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Two-dimensional materials are extraordinarily sensitive to external stimuli, making them ideal for studying fundamental properties and for engineering devices with new functionalities. One such stimulus, strain, affects the magnetic properties of the layered magnetic semiconductor CrSBr to such a degree that it can induce a reversible antiferromagnetic-to-ferromagnetic phase transition. Given the pervasiveness of non-uniform strain in exfoliated two-dimensional magnets, it is crucial to understand its impact on their magnetic behavior. Using scanning SQUID-on-lever microscopy, we directly image the effects of spatially inhomogeneous strain on the magnetization of layered CrSBr as it is polarized by a field applied along its easy axis. The evolution of this magnetization and the formation of domains is reproduced by a micromagnetic model, which incorporates the spatially varying strain and the corresponding changes in the local interlayer exchange stiffness. The observed sensitivity to small strain gradients along with similar images of a nominally unstrained CrSBr sample suggest that unintentional strain inhomogeneity influences the magnetic behavior of exfoliated samples and must be considered in the design of future devices., Comment: 15 pages, 4 figures, 1 supplementary section

Magnetism of oxide antiferromagnets (AFMs) has been studied in single crystals and extended thin films. The properties of AFM nanostructures still remain underexplored. Here, we report on the fabrication and magnetic imaging of granular 100-nm-thick magnetoelectric Cr$_2$O$_3$ films patterned in circular bits with diameters ranging from 500 down to 100 nm. With the change of the lateral size, the domain structure evolves from a multidomain state for larger bits to a single domain state for the smallest bits. Based on spin-lattice simulations, we show that the physics of the domain pattern formation in granular AFM bits is primarily determined by the energy dissipation upon cooling, which results in motion and expelling of AFM domain walls of the bit. Our results provide a way towards the fabrication of single domain AFM-bit-patterned memory devices and the exploration of the interplay between AFM nanostructures and their geometric shape.

Nitrogen vacancy (NV) centre quantum sensors provide unique opportunities in studying condensed matter systems: they are quantitative, noninvasive, physically robust, offer nanoscale resolution, and may be used across a wide range of temperatures. These properties have been exploited in recent years to obtain nanoscale resolution measurements of static magnetic fields arising from spin order and current flow in condensed matter systems. Compared with other nanoscale magnetic-field sensors, NV centres have the unique advantage that they can probe quantities that go beyond average magnetic fields. Leveraging techniques from magnetic resonance, NV centres can perform high precision noise sensing, and have given access to diverse systems, such as fluctuating electrical currents in simple metals and graphene, as well as magnetic dynamics in yttrium iron garnet. In this review we summarise unique opportunities in condensed matter sensing by focusing on the connections between specific NV measurements and previously established physical characteristics that are more readily understood in the condensed matter community, such as correlation functions and order parameters that are inaccessible by other techniques, and we describe the technical frontier enabled by NV centre sensing.

The exchange bias phenomenon, inherent in exchange-coupled ferromagnetic and antiferromagnetic systems, has intrigued researchers for decades. Van der Waals materials, with their layered structure, provide an optimal platform for probing such physical phenomena. However, achieving a facile and effective means to manipulate exchange bias in pristine van der Waals heterostructures remains challenging. In this study, we investigate the origin of exchange bias in MnPS3/Fe3GeTe2 van der Waals heterostructures. Our work demonstrates a method to modulate unidirectional exchange anisotropy, achieving an unprecedented nearly 1000% variation through simple thermal cycling. Despite the compensated interfacial spin configuration of MnPS3, magneto-transport measurements reveal a huge 170 mT exchange bias at 5 K, the largest observed in pristine van der Waals antiferromagnet-ferromagnet interfaces. This substantial magnitude of the exchange bias is linked to an anomalous weak ferromagnetic ordering in MnPS3 below 40 K. On the other hand, the tunability of exchange bias during thermal cycling is ascribed to the modified arrangement of interfacial atoms and changes in the vdW gap during field cooling. Our findings highlight a robust and easily adjustable exchange bias in van der Waals antiferromagnetic/ferromagnetic heterostructures, presenting a straightforward approach to enhance other interface related spintronic phenomena for practical applications.

