Antiferromagnets have recently emerged as attractive platforms for spintronics applications, offering fundamentally new functionalities compared with their ferromagnetic counterparts. Whereas nanoscale thin-film materials are key to the development of future antiferromagnetic spintronic technologies, existing experimental tools tend to suffer from low resolution or expensive and complex equipment requirements. We offer a simple, high-resolution alternative by addressing the ubiquitous surface magnetization of magnetoelectric antiferromagnets in a granular thin-film sample on the nanoscale using single-spin magnetometry in combination with spin-sensitive transport experiments. Specifically, we quantitatively image the evolution of individual nanoscale antiferromagnetic domains in 200 nm thin films of Cr2O3 in real space and across the paramagnet-to-antiferromagnet phase transition, finding an average domain size of 230 nm, several times larger than the average grain size in the film. These experiments allow us to discern key properties of the Cr2O3 thin film, including the boundary magnetic moment density, the variation of critical temperature throughout the film, the mechanism of domain formation, and the strength of exchange coupling between individual grains comprising the film. Our work offers novel insights into the magnetic ordering mechanism of Cr2O3 and firmly establishes single-spin magnetometry as a versatile and widely applicable tool for addressing antiferromagnetic thin films on the nanoscale.

Thin-film antiferromagnets are key contenders in the development of spintronic devices. We investigate one such material, Cr2O3 using nitrogen vacancy scanning magnetometry and present quantitative magnetic field maps showing the structure and nucleation of domain walls over the paramagnet to antiferromagnet transition, allowing us to extract critical material properties.

Microwave devices and circuits are an essential part of modern communication technology and precision instrumentation, with numerous applications in wireless networks, mobile phones, satellite communication, navigation, radar systems and precision measurement. For the development and testing of microwave devices, a calibrated technique for high-resolution non-perturbative imaging of microwave fields is needed. Microwave detectors with high spatial resolution and low crosstalk are also essential for emerging applications of microwaves in medical imaging [1]. We have developed a novel technique for high-resolution imaging of microwave fields using atoms in miniaturized vapor cells as sensors [2], see Fig. 1. In this technique, the microwave field to be measured drives Rabi oscillations on atomic hyperfine transitions. The oscillations are recorded in a spatially resolved way by absorption imaging with a laser and a camera. From the measured distribution of Rabi frequencies we obtain an image of the microwave field distribution. All vector components of the microwave magnetic field can be imaged and the technique is intrinsically calibrated because the properties of the atomic transitions are precisely known. Using a custom vapor cell with thin walls (see Fig. 1) our technique provides a spatial resolution of < 100 μm [3]. By applying a static magnetic field to tune the hyperfine levels, microwave fields with frequencies ranging from a few GHz to a few tens of GHz can be detected [4]. The experimental apparatus is simple and compact and does not require cryogenics or ultra-high vacuum, making the technique attractive for applications outside the laboratory. Our imaging method can also be applied to other systems that feature optical and microwave transitions. In a collaboration with the Maletinsky group we have recently implemented our microwave field imaging scheme with high-density layers of nitrogen vacancy centers in diamond [5].

Many promising applications of single crystal diamond and its color centers as sensor platform and in photonics require free-standing membranes with a thickness ranging from several micrometers to the few 100 nm range. In this work, we present an approach to conveniently fabricate such thin membranes with up to about one millimeter in size. We use commercially available diamond plates (thickness 50 μ m) in an inductively coupled reactive ion etching process which is based on argon, oxygen and SF 6 . We thus avoid using toxic, corrosive feed gases and add an alternative to previously presented recipes involving chlorine-based etching steps. Our membranes are smooth (RMS roughness <1 nm) and show moderate thickness variation (central part: <1 μ m over ≈200 × 200 μ m 2 ). Due to an improved etch mask geometry, our membranes stay reliably attached to the diamond plate in our chlorine-based as well as SF 6 -based processes. Our results thus open the route towards higher reliability in diamond device fabrication and up-scaling.

