Solid-state quantum sensors are attracting wide interest because of their sensitivity at room temperature. In particular, the spin properties of individual nitrogen-vacancy (NV) colour centres in diamond 1-3 make them outstanding nanoscale sensors of magnetic fields 4-9 , electric fields 10 and temperature 11-13 under ambient conditions. Recent work on NV ensemble-based magnetometers 14-16 , inertial sensors 17 , and clocks 18 has employed unentangled colour centres to realize significant improvements in sensitivity 15,19 . However, to achieve this potential sensitivity enhancement in practice, new techniques are required to excite e ciently and to collect the optical signal from large NV ensembles. Here, we introduce a light-trapping diamond waveguide geometry with an excitation e ciency and signal collection that enables in excess of 5% conversion e ciency of pump photons into optically detected magnetic resonance 20 (ODMR) fluorescence-an improvement over previous single-pass geometries of more than three orders of magnitude. This marked enhancement of the ODMR signal enables precision broadband measurements of magnetic field and temperature in the low-frequency range, otherwise inaccessible by dynamical decoupling techniques.The NV's low absorption cross-section 21 has thus far prevented high conversion efficiency from excitation power to NV ODMR signal in bulk diamond samples using the typical single-pass excitation geometry illustrated in Fig. 1a. For NV densities around 10 15 cm −3 (refs 5,22), for which the NV spacing in the diamond lattice is sufficiently sparse to maintain long spin coherence times 6 , up to metre-long path length would be necessary for significant excitation absorption (see Supplementary Information). Efficient collection has been demonstrated from NVs in singlepass excitation geometries 14,15 , but absorption is less than 1% for a typical 300-µm-thick, electron-irradiated, type IIa diamond sample. Wide-field microscopy implemented with CCD (chargecoupled device) cameras can collect fluorescence from ensembles of up to 10 3 NV centres 23 in the focal volume of the objective, but suffers from low collection efficiency with most fluorescence emission trapped within the diamond owing to total internal reflection (TIR) at the diamond-air interface. External Fabry-Perot cavities could increase the optical depth 16 for green excitation, but would introduce bandwidth restrictions as well as the additional complexity of stabilized, narrow-linewidth excitation lasers.We have designed and realized a new ensemble measurement scheme that overcomes these limitations. The light-trapping diamond waveguide (LTDW) consists of a rectangular diamond slab with a small angled facet at one corner for input coupling of the pump beam, as illustrated in Fig. 1b. The input facet has a length a at an angle 45 • relative to the square sample sides, allowing pump light to couple into the structure while being confined by TIR (θ > θ c = 24.6 • ) on the other surfaces. Figure 1c shows an optical image of a pump beam...

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

We demonstrate a photonic circuit with integrated long-lived quantum memories. Precharacterized quantum nodes-diamond microwaveguides containing single, stable, negatively charged nitrogenvacancy centers-are deterministically integrated into low-loss silicon nitride waveguides. These quantum nodes efficiently couple into the single-mode waveguides with >1 Mcps collected into the waveguide, have narrow single-scan linewidths below 400 MHz, and exhibit long electron spin coherence times up to 120 μs. Our system facilitates the assembly of multiple quantum nodes with preselected properties into a photonic integrated circuit with near unity yield, paving the way towards the scalable fabrication of quantum information processors. DOI: 10.1103/PhysRevX.5.031009 Subject Areas: Photonics, Quantum InformationAdvanced quantum information systems, such as quantum computers [1] and quantum repeaters [2], require multiple entangled quantum memories that can be individually controlled [3]. Over the past decade, there has been rapid theoretical and experimental progress in developing such entangled networks using stationary quantum bits (qubits) connected via photons [4][5][6]. Photonic integrated circuits (PICs) could provide a compact, phase-stable, and scalable architecture for such quantum networks. However, the realization of this promise requires near-unity-yield fabrication of high-quality solid-state quantum memories efficiently coupled to low-loss single-mode waveguides.A promising solid-state quantum memory with secondscale spin coherence times is the negatively charged nitrogen-vacancy (NV) center in diamond [7,8]. Its electronic spin state can be optically initialized, manipulated, measured [9,10], and mapped onto nearby auxiliary nuclear memories [11], which allows for quantum error correction [12]. Quantum network protocols based on these unique qualities have been proposed [13,14], and entanglement generation and state teleportation between two spatially separated quantum memories have been demonstrated [15,16]. Translating such entanglement techniques into an on-chip architecture promises scalability, but can only succeed if an efficient photonic architecture can be fabricated with a high yield. So far, waveguide patterning in diamond is challenging, preventing low-loss waveguides in the optical domain around 637 nm, the zero phonon line (ZPL) of the NV. This is in contrast to silicon nitride (SiN)-based photonics, which relies on well-developed fabrication processes [17], is CMOS compatible [18], and has a large band gap (∼5 eV) and high index of refraction (n ¼ 2.1), which makes it ideal for routing the visible emission of NVs.A second challenge in creating a scalable solid-state quantum network architecture is the inherently low yield production of high-quality quantum nodes due to the stochastic process of NV creation. The number of fabrication attempts necessary to create a monolithic network in which all nodes contain NV quantum memories with the desired spectral and spin properties scales exponentially...

Efficient collection of the broadband fluorescence from the diamond nitrogen vacancy (NV) center is essential for a range of applications in sensing, on-demand single photon generation, and quantum information processing. Here, we introduce a circular "bullseye" diamond grating which enables a collected photon rate of (2.7 ± 0.09) × 10(6) counts per second from a single NV with a spin coherence time of 1.7 ± 0.1 ms. Back-focal-plane studies indicate efficient redistribution of the NV photoluminescence into low-NA modes by the bullseye grating.