The nitrogen-vacancy center in diamond, owing to its optically addressable and long-lived electronic spin, is an attractive resource for the generation of remote entangled states. However, the center's low native fraction of coherent photon emission, $\sim$3\%, strongly reduces the achievable spin-photon entanglement rates. Here, we couple a nitrogen-vacancy center with a narrow extrinsically broadened linewidth (\unit[159]{MHz}), hosted in a micron-thin membrane, to the mode of an open optical microcavity. The resulting Purcell factor of $\sim$1.8 increases the fraction of zero-phonon line photons to above 44\%, leading to coherent photon emission rates exceeding four times the state of the art under non-resonant excitation. Bolstered by the enhancement provided by the cavity, we for the first time measure resonance fluorescence without any temporal filtering with $>$10 signal-to-laser background ratio. Our microcavity platform would increase spin-spin entanglement success probabilities by more than an order of magnitude compared to existing implementations. Selective enhancement of the center's zero-phonon transitions could furthermore unlock efficient application of quantum optics techniques such as wave-packet shaping or all-optical spin manipulation., Comment: Main manuscript: 10 pages, 4 figures. Supplementary Information: 9 pages, 5 figures

Josephson junctions enable dissipation-less electrical current through metals and insulators below a critical current. Despite being central to quantum technology based on superconducting quantum bits and fundamental research into self-conjugate quasiparticles, the spatial distribution of super current flow at the junction and its predicted evolution with current bias and external magnetic field remain experimentally elusive. Revealing the hidden current flow, featureless in electrical resistance, helps understanding unconventional phenomena such as the nonreciprocal critical current, i.e., Josephson diode effect. Here we introduce a platform to visualize super current flow at the nanoscale. Utilizing a scanning magnetometer based on nitrogen vacancy centers in diamond, we uncover competing ground states electrically switchable within the zero-resistance regime. The competition results from the superconducting phase re-configuration induced by the Josephson current and kinetic inductance of thin-film superconductors. We further identify a new mechanism for the Josephson diode effect involving the Josephson current induced phase. The nanoscale super current flow emerges as a new experimental observable for elucidating unconventional superconductivity, and optimizing quantum computation and energy-efficient devices.

Magnetism in two-dimensional materials reveals phenomena distinct from bulk magnetic crystals, with sensitivity to charge doping and electric fields in monolayer and bilayer van der Waals magnet CrI3. Within the class of layered magnets, semiconducting CrSBr stands out by featuring stability under ambient conditions, correlating excitons with magnetic order and thus providing strong magnon-exciton coupling, and exhibiting peculiar magneto-optics of exciton-polaritons. Here, we demonstrate that both exciton and magnetic transitions in bilayer and trilayer CrSBr are sensitive to voltage-controlled field-effect charging, exhibiting bound exciton-charge complexes and doping-induced metamagnetic transitions. Moreover, we demonstrate how these unique properties enable optical probes of local magnetic order, visualizing magnetic domains of competing phases across metamagnetic transitions induced by magnetic field or electrostatic doping. Our work identifies few-layer CrSBr as a rich platform for exploring collaborative effects of charge, optical excitations, and magnetism., Comment: 18 pages, 11 figures

Since their first observation in 2017, atomically thin van der Waals (vdW) magnets have attracted significant fundamental, and application-driven attention. However, their low ordering temperatures, $T_c$, sensitivity to atmospheric conditions and difficulties in preparing clean large-area samples still present major limitations to further progress. The remarkably stable high-$T_c$ vdW magnet CrSBr has the potential to overcome these key shortcomings, but its nanoscale properties and rich magnetic phase diagram remain poorly understood. Here we use single spin magnetometry to quantitatively characterise saturation magnetization, magnetic anisotropy constants, and magnetic phase transitions in few-layer CrSBr by direct magnetic imaging. We show pristine magnetic phases, devoid of defects on micron length-scales, and demonstrate remarkable air-stability down the monolayer limit. We address the spin-flip transition in bilayer CrSBr by direct imaging of the emerging antiferromagnetic (AFM) to ferromagnetic (FM) phase wall and elucidate the magnetic properties of CrSBr around its ordering temperature. Our work will enable the engineering of exotic electronic and magnetic phases in CrSBr and the realisation of novel nanomagnetic devices based on this highly promising vdW magnet., Comment: 8 pages, 4 figures, plus supplementary material. Questions and comments are welcome