Devices relying on microwave circuitry form a cornerstone of many classical and emerging quantum technologies. A capability to provide in-situ, noninvasive and direct imaging of the microwave fields above such devices would be a powerful tool for their function and failure analysis. In this work, we build on recent achievements in magnetometry using ensembles of nitrogen vacancy centres in diamond, to present a widefield microwave microscope with few-micron resolution over a millimeter-scale field of view, 130nT/sqrt-Hz microwave amplitude sensitivity, a dynamic range of 48 dB, and sub-ms temporal resolution. We use our microscope to image the microwave field a few microns above a range of microwave circuitry components, and to characterise a novel atom chip design. Our results open the way to high-throughput characterisation and debugging of complex, multi-component microwave devices, including real-time exploration of device operation.

A non-invasive scanning magnetometer, based on a single nitrogen–vacancy defect in diamond, visualizes antiferromagnetic order at the nanometre scale in thin films of bismuth ferrite at room temperature. Antiferromagnets are of substantial interest for spintronic applications. They can harbour magnetic configurations that can be controlled in new and more energy-efficient ways than the magnetic configurations in ferromagnets, such as with electric rather than magnetic fields. However, exploring and characterizing antiferromagnetic structures with high-resolution magnetic imaging is challenging as the antiparallel order of neighbouring spins results in a net vanishing value of magnetization. Vincent Jacques and colleagues develop a non-invasive scanning magnetometer, based on a single nitrogen–vacancy defect in diamond, and use it to visualize antiferromagnetic order at the nanometre scale in thin films of bismuth ferrite, at room temperature. Bismuth ferrite is a multiferroic material, meaning that the magnetic configuration can be controlled with an electric field, and is of great interest for developing spintronic devices with reconfigurable nanoscale spin textures. Although ferromagnets have many applications, their large magnetization and the resulting energy cost for switching magnetic moments bring into question their suitability for reliable low-power spintronic devices. Non-collinear antiferromagnetic systems do not suffer from this problem, and often have extra functionalities: non-collinear spin order1 may break space-inversion symmetry2,3 and thus allow electric-field control of magnetism4,5, or may produce emergent spin–orbit effects6 that enable efficient spin–charge interconversion7. To harness these traits for next-generation spintronics, the nanoscale control and imaging capabilities that are now routine for ferromagnets must be developed for antiferromagnetic systems. Here, using a non-invasive, scanning single-spin magnetometer based on a nitrogen–vacancy defect in diamond8,9,10, we demonstrate real-space visualization of non-collinear antiferromagnetic order in a magnetic thin film at room temperature. We image the spin cycloid of a multiferroic bismuth ferrite (BiFeO3) thin film and extract a period of about 70 nanometres, consistent with values determined by macroscopic diffraction11,12. In addition, we take advantage of the magnetoelectric coupling present in BiFeO3 to manipulate the cycloid propagation direction by an electric field. Besides highlighting the potential of nitrogen–vacancy magnetometry for imaging complex antiferromagnetic orders at the nanoscale, these results demonstrate how BiFeO3 can be used in the design of reconfigurable nanoscale spin textures.

A key requirement for many applications in solid-state quantum sciences is a high fluence of indistinguishable photons. The spontaneous emission of these photons is governed by the coupling of an excited quantum system to electromagnetic vacuum fluctuations. In our experiment, we enhance this coupling by engineering a tunable Fabry-Perot microcavity. The quantum system we study is the nitrogen-vacancy (NV) center in diamond, a workhorse for quantum science and engineering, due to its optical transitions and the coherent electron spin system it hosts. Our device consists of a high-quality, nano-fabricated, single-crystalline diamond membrane bonded to a planar mirror; the cavity is completed by a second, concave mirror. Using piezo positioners, we achieve full spectral and spatial tunability and freedom in selecting NVs with favorable emission properties in our low-temperature (4 Kelvin) experiments. Upon tuning of the cavity into resonance, we find significant enhancement of the 637 nm zero phonon line for several individual NVs which is accompanied by a strong reduction of the overall photoluminescence (PL) lifetime. We infer a 30-fold enhancement of the zero-phonon transition rate at best. The fraction of the PL emission associated to this resonant transition is thereby increased from 3% to 46%. Our results constitute a significant leap on the route towards the implementation of fast long-distance quantum networks, which are currently limited by the photon emission rate in their nodes. Furthermore, our versatile design is readily applicable to other solid-state quantum emitters like color centers in silicon carbide.

