Extracting photons from defect centers is challenging due to the high refractive index of typical substrates. For nitrogen-vacancy centers in diamond, reaching saturation count rates above 2.5 × 105 counts/s so far requires nanofabricated optics, such as diamond waveguides or solid immersion lenses. Here, we present an experimental and theoretical study of defect center emission at unmodified planar dielectric surfaces by quantitative back-focal-plane imaging and analytical modeling. Our results indicate that photon count rates approaching those of nanofabricated optics can also be achieved by oil-immersion optics. This is due to a dielectric antenna effect, which directs the majority of the emission into the substrate within a narrow angle window suitable for back-side collection. By quantification of the collection efficiency of back-side detection, our work also enables a novel measurement method for the quantum efficiency of shallow defect centers. Its result challenges established values. Possible reasons for the discrepancy are discussed.

Extended two-dimensional (2D) crystals of dye molecules adsorbed on 2D material substrates such as boron nitride have recently become a subject of intense study, with potential applications ranging from quantum technology to optoelectronics. The most established technique for the production of these films is physical vapor transport in a vacuum. We demonstrate that few-layer crystalline films of the organic dye molecule perylenetetracarboxylic diimide (PTCDI) on boron nitride can be produced by microspacing in-air sublimation, a radically simplified technique that does not require complicated vacuum systems. The resulting layers display clearly resolved atomic step terraces in atomic force microscopy and a clear polarization anisotropy in their fluorescence, confirming molecular alignment and long-range order. Using density functional theory and classical molecular dynamics simulations, the canted motif is identified as the most likely building block for the morphology of a PTDCI monolayer on the hBN substrate.

Planar scanning probe microscopy is a recently emerging alternative approach to tip-based scanning probe imaging. It can scan an extended planar sensor, such as a polished bulk diamond doped with magnetic-field-sensitive nitrogen-vacancy (NV) centers, in nanometer-scale proximity of a planar sample. So far, this technique has been limited to optical near-field microscopy, and has required nanofabrication of the sample of interest. Here we extend this technique to magnetometry using NV centers, and present a modification that removes the need for sample-side nanofabrication. We harness this new ability to perform a hitherto infeasible measurement - direct imaging of the three-dimensional vector magnetic field of magnetic vortices in a thin film magnetic heterostructure, based on repeated scanning with NV centers with different orientations within the same scanning probe. Our result opens the door to quantum sensing using multiple qubits within the same scanning probe, a prerequisite for the use of entanglement-enhanced and massively parallel schemes.

An unorthodox optical approach to scanning probe positioning is opening a realm of novel applications in various fields of research. By using optical interferometry to carefully align a sensor parallel to a sample, even millimeter-sized sensors can be brought into nanometer-scale proximity of samples. Extended circuits and massively parallel arrays of sensors can thus be used as a scanning probe, a feat that would be impossible using established sharp tips.

We demonstrate three-dimensional magnetic resonance tomography with a resolution down to 5.9 ± 0.1 nm. Our measurements use lithographically fabricated microwires as a source of three-dimensional magnetic field gradients, which we use to image NV centers in a densely doped diamond by Fourier-accelerated magnetic resonance tomography. We also demonstrate a compressed sensing scheme, which allows for direct visual interpretation without numerical optimization and implements an effective zoom into a spatially localized volume of interest, such as a localized cluster of NV centers. It is based on aliasing induced by equidistant undersampling of k-space. The resolution achieved in our work is comparable to the best existing schemes of super-resolution microscopy and approaches the positioning accuracy of site-directed spin labeling, paving the way to three-dimensional structure analysis by magnetic-gradient based tomography.