The negatively charged silicon vacancy center (SiV$^-$) in diamond is a promising, yet underexplored candidate for single-spin quantum sensing at sub-kelvin temperatures and tesla-range magnetic fields. A key ingredient for such applications is the ability to perform all-optical, coherent addressing of the electronic spin of near-surface SiV$^-$ centers. We present a robust and scalable approach for creating individual, $\sim$50nm deep SiV$^-$ with lifetime-limited optical linewidths in diamond nanopillars through an easy-to-realize and persistent optical charge-stabilization scheme. The latter is based on single, prolonged 445nm laser illumination that enables continuous photoluminescence excitation spectroscopy, without the need for any further charge stabilization or repumping. Our results constitute a key step towards the use of near-surface, optically coherent SiV$^-$ for sensing under extreme conditions, and offer a powerful approach for stabilizing the charge-environment of diamond color centers for quantum technology applications., Comment: 15 pages, 13 figures including supplementary information

Magnetic random access memory (MRAM) is a leading emergent memory technology that is poised to replace current non-volatile memory technologies such as eFlash. However, the scaling of MRAM technologies is heavily affected by device-to-device variability rooted in the stochastic nature of the MRAM writing process into nanoscale magnetic layers. Here, we introduce a non-contact metrology technique deploying Scanning NV Magnetometry (SNVM) to investigate MRAM performance at the individual bit level. We demonstrate magnetic reversal characterization in individual, < 60 nm sized bits, to extract key magnetic properties, thermal stability, and switching statistics, and thereby gauge bit-to-bit uniformity. We showcase the performance of our method by benchmarking two distinct bit etching processes immediately after pattern formation. Unlike previous methods, our approach unveils marked differences in switching behaviour of fully contacted MRAM devices stemming from these processes. Our findings highlight the potential of nanoscale quantum sensing of MRAM devices for early-stage screening in the processing line, paving the way for future incorporation of this nanoscale characterization tool in the semiconductor industry.

We present a comprehensive study of the temperature and magnetic-field dependent photoluminescence (PL) of individual NV centers in diamond, spanning the temperature-range from cryogenic to ambient conditions. We directly observe the emergence of the NV's room-temperature effective excited state structure and provide a clear explanation for a previously poorly understood broad quenching of NV PL at intermediate temperatures around 50 K. We develop a model that quantitatively explains all of our findings, including the strong impact that strain has on the temperaturedependence of the NV's PL. These results complete our understanding of orbital averaging in the NV excited state and have significant implications for the fundamental understanding of the NV center and its applications in quantum sensing., Comment: 5 pages, 4 figures plus Supplementary Material. Questions and comments are welcome. arXiv admin note: text overlap with arXiv:2105.08075

Solid state spins have demonstrated significant potential in quantum sensing with applications including fundamental science, medical diagnostics and navigation. The quantum sensing schemes showing best performance under ambient conditions all utilize microwave or radio-frequency driving, which poses a significant limitation for miniaturization, energy-efficiency and non-invasiveness of quantum sensors. We overcome this limitation by demonstrating a purely optical approach to coherent quantum sensing. Our scheme involves the $^{15}$N nuclear spin of the Nitrogen-Vacancy (NV) center in diamond as a sensing resource, and exploits NV spin dynamics in oblique magnetic fields near the NV's excited state level anti-crossing to optically pump the nuclear spin into a quantum superposition state. We demonstrate all-optical free-induction decay measurements - the key protocol for low-frequency quantum sensing - both on single spins and spin ensembles. Our results pave the way for highly compact quantum sensors to be employed for magnetometry or gyroscopy applications in challenging environments., Comment: 6 pages, 4 figures, plus supplementary material. Questions and comments are welcome

Neutral silicon vacancy centers (SiV0) in diamond are promising candidates for quantum networks because of their long spin coherence times and stable, narrow optical transitions. However, stabilizing SiV0 requires high purity, boron doped diamond, which is not a readily available material. Here, we demonstrate an alternative approach via chemical control of the diamond surface. We use low-damage chemical processing and annealing in a hydrogen environment to realize reversible and highly stable charge state tuning in undoped diamond. The resulting SiV0 centers display optically detected magnetic resonance and bulk-like optical properties. Controlling the charge state tuning via surface termination offers a route for scalable technologies based on SiV0 centers, as well as charge state engineering of other defects.