Magnetic random access memory schemes employing magnetoelectric coupling to write binary information promise outstanding energy efficiency. We propose and demonstrate a purely antiferromagnetic magnetoelectric random access memory (AF-MERAM) that offers a remarkable 50-fold reduction of the writing threshold compared with ferromagnet-based counterparts, is robust against magnetic disturbances and exhibits no ferromagnetic hysteresis losses. Using the magnetoelectric antiferromagnet Cr 2 O 3 , we demonstrate reliable isothermal switching via gate voltage pulses and all-electric readout at room temperature. As no ferromagnetic component is present in the system, the writing magnetic field does not need to be pulsed for readout, allowing permanent magnets to be used. Based on our prototypes, we construct a comprehensive model of the magnetoelectric selection mechanisms in thin films of magnetoelectric antiferromagnets, revealing misfit induced ferrimagnetism as an important factor. Beyond memory applications, the AF-MERAM concept introduces a general all-electric interface for antiferromagnets and should find wide applicability in antiferromagnetic spintronics.

The paper presents progress in developing a widefield imaging system for microwave fields based on dense ensembles of NV centres. Widefield microscopy of dc and low-frequency magnetic fields using NV centres is an already promising tool for applications ranging from biological imaging to imaging current flows in graphene. The paper expects to realise a >1mm 2 field of view and sub-ms temporal resolution, exceeding the state-of-the art for widefield NV microscopy. The microscope provides 5 μm spatial resolution, given by the thickness of the near-uniaxial NV layer, and the current magnetic field sensitivity is hundreds of nT·Hz -1/2 , which we expect to improve by an order of magnitude. These combined performance characteristics will provide a capability unmatched by any other microwave imaging technology. Expanding on our current focus on microwave device characterisation, microwave imaging with NV centres could also find application in medical microwave sensing and imaging, a promising new diagnostic field which is currently limited by its microwave sensor technology. Atom- or NV-based sensors would seem to be ideal candidates to augment or replace current antenna based measurements.

The nitrogen-vacancy (NV) center in diamond has an optically addressable, highly coherent spin. However, an NV center even in high quality single-crystalline material is a very poor source of single photons: extraction out of the high-index diamond is inefficient, the emission of coherent photons represents just a few per cent of the total emission, and the decay time is large. In principle, all three problems can be addressed with a resonant microcavity. In practice, it has proved difficult to implement this concept: photonic engineering hinges on nano-fabrication yet it is notoriously difficult to process diamond without degrading the NV centers. We present here a microcavity scheme which uses minimally processed diamond, thereby preserving the high quality of the starting material, and a tunable microcavity platform. We demonstrate a clear change in the lifetime for multiple individual NV centers on tuning both the cavity frequency and anti-node position, a Purcell effect. The overall Purcell factor $F_{\rm P}=2.0$ translates to a Purcell factor for the zero phonon line (ZPL) of $F_{\rm P}^{\rm ZPL}\sim30$ and an increase in the ZPL emission probability from $\sim 3 \%$ to $\sim 46 \%$. By making a step-change in the NV's optical properties in a deterministic way, these results pave the way for much enhanced spin-photon and spin-spin entanglement rates.

