Skyrmions: Room temperature and beyond.

The nitrogen-vacancy defect centre in diamond1–4 has potential applications in nanoscale electric and magnetic-field sensing2–6, single-photon microscopy7,8, quantum information processing9 and bioimaging10. These applications rely on the ability to position a single nitrogen-vacancy centre within a few nanometres of a sample, and then scan it across the sample surface, while preserving the centre’s spin coherence and readout fidelity. However, existing scanning techniques, which use a single diamond nanocrystal grafted onto the tip of a scanning probe microscope2,8,11,12, suffer from short spin coherence times due to poor crystal quality, and from inefficient far-field collection of the fluorescence from the nitrogen-vacancy centre. Here, we demonstrate a robust method for scanning a single nitrogen-vacancy centre within tens of nanometres from a sample surface that addresses both of these concerns. This is achieved by positioning a single nitrogen-vacancy centre at the end of a high-purity diamond nanopillar, which we use as the tip of an atomic force microscope. Our approach ensures long nitrogen-vacancy spin coherence times (∼75 ms), enhanced nitrogen-vacancy collection efficiencies due to waveguiding, and mechanical robustness of the device (several weeks of scanning time). We are able to image magnetic domains with widths of 25 nm, and demonstrate a magnetic field sensitivity of 56 nT Hz at a frequency of 33 kHz, which is unprecedented for scanning nitrogen-vacancy centres. Nitrogen-vacancy (NV)-based nanoscale sensing is possible because the NV centre forms a bright and stable single-photon source13 for optical imaging and has a spin-triplet ground state that offers excellent magnetic3 and electric5 field sensing capabilities. The remarkable performance of the NV centre in such spin-based sensing schemes is the result of the long NV spin coherence time14, combined with efficient optical spin preparation and readout15. These properties persist from cryogenic temperatures to ambient conditions, a feature that distinguishes the NV centre from other systems proposed as quantum sensors, such as single molecules16 or quantum dots1. Reducing the distance between the NV centre and the sample of interest is crucial for improving spatial resolution. Past experiments aimed at implementing scanning NV microscopes were focused on grafting diamond nanocrystals onto scanning probe tips2,8. Although used successfully in the past, this approach suffers from the poor sensing performance of nanocrystal-based NV centres, for which the spin coherence times are typically orders of magnitude shorter than for NVs in bulk diamond3. Here, we present a novel approach that overcomes these drawbacks and thereby realizes the full potential of bulk NV-based sensing schemes in the scanning geometry relevant for nanoscale imaging. In particular, we have developed a monolithic ‘scanning NV sensor’ (Fig. 1a), which uses a diamond nanopillar as the scanning probe, with an individual NV centre artificially created within 10 nm of the pillar tip through ion implantation17. Long NV spin coherence times are achieved as our devices are fabricated from high-purity, single-crystalline bulk diamond, which brings the additional advantage of high mechanical robustness. Furthermore, diamond nanopillars are efficient waveguides for the NV fluorescence band18, which for a scanning NV device yields record-high NV signal collection efficiencies. Figure 1b shows a representative scanning electron microscope (SEM) image of a single-crystalline diamond scanning probe containing a single NV centre within 10 nm of its tip. To prepare such devices, a series of fabrication steps are performed sequentially, including low-energy ion implantation for NV creation, several successively aligned electron-beam lithography steps and reactive ion etching19. An essential element to this sequence is the fabrication of micrometre-thin, single-crystalline diamond slabs that form the basis of the scanning probe device shown in Fig. 1b. A detailed description of the fabrication procedure of these slabs and the resulting devices can be found in the Methods. Our scanning diamond nanopillars have typical diameters of 200 nm and lengths of 1 mm and are fabricated on few-micrometre-sized diamond platforms that are individually attached to atomic force microscope (AFM) tips for scanning (see Fig. 1b and Methods for details of the attaching process). Our fabrication procedure (Fig. 1c) allows for highly parallel processing, as shown in the array of diamond devices depicted in the SEM image in Fig. 1d. Close to 30% of the diamond nanopillars in our samples contain single, negatively charged NV centres. Other devices contain more than one NV, or one NV in a charge-neutral state, which is unsuitable for magnetometry. From these 30%, we select the NV centres that exhibit the highest photon count rates and longest spin coherence times and mount these single-NV nanopillars onto AFM tips to yield the finalized scanning probe shown in the SEM picture in Fig. 1b. We note that these scanning devices were fabricated from a [001]-oriented diamond crystal, resulting in NV orientation and magnetic-field sensing along an axis tilted by 54.78 from the nanopillar direction. To use the scanning NV sensor and characterize its basic spin and optical properties, we used a combined confocal and atomic force microscope as sketched in Fig. 1a. The set-up was equipped with piezo positioners for the sample and an AFM head to allow for independent scanning with respect to the optical axis. Optical addressing and readout of the NV centre was performed through a long-working-distance microscope objective (numerical aperture,