We present room-temperature measurements of magnon spin diffusion in epitaxial ferrimagnetic insulator MgAl$_{0.5}$Fe$_{1.5}$O$_{4}$ (MAFO) thin films near zero applied magnetic field where the sample forms a multi-domain state. Due to a weak uniaxial magnetic anisotropy, the domains are separated primarily by 180$^\circ$ domain walls. We find, surprisingly, that the presence of the domain walls has very little effect on the spin diffusion -- nonlocal spin transport signals in the multi-domain state retain at least 95% of the maximum signal strength measured for the spatially-uniform magnetic state, over distances at least five times the typical domain size. This result is in conflict with simple models of interactions between magnons and static domain walls, which predict that the spin polarization carried by the magnons reverses upon passage through a 180$^\circ$ domain wall.

Raman lasers based on bulk diamond are a valuable resource for generating coherent light in wavelength regimes where no common laser diodes are available. Nevertheless, the widespread use of such lasers is limited by their high threshold power requirements on the order of several Watts. Using on-chip microresonators, a significant reduction of the lasing threshold by more than two orders of magnitude has been shown. However, these resonators lack a continuous tuning mechanism and, mainly due to fabrication limitations, their implementation in the visible remains elusive. Here, we propose a platform for a diamond Raman laser in the visible. The device is based on a miniaturized, open-access Fabry-Perot cavity. Our microcavity provides widely-tunable doubly-resonant enhancement of Raman scattering from high quality single-crystalline diamond. We demonstrate a $>$THz continuous tuning range of doubly-resonant Raman scattering, a range limited only by the reflective stopband of the mirrors. Based on the experimentally determined quality factors exceeding $300\,000$, our theoretical analysis suggests that, with realistic improvements, a sub-mW threshold is readily within reach. Our findings pave the way to the creation of a universal low-power frequency shifter, a potentially valuable addition to the nonlinear optics toolbox., Comment: 13 pages, 9 figures

With a highly coherent, optically addressable electron spin, the nitrogen vacancy (NV) centre in diamond is a promising candidate for a node in a quantum network. However, the NV centre is a poor source of coherent single photons owing to a long radiative lifetime, a small branching ratio into the zero-phonon line (ZPL) and a poor extraction efficiency out of the high-index host material. In principle, these three shortcomings can be addressed by resonant coupling to a single mode of an optical cavity. Utilising the weak-coupling regime of cavity electrodynamics, resonant coupling between the ZPL and a single cavity-mode enhances the transition rate and branching ratio into the ZPL. Furthermore, the cavity channels the light into a well-defined mode thereby facilitating detection with external optics. Here, we present an open Fabry-Perot microcavity geometry containing a single-crystal diamond membrane, which operates in a regime where the vacuum electric field is strongly confined to the diamond membrane. There is a field anti-node at the diamond-air interface. Despite the presence of surface losses, quality factors exceeding $120\,000$ and a finesse $\mathcal{F}=11\,500$ were observed. We investigate the interplay between different loss mechanisms, and the impact these loss channels have on the performance of the cavity. This analysis suggests that the "waviness" (roughness with a spatial frequency comparable to that of the microcavity mode) is the mechanism preventing the quality factors from reaching even higher values. Finally, we apply the extracted cavity parameters to the NV centre and calculate a predicted Purcell factor exceeding 150., Comment: 12 pages, 6 figures

We investigate the magnetic field dependent photo-physics of individual Nitrogen-Vacancy (NV) color centers in diamond under cryogenic conditions. At distinct magnetic fields, we observe significant reductions in the NV photoluminescence rate, which indicate a marked decrease in the optical readout efficiency of the NV's ground state spin. We assign these dips to excited state level anti-crossings, which occur at magnetic fields that strongly depend on the effective, local strain environment of the NV center. Our results offer new insights into the structure of the NVs' excited states and a new tool for their effective characterization. Using this tool, we observe strong indications for strain-dependent variations of the NV's orbital g-factor, obtain new insights into NV charge state dynamics, and draw important conclusions regarding the applicability of NV centers for low-temperature quantum sensing., Comment: 5 pages, 4 figures plus Supplementary Material. Questions and comments are welcome

We report on pulsed-laser induced generation of nitrogen-vacancy (NV) centers in diamond facilitated by a solid-immersion lens (SIL). The SIL enables laser writing at energies as low as 5.8 nJ per pulse and allows vacancies to be formed close to a diamond surface without inducing surface graphitization. We operate in the previously unexplored regime where lattice vacancies are created following tunneling breakdown rather than multiphoton ionization. We present three samples in which NV-center arrays were laser-written at distances between ~1 $\mu$m and 40 $\mu$m from a diamond surface, all presenting narrow distributions of optical linewidths with means between 62.1 MHz and 74.5 MHz. The linewidths include the effect of long-term spectral diffusion induced by a 532 nm repump laser for charge-state stabilization, thereby emphasizing the particularly low charge-noise environment of the created color centers. Such high-quality NV centers are excellent candidates for practical applications employing two-photon quantum interference with separate NV centers. Finally, we propose a model for disentangling power broadening from inhomogeneous broadening in the NV center optical linewidth.