There is considerable evidence that action potentials are accompanied by "intrinsic optical signals", such as a nanometer-scale motion of the cell membrane. Here we present ChiSCAT, a technically simple imaging scheme that detects such signals with interferometric sensitivity. ChiSCAT combines illumination by a chaotic speckle pattern and interferometric scattering microscopy (iSCAT) to sensitively detect motion in any direction. The technique features reflective high-NA illumination, common-path suppression of vibrations, and a large field of view. This approach maximizes sensitivity to motion, but does not produce a visually interpretable image. We show that unsupervised learning based on matched filtering and motif discovery can recover underlying motion patterns and detect action potentials. We demonstrate these claims in an experiment on blebbistatin-paralyzed cardiomyocytes. ChiSCAT opens the door to action potential measurement in scattering tissue, including a living brain.

We present and analyze a simple scheme to calibrate single–qubit gates. It determines the amplitude and phase difference between a quadrature pair of drives, as well as their common detuning from the qubit resonance. The method is based on a two–dimensional Rabi oscillation, a sequence of two pulses of varying length sourced from the drive pair. We demonstrate error diagnosis using this scheme on an ensemble of nitrogen-vacancy centers in diamond and point out subtle pitfalls in its implementation.

We present a scheme to neutralize the dephasing effect induced by classical noise on a qubit. The scheme builds upon the key idea that this kind of noise can be recorded by a classical device during the qubit evolution, and that its effect can be undone by a suitable control sequence that is conditioned on the measurement result. We specifically demonstrate this scheme on a nitrogen-vacancy (NV) center that strongly couples to current noise in a nearby conductor. By conditioning the readout observable on a measurement of the current, we recover the full qubit coherence and its intrinsic coherence time $T_2$. We demonstrate that this scheme provides a simple way to implement single-qubit gates with an infidelity of $10^{-2}$ even if they are driven by noisy sources, and we estimate that an infidelity of $10^{-5}$ could be reached with additional improvements. We anticipate this method to find widespread adoption in experiments using fast control pulses driven from strong currents, in particular in nanoscale magnetic resonance imaging, where control of peak current s of 100 mA with a bandwidth of 100 MHz is required.

High-fidelity projective readout of a qubit’s state in a single experimental repetition is a prerequisite for various quantum protocols of sensing and computing. Achieving single-shot readout is challenging for solid-state qubits. For Nitrogen-Vacancy (NV) centers in diamond, it has been realized using nuclear memories or resonant excitation at cryogenic temperature. All of these existing approaches have stringent experimental demands. In particular, they require a high efficiency of photon collection, such as immersion optics or all-diamond micro-optics. For some of the most relevant applications, such as shallow implanted NV centers in a cryogenic environment, these tools are unavailable. Here we demonstrate an all-optical spin readout scheme that achieves single-shot fidelity even if photon collection is poor (delivering less than 10 3 clicks/second). The scheme is based on spin-dependent resonant excitation at cryogenic temperature combined with spin-to-charge conversion, mapping the fragile electron spin states to the stable charge states. We prove this technique to work on shallow implanted NV centers, as they are required for sensing and scalable NV-based quantum registers.

We present ab initio calculations of photoionization thresholds and cross sections of the negatively charged nitrogen-vacancy (NV) center in diamond. We consider photoionization from the ground (3)A(2) and the excited E-3 states. After the ionization from the E-3 level, we show that the NV center transitions into the metastable (4)A(2) electronic state of the neutral defect. We reveal how spin polarization of NV- gives rise to spin polarization of the (4)A(2) state, explaining electron spin resonance experiments. We obtain smooth curves of photoionization cross sections as a function of energy by employing dense k-point meshes for the Brillouin-zone integration together with the band unfolding technique to rectify the distortions of the band structure induced by the artificial periodicity of the supercell approach. Our calculations provide a comprehensive picture of photoionization mechanisms of NV-. They will be useful in interpreting and designing experiments on charge-state dynamics at NV centers. In particular, we offer a consistent explanation of recent results of spin-to-charge conversion of NV centers.