The electronic spin of the nitrogen vacancy (NV) center in diamond forms an atomically sized, highly sensitive sensor for magnetic fields. To harness the full potential of individual NV centers for sensing with high sensitivity and nanoscale spatial resolution, NV centers have to be incorporated into scanning probe structures enabling controlled scanning in close proximity to the sample surface. Here, we present an optimized procedure to fabricate single-crystal, all-diamond scanning probes starting from commercially available diamond and show a highly efficient and robust approach for integrating these devices in a generic atomic force microscope. Our scanning probes consisting of a scanning nanopillar (200 nm diameter, 1-2 μm length) on a thin (<1 μm) cantilever structure enable efficient light extraction from diamond in combination with a high magnetic field sensitivity (ηAC≈50±20nT/Hz). As a first application of our scanning probes, we image the magnetic stray field of a single Ni nanorod. We show that this stray field can be approximated by a single dipole and estimate the NV-to-sample distance to a few tens of nanometer, which sets the achievable resolution of our scanning probes.

Silicon photonics enables large-scale photonic–electronic integration by leveraging highly developed fabrication processes from the microelectronics industry. However, while a rich portfolio of devices has already been demonstrated on the silicon platform, on-chip light sources still remain a key challenge since the indirect bandgap of the material inhibits efficient photon emission and thus impedes lasing. Here we demonstrate a class of infrared lasers that can be fabricated on the silicon-on-insulator (SOI) integration platform. The lasers are based on the silicon–organic hybrid (SOH) integration concept and combine nanophotonic SOI waveguides with dye-doped organic cladding materials that provide optical gain. We demonstrate pulsed room-temperature lasing with on-chip peak output powers of up to 1.1 W at a wavelength of 1,310 nm. The SOH approach enables efficient mass-production of silicon photonic light sources emitting in the near infrared and offers the possibility of tuning the emission wavelength over a wide range by proper choice of dye materials and resonator geometry.

The past decade has witnessed enormous progress in the design and fabrication of nanostructured materials with unique optical properties. These tiny physical structures allow geometrical control over the electromagnetic shape of light well below its own wavelength. A large variety of applications for this field of subwavelength optics known as nanophotonics have already begun to emerge, including solar photovoltaics, chemical sensing, quantum cryptography, and LED lighting. Many fundamental optical interactions are hidden beneath the diffraction limit, however, which makes the retrieval of subwavelength optical behavior impossible with standard microscopy. An alternative approach, known as cathodoluminescence (CL) [1], is to probe nanostructures with an electron beam, which excites optical resonances and transitions and then collects the emitted light. It has recently been used to characterize plasmonic structure [2] or III‐N heterostructures [3]. Thanks to a very efficient CL system, in a scanning electron microscope (SEM), working in free space, we can directly probed many key dimensions of light – including intensity, angle, polarization, and frequency – with ~10 nm resolution [4]. In this presentation we will show how we can use a blanker to modulate a continuous electron beam to generate electron pulses with a resolution of ~10 ns. This approach has several advantages compared to alternatives such as a laser driven pulsed electron gun [5]. It can be easily implemented in any SEM, it requires few experimental modifications, it does not require a dedicated microscope and the pulsed electron beam can be controlled at will (pulse duration and repetition rate). We will show how this pulsed electron gun can be used to explore the dynamics of a single photon emitter, such as a rare earth ion or the NV 0 center of nano‐diamond. An HBT interferometer allows recording of the autocorrelation function of the CL signal [6][7]. Combining secondary electron images with measurements of the CL spectra, emission polarizations, lifetimes and second order correlation function, we are able to fully characterize optically active systems at the nanometer scale.

We demonstrate the controlled preparation of heteroepitaxial diamond nano- and microstructures on silicon wafer based iridium films as hosts for single color centers. Our approach uses electron beam lithography followed by reactive ion etching to pattern the carbon layer formed by bias enhanced nucleation on the iridium surface. In the subsequent chemical vapor deposition process, the patterned areas evolve into regular arrays of (001) oriented diamond nano-islands with diameters of <500nm and a height of approx. 60 nm. In the islands, we identify single SiV color centers with narrow zero phonon lines down to 1 nm at room temperature.