Antiferromagnets offer remarkable promise for future spintronics devices, where antiferromagnetic order is exploited to encode information. The control and understanding of antiferromagnetic domain walls (DWs) - the interfaces between domains with differing order parameter orientations - is a key ingredient for advancing such antiferromagnetic spintronics technologies. However, studies of the intrinsic mechanics of individual antiferromagnetic DWs remain elusive since they require sufficiently pure materials and suitable experimental approaches to address DWs on the nanoscale. Here we nucleate isolated, 180{\deg} DWs in a single-crystal of Cr$_2$O$_3$, a prototypical collinear magnetoelectric antiferromagnet, and study their interaction with topographic features fabricated on the sample. We demonstrate DW manipulation through the resulting, engineered energy landscape and show that the observed interaction is governed by the DW's elastic properties. Our results advance the understanding of DW mechanics in antiferromagnets and suggest a novel, topographically defined memory architecture based on antiferromagnetic DWs., Comment: 3 pages, 3 figures plus Supplementary Material. Questions and comments are welcome

We investigate the effects of a novel approach to diamond nanofabrication and nitrogen vacancy (NV) center formation on the optical linewidth of the NV zero-phonon line (ZPL). In this post-implantation method, nitrogen is implanted after all fabrication processes have been completed. We examine three post-implanted samples, one implanted with $^{14}$N and two with $^{15}$N isotopes. We perform photoluminescence excitation (PLE) spectroscopy to assess optical linewidths and optically detected magnetic resonance (ODMR) measurements to isotopically classify the NV centers. From this, we find that NV centers formed from nitrogen naturally occuring in the diamond lattice are characterized by a linewidth distribution peaked at an optical linewidth nearly two orders of magnitude smaller than the distribution characterizing most of the NV centers formed from implanted nitrogen. Surprisingly, we also observe a number of $^{15}$NV centers with narrow ($<500\,\mathrm{MHz}$) linewidths, implying that implanted nitrogen can yield NV centers with narrow optical linewidths. We further use a Bayesian approach to statistically model the linewidth distributions, to accurately quantify the uncertainty of fit parameters in our model, and to predict future linewidths within a particular sample. Our model is designed to aid comparisons between samples and research groups, in order to determine the best methods of achieving narrow NV linewidths in structured samples.

We present a new approach combining top down fabrication and bottom up overgrowth to create diamond photonic nanostructures in form of single-crystalline diamond nanopyramids. Our approach relies on diamond nanopillars, that are overgrown with single-crystalline diamond to form pyramidal structures oriented along crystal facets. To characterize the photonic properties of the pyramids, color centers are created in a controlled way using ion implantation and annealing. We find very high collection efficiency from color centers close to the pyramid apex. We further show excellent smoothness and sharpness of our diamond pyramids with measured tip radii on the order of 10 nm. Our results offer interesting prospects for nanoscale quantum sensing using diamond color centers, where our diamond pyramids could be used as scanning probes for nanoscale imaging. There, our approach would offer signifikant advantages compared to the cone-shaped scanning probes which define the current state of the art., Comment: 6 pages, 4 figures

We report cavity-enhanced Raman scattering from a single-crystal diamond membrane embedded in a highly miniaturized fully-tunable Fabry-P\'{e}rot cavity. The Raman intensity is enhanced 58.8-fold compared to the corresponding confocal measurement. The strong signal amplification results from the Purcell effect. We show that the cavity-enhanced Raman scattering can be harnessed as a narrowband, high-intensity, internal light-source. The Raman process can be triggered in a simple way by using an optical excitation frequency outside the cavity stopband and is independent of the lateral positioning of the cavity mode with respect to the diamond membrane. The strong Raman signal emerging from the cavity output facilitates in situ mode-matching of the cavity mode to single-mode collection optics; it also represents a simple way of measuring the dispersion and spatial intensity-profile of the cavity modes. The optimization of the cavity performance via the strong Raman process is extremely helpful in achieving efficient cavity-outcoupling of the relatively weak emission of single color-centers such as nitrogen-vacancy centers in diamond or rare-earth ions in crystalline hosts with low emitter density.

