated with a highly reflective dielectric mirror stack (Figure 1(A)). Microwave striplines and marker arrays are fabricated on the planar mirror surface...

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