We demonstrate dispersive readout of the spin of an ensemble of Nitrogen-Vacancy centers in a high-quality dielectric microwave resonator at room temperature. The spin state is inferred from the reflection phase of a microwave signal probing the resonator. Time-dependent tracking of the spin state is demonstrated, and is employed to measure the T1 relaxation time of the spin ensemble. Dispersive readout provides a microwave interface to solid state spins, translating a spin signal into a microwave phase shift. We estimate that its sensitivity can outperform optical readout schemes, owing to the high accuracy achievable in a measurement of phase. The scheme is moreover applicable to optically inactive spin defects and it is non-destructive, which renders it insensitive to several systematic errors of optical readout and enables the use of quantum feedback.

J. Ebel, 2, ∗ T. Joas, 2, ∗ M. Schalk, 2 Andreas Angerer, J. Majer, 4, 5 and F. Reinhard 2, † TU München, Walter Schottky Institut and Physik–Department, Am Coulombwall 4, 85748 Garching, Germany Munich Center for Quantum Science and Technology (MCQST), Schellingstr. 4, D-80799 München, Germany Vienna Center for Quantum Science and Technology, Atominstitut, TU Wien, Vienna, Austria. Shanghai Branch, CAS Center for Excellence and Synergetic Innovation Center in Quantum Information and Quantum Physics, University of Science and Technology of China, Shanghai 201315, China National Laboratory for Physical Sciences at Microscale and Department of Modern Physics, University of Science and Technology of China, Hefei 230026, China (Dated: June 24, 2021)

Scanning probe microscopy (SPM) traditionally employs a sharp tip as a sensor. This geometry is a problem for many modern near-field probes, such as NV centers in diamond, which cannot easily be placed on a tip. Here we present a novel, tipless approach - a technique to scan a planar probe parallel to a planar sample at a distance of few tens of nanometers. The core of our scheme are optical far-field techniques to measure both distance and tilt between the probe and the sample with sub-nm and sub-mrad precision. These measurements are employed as a feedback signal for positioning. Using this scheme, we demonstrate scanning near-field optical microscopy (SNOM) of plasmonic modes in silver nanowires using shallow NV centers in a bulk diamond. We will equally present ongoing experiments to implement a scanning nanogap cavity. S. Ernst et al., ACS Photonics 6, 327 (2019)

Small solid state qubits, most prominently single spins in solids, can be remarkable sensors for various physical quantities ranging from magnetic fields to temperature. They package the performance of their bulk semiconductor counterparts into a nanoscale device, sometimes as small as a single atom. This review is a minimalist introduction into this concept. It gives a brief summary of quantum coherence, Ramsey spectroscopy and a derivation of the "standard quantum limit" of the sensitivity that a single-qubit sensor can reach. It goes on to discuss the surprising improvement that dynamical decoupling has brought about and concludes with an outlook to the major frontiers of the field.

It has been reported that the conversion yield and coherence time of ion-implanted NV centers improve if the Fermi level is raised or lowered during the annealing step following implantation. Here, we investigate whether surface transfer doping and surface charging, by UV light, can be harnessed to induce this effect. We analyze the coherence times and the yield of NV centers created by ion implantation and annealing, applying various conditions during annealing. Specifically, we study coating diamond with nickel, palladium, or aluminum oxide, to induce positive surface transfer doping, as well as annealing under UV illumination to trigger vacancy charging. The metal-coated diamonds display a two times higher formation yield than the other samples. The coherence time T-2 varies by less than a factor of two between the investigated samples. Both effects are weaker than previous reports, suggesting that stronger modifications of the band structure are necessary to find a pronounced effect. UV irradiation has no effect on the yield and T-2 times.

We demonstrate dispersive readout of the spin of an ensemble of Nitrogen-Vacancy centers in a high-quality dielectric microwave resonator at room temperature. The spin state is inferred from the reflection phase of a microwave signal probing the resonator. Time-dependent tracking of the spin state is demonstrated, and is employed to measure the T1 relaxation time of the spin ensemble