We present high-resolution optically detected magnetic resonance (ODMR) spectroscopy on single nitrogen-vacancy (NV) center spins in diamond at and around zero magnetic field. The experimentally observed transitions depend sensitively on the interplay between the microwave (MW) probing field and the local intrinsic effective field comprising strain and electric fields, which act on the NV spin. Based on a theoretical model of the magnetic dipole transitions and the MW driving field, we extract both the strength and the direction of the transverse component of the effective field. Our results reveal that for the diamond crystal under study, strain is the dominant contribution to the effective field. Our experiments further yield a method for MW polarization analysis in a tunable, linear basis, which we demonstrate on a single NV spin. Our results are of importance to low-field quantum sensing applications using NV spins and form a relevant addition to the ever-growing toolset of spin-based quantum sensing., Comment: 12 pages, 5 figures. Including appendix

The recent discovery of ferromagnetism in 2D van der Waals (vdw) crystals has generated widespread interest, owing to their potential for fundamental and applied research. Advancing the understanding and applications of vdw magnets requires methods to quantitatively probe their magnetic properties on the nanoscale. Here, we report the study of atomically thin crystals of the vdw magnet CrI$_3$ down to individual monolayers using scanning single-spin magnetometry, and demonstrate quantitative, nanoscale imaging of magnetisation, localised defects and magnetic domains. We determine the magnetisation of CrI$_3$ monolayers to be $\approx16~\mu_B/$nm$^2$ and find comparable values in samples with odd numbers of layers, whereas the magnetisation vanishes when the number of layers is even. We also establish that this inscrutable even-odd effect is intimately connected to the material structure, and that structural modifications can induce switching between ferro- and anti-ferromagnetic interlayer ordering. Besides revealing new aspects of magnetism in atomically thin CrI$_3$ crystals, these results demonstrate the power of single-spin scanning magnetometry for the study of magnetism in 2D vdw magnets., Comment: Questions and comments are welcome. For further information and related job-openings, please visit www.quantum-sensing.ch

We demonstrate the use of shortcuts to adiabaticity protocols for initialisation, readout, and coherent control of dressed states generated by closed-contour, coherent driving of a single spin. Such dressed states have recently been shown to exhibit efficient coherence protection, beyond what their two-level counterparts can offer. Our state transfer protocols yield a transfer fidelity of ~ 99.4(2) % while accelerating the transfer speed by a factor of 2.6 compared to the adiabatic approach. We show bi-directionality of the accelerated state transfer, which we employ for direct dressed state population readout after coherent manipulation in the dressed state manifold. Our results enable direct and efficient access to coherence-protected dressed states of individual spins and thereby offer attractive avenues for applications in quantum information processing or quantum sensing., Comment: 16 pages, 9 figures. Including supplementary material

Modern neuroscience is currently limited in its capacity to perform long term, wide-field measurements of neuron electromagnetics with nanoscale resolution. Quantum microscopy using the nitrogen vacancy centre (NV) can provide a potential solution to this problem with electric and magnetic field sensing at nano-scale resolution and good biocompatibility. However, the performance of existing NV sensing technology does not allow for studies of small mammalian neurons yet. In this paper, we propose a solution to this problem by engineering NV quantum sensors in diamond nanopillar arrays. The pillars improve light collection efficiency by guiding excitation/emission light, which improves sensitivity. More importantly, they also improve the size of the signal at the NV by removing screening charges as well as coordinating the neuron growth to the tips of the pillars where the NV is located. Here, we provide a growth study to demonstrate coordinated neuron growth as well as the first simulation of nano-scopic neuron electric and magnetic fields to assess the enhancement provided by the nanopillar geometry., Comment: 18 pages including supplementary and references, 12 figures

