A central aim of quantum information processing is the efficient entanglement of multiple stationary quantum memories via photons. Among solid-state systems, the nitrogen-vacancy centre in diamond has emerged as an excellent optically addressable memory with second-scale electron spin coherence times. Recently, quantum entanglement and teleportation have been shown between two nitrogen-vacancy memories, but scaling to larger networks requires more efficient spin-photon interfaces such as optical resonators.Here we report such nitrogen-vacancy-nanocavity systems in the strong Purcell regime with optical quality factors approaching 10,000 and electron spin coherence times exceeding 200 ms using a silicon hard-mask fabrication process. This spin-photon interface is integrated with on-chip microwave striplines for coherent spin control, providing an efficient quantum memory for quantum networks.

The past decade has seen great advances in developing color centers in diamond for sensing, quantum information processing, and tests of quantum foundations. Increasingly, the success of these applications as well as fundamental investigations of light-matter interaction depend on improved control of optical interactions with color centers -from better fluorescence collection to efficient and precise coupling with confined single optical modes. Wide ranging research efforts have been undertaken to address these demands through advanced nanofabrication of diamond. This review will cover recent advances in diamond nano-and microphotonic structures for efficient light collection, color center to nanocavity coupling, hybrid integration of diamond devices with other material systems, and the wide range of fabrication methods that have enabled these complex photonic diamond systems. CONTENTS

Diamond photonics provides an attractive architecture to explore room temperature cavity quantum electrodynamics and to realize scalable multi-qubit computing. Here we review the present state of diamond photonic technology. The design, fabrication and characterization of a novel triangular cross section nanobeam cavity produced in a single crystal diamond is demonstrated. The present cavity design, based on a triangular cross section allows vertical confinement and better signal collection efficiency than that of slab-based nanocavities, and eliminates the need for a pre-existing membrane. The nanobeam is fabricated by Focused-Ion-Beam (FIB) patterning. The cavity is characterized by a confocal photoluminescence. The modes display quality factors of Q ~220 and are deviated in wavelength by only ~1.7nm from the NVcolor center zero phonon line (ZPL). The measured results are found in good agreement with 3D Finite-Difference-Time-Domain (FDTD) calculations. A more advanced cavity design with Q=22,000 is modeled, showing the potential for high-Q implementations using the triangular cavity design. The prospects of this concept and its application to spin nondemolition measurement and quantum computing are discussed.

A central challenge in developing magnetically coupled quantum registers in diamond is the fabrication of nitrogen vacancy (NV) centers with localization below ∼20 nm to enable fast dipolar interaction compared to the NV decoherence rate. Here, we demonstrate the targeted, high throughput formation of NV centers using masks with a thickness of 270 nm and feature sizes down to ∼1 nm. Superresolution imaging resolves NVs with a full-width maximum distribution of 26 ± 7 nm and a distribution of NV−NV separations of 16 ± 5 nm.A mong the hundreds of color centers in diamond, 1 only a few defects are known to have electron spin states that can be initialized 2 and measured optically. 2−5 These include as yet unidentified centers 5 and silicon vacancies 4 as well as the nitrogen vacancy (NV) center, which to date has been studied most extensively. 2,3 The NV electron spin triplet levels can be manipulated by microwave pulses 6 and exhibit coherence times exceeding milliseconds at room temperature 7,8 and approaching one second at liquid nitrogen temperature. 13 These exceptional properties have enabled demonstrations of qubit gates, 9,10 quantum registers, 11−13 and NV−photon 14 and NV−NV 15 entanglement. In particular, entanglement between two NV centers coupled via dipolar interactions was recently demonstrated at room temperature. 15 However, to extend this approach to larger numbers of coupled qubits, a technique for fabricating small ensembles of several NVs with separations of 5−20 nm is required to enable dipolar coupling faster than the NV electron spin decoherence rate. 16−23 These ensembles need to be sufficiently isolated from other NVs and other atomic defects, in particular nitrogen atoms, to avoid unnecessary background fluorescence and magnetic noise. One established way to realize such isolated clusters of spins is by nitrogen implantation and subsequent annealing. 24 Recently, spatially selective implantation of NVs has been reported based on nitrogen implantation through an electronbeam patterned resist mask (made of poly(methyl methacrylate) (PMMA)), 16,25 a mica nanochannel hard mask, 26 a pierced atomic force microscope (AFM) tip, 27 and directly by focused ion beam (FIB). 28 The smallest reported full-width half-maximum (FWHM) of implanted NV ensembles to date is ∼25 nm, achieved by sequential N implantation through a pierced AFM tip; however, individual NV centers were not resolved in this study. 27 By contrast, PMMA masks produced by electron beam lithography (EBL) enable high fabrication rate and implantation with single NV localization, but with a much broader FWHM of ∼60−80 nm. 6,20 Furthermore, to achieve high implantation isolation outside of the defined apertures, a high aspect ratio of the mask is required, as is the case for mica masks. 26 Here, we present an implantation technique based on masks produced from 270 nm thick silicon-on-oxide (SOI) membranes by a combination of EBL lithography and atomic layer deposition (ALD), enabling arbitrarily narrow (measured here below 1 nm) implant...