Microscopic studies of superconductors and their vortices play a pivotal role in our understanding of the mechanisms underlying superconductivity. Local measurements of penetration depths or magnetic stray-fields enable access to fundamental aspects of superconductors such as nanoscale variations of superfluid densities or the symmetry of their order parameter. However, experimental tools, which offer quantitative, nanoscale magnetometry and operate over the large range of temperature and magnetic fields relevant to address many outstanding questions in superconductivity, are still missing. Here, we demonstrate quantitative, nanoscale magnetic imaging of Pearl vortices in the cuprate superconductor YBCO, using a scanning quantum sensor in form of a single Nitrogen-Vacancy (NV) electronic spin in diamond. The sensor-to-sample distance of ~10nm we achieve allows us to observe striking deviations from the prevalent monopole approximation in our vortex stray-field images, while we find excellent quantitative agreement with Pearl's analytic model. Our experiments yield a non-invasive and unambiguous determination of the system's local London penetration depth, and are readily extended to higher temperatures and magnetic fields. These results demonstrate the potential of quantitative quantum sensors in benchmarking microscopic models of complex electronic systems and open the door for further exploration of strongly correlated electron physics using scanning NV magnetometry.

We report on imaging of microwave (MW) magnetic fields using a magnetometer based on the electron spin of a nitrogen vacancy (NV) center in diamond. We quantitatively image the magnetic field generated by high frequency (GHz) MW current with nanoscale resolution using a scanning probe technique. Together with a shot noise limited MW magnetic field sensitivity of 680 nT Hz−1/2 our room temperature experiments establish the NV center as a versatile and high performance tool for MW imaging, which furthermore offers polarization selectivity and broadband capabilities. As a first application of this scanning MW detector, we image the MW stray field around a stripline structure and thereby locally determine the MW current density with a MW current sensitivity of a few nA Hz−1/2.

We report on single electronic spins coupled to the motion of mechanical resonators by a novel mechanism based on crystal strain. Our device consists of single-crystal diamond cantilevers with embedded nitrogen-vacancy center spins. Using optically detected electron spin resonance, we determine the unknown spin-strain coupling constants and demonstrate that our system resides well within the resolved sideband regime. We realize coupling strengths exceeding 10 MHz under mechanical driving and show that our system has the potential to reach strong coupling. Our novel hybrid system forms a resource for future experiments on spin-based cantilever cooling and coherent spin-oscillator coupling.

We demonstrate the fabrication of single-crystalline diamond nanopillars on a (111)-oriented chemical vapor deposited diamond substrate. This crystal orientation offers optimal coupling of nitrogen-vacancy (NV) center emission to the nanopillar mode and is thus advantageous over previous approaches. We characterize single native NV centers in these nanopillars and find one of the highest reported saturated fluorescence count rates in single crystalline diamond in excess of 106 counts per second. We show that our nano-fabrication procedure conserves the preferential alignment as well as the spin coherence of the NVs in our structures. Our results will enable a new generation of highly sensitive probes for NV magnetometry and pave the way toward photonic crystals with optimal orientation of the NV center's emission dipole.

We report the creation of a low-loss broadband optical antenna giving highly directed output from a coherent single spin in the solid state. The device, a crystalline solid-state realization of a dielectric antenna, is engineered for individual nitrogen-vacancy electronic spins in diamond. We demonstrate a directionality close to 10. The photonic structure preserves the high spin coherence of single-crystal diamond (T-2 greater than or similar to 100 mu s). The single-photon count rate approaches a megahertz facilitating efficient spin readout. We thus demonstrate a key enabling technology for quantum applications such as high-sensitivity magnetometry and long-distance spin entanglement.

Lasing in a silicon-organic hybrid (SOH) waveguide is demonstrated for the first time. The device combines nanophotonic silicon-on-insulator (SOI) waveguides with active organic cladding materials, thereby enabling silicon-based optical sources at infrared telecommunication wavelengths.