Hybrid quantum systems, which combine quantum-mechanical systems with macroscopic mechanical oscillators, have attracted increasing interest as they are well suited as high-performance sensors or transducers in quantum computers. A promising candidate is based on diamond cantilevers, whose motion is coupled to embedded Nitrogen-Vacancy (NV) centers through crystal deformation. Even though this type of coupling has been investigated intensively in the past, several inconsistencies exist in available literature, and no complete and consistent theoretical description has been given thus far. To clarify and resolve these issues, we here develop a complete and consistent formalism to describe the coupling between the NV spin degree of freedom and crystal deformation in terms of stress, defined in the crystal coordinate system XYZ, and strain, defined in the four individual NV reference frames. We find that the stress-based approach is straightforward, yields compact expressions for stress-induced level shifts and therefore constitutes the preferred approach to be used in future advances in the field. In contrast, the strain-based formalism is much more complicated and requires extra care when transforming into the employed NV reference frames. Furthermore, we illustrate how the developed formalism can be employed to extract values for the spin-stress and spin-strain coupling constants from data published by Teissier et al.., Comment: 14 pages, 3 figures; SOM available for download under https://quantum-sensing.physik.unibas.ch/publications/research-articles.html

Magnetic imaging using color centers in diamond through both scanning and wide-field methods offers a combination of unique capabilities for studying superconductivity, for example, enabling accurate vector magnetometry at high temperature or high pressure, with spatial resolution down to the nanometer scale. The paper briefly reviews various experimental modalities in this rapidly developing nascent field and provides an outlook towards possible future directions., Comment: 11 pages, 8 figures. Comments are welcome

Antiferromagnets have recently emerged as attractive platforms for spintronics applications, offering fundamentally new functionalities compared to their ferromagnetic counterparts. While nanoscale thin film materials are key to the development of future antiferromagnetic spintronics technologies, experimental tools to explore such films on the nanoscale are still sparse. Here, we offer a solution to this technological bottleneck, by addressing the ubiquitous surface magnetisation of magnetoelectic 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 Cr$_2$O$_3$ in real space and across the paramagnet-to-antiferromagnet phase transition. These experiments allow us to discern key properties of the Cr$_2$O$_3$ thin film, including the mechanism of domain formation and the strength of exchange coupling between individual grains comprising the film. Our work offers novel insights into Cr$_2$O$_3$'s magnetic ordering mechanism and establishes single spin magnetometry as a novel, widely applicable tool for nanoscale addressing of antiferromagnetic thin films., Comment: 22 pages, 7 figures

Understanding the mechanisms behind high-$T_{c}$ Type-II superconductors (SC) is still an open task in condensed matter physics. One way to gain further insight into the microscopic mechanisms leading to superconductivity is to study the magnetic properties of the SC in detail, for example by studying the properties of vortices and their dynamics. In this work we describe a new method of wide-field imaging magnetometry using nitrogen-vacancy (NV) centers in diamond to image vortices in an yttrium barium copper oxide (YBCO) thin film. We demonstrate quantitative determination of the magnetic field strength of the vortex stray field, the observation of vortex patterns for different cooling fields and direct observation of vortex pinning in our disordered YBCO film. This method opens prospects for imaging of the magnetic-stray fields of vortices at frequencies from DC to several megahertz within a wide range of temperatures which allows for the study of both high-$T_{C}$ and low-$T_{C}$ SCs. The wide temperature range allowed by NV center magnetometry also makes our approach applicable for the study of phenomena like island superconductivity at elevated temperatures (e.g. in metal nano-clusters)., Comment: Main text (4 pages, 3 figures)

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 $\mu$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 $\mu$m over $\approx \,$200x200 $\mu$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., Comment: 9 pages, 4 figures, version 2 accepted for publication in MDPI micromachines

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.

Three-level quantum systems have formed a cornerstone of quantum optics since the discovery of coherent population trapping (CPT) and electromagnetically induced transparency. Key to these phenomena is quantum interference, which arises if two of the three available transitions are coherently driven at well-controlled amplitudes and phases. The additional coherent driving of the third available transition would form a closed-contour interaction (CCI) from which fundamentally new phenomena would emerge, including phase-controlled CPT and one atom interferometry. However, due to the difficulty in experimentally realising a fully coherent CCI, such aspects of three-level systems remain unexplored as of now. Here, we exploit recently developed methods for coherent driving of single Nitrogen-Vacancy (NV) electronic spins to implement highly coherent CCI driving. Our experiments reveal phase-controlled, single spin quantum interference fringes, reminiscent of electron dynamics on a triangular lattice, with the driving field phases playing the role of a synthetic magnetic flux. We find that for suitable values of this phase, CCI driving leads to efficient coherence protection of the NV spin, yielding a nearly two orders of magnitude improvement of the coherence time, even for moderate drive strengths <~1MHz. Our results establish CCI driving as a novel paradigm in coherent control of few-level systems that offers attractive perspectives for applications in quantum sensing or quantum information processing., Comment: 18 pages, 11 figures. Including supplementary material. Comments are welcome. For further information visit https://quantum-sensing.physik.unibas.ch/news.html

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., Comment: 6 pages, 4 figures

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 to ferromagnet-based counterparts, is robust against magnetic disturbances and exhibits no ferromagnetic hysteresis losses. Using the magnetoelectric antiferromagnet Cr2O3, 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 of these novel systems, we construct a comprehensive model of the magnetoelectric selection mechanism in thin films of magnetoelectric antiferromagnets. We identify that growth induced effects lead to emergent ferrimagnetism, which is detrimental to the robustness of the storage. After pinpointing lattice misfit as the likely origin, we provide routes to enhance or mitigate this emergent ferrimagnetism as desired. Beyond memory applications, the AF-MERAM concept introduces a general all-electric interface for antiferromagnets and should find wide applicability in purely antiferromagnetic spintronics devices., Comment: Main text (4 figures) + supplementary information (7 figures)

We study the parametric interaction between a single Nitrogen-Vacancy electronic spin and a diamond mechanical resonator in which the spin is embedded. Coupling between spin and oscillator is achieved by crystal strain, which is generated upon actuation of the oscillator and which parametrically modulates the spins' energy splitting. Under coherent microwave driving of the spin, this parametric drive leads to a locking of the spin Rabi frequency to the oscillator mode in the megahertz range. Both the Rabi oscillation decay time and the inhomogeneous spin dephasing time increase by two orders of magnitude under this spin-locking condition. We present routes to prolong the dephasing times even further, potentially to the relaxation time limit. The remarkable coherence protection that our hybrid spin-oscillator system offers is reminiscent of recently proposed concatenated continuous dynamical decoupling schemes and results from our robust, drift-free strain-coupling mechanism and the narrow linewidth of the high-quality diamond mechanical oscillator employed. Our findings suggest feasible applications in quantum information processing and sensing., Comment: 6 pages, 4 figures

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, $1-2\,\mu$m length) on a thin ($< 1\mu$m) cantilever structure, enable efficient light extraction from diamond in combination with a high magnetic field sensitivity ($\mathrm{\eta_{AC}}\approx50\pm20\,\mathrm{nT}/\sqrt{\mathrm{Hz}}$). 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.

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., Comment: version 2 accepted for publication in APL

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., Comment: Main text (5 pages, 4 figures) plus supplementary material (5 pages, 6 figures). Comments welcome. Further information under http://www.quantum-sensing.ch

We report on imaging of microwave (MW) magnetic fields using a magnetometer based on the electron spin of a nitrogen vacancy center in diamond. We quantitatively image the magnetic field generated by high frequency (GHz) MW current with nanoscale resolution using a scanning probe technique. We demonstrate a MW magnetic field sensitivity in the range of a few nT/$\sqrt{\text{Hz}}$, polarization selection and broadband capabilities under ambient conditions and thereby establish the nitrogen vacancy center a versatile and high performance tool for the detection of MW fields. As a first application of this scanning MW detector, we determine the MW current density in a stripline and demonstrate a MW current sensitivity of a few nA/$\sqrt{\text{Hz}}$

Quantum devices for sensing and computing applications require coherent quantum systems which can be manipulated in a fast and robust way. Such quantum control is typically achieved using external electric or magnetic fields which drive the system's orbital or spin degrees of freedom. However, most of these approaches require complex and unwieldy antenna or gate structures, and with few exceptions are limited to the regime of weak driving. Here, we present a novel approach to strongly and coherently drive a single electron spin in the solid state using internal strain fields in an integrated quantum device. Specifically, we study individual Nitrogen-Vacancy (NV) spins embedded in diamond mechanical oscillators and exploit the intrinsic strain coupling between spin and oscillator to strongly drive the spins. As hallmarks of the strong driving regime, we directly observe the energy spectrum of the emerging phonon-dressed states and employ our strong, continuous driving for enhancement of the NV spin coherence time. Our results constitute a first step towards strain-driven, integrated quantum devices and open new perspectives to investigate unexplored regimes of strongly driven multi-level systems and to study exotic spin dynamics in hybrid spin-oscillator devices., Comment: 7 pages, 4 figures. Including supplementary material. Comments welcome. Further information under http://quantum-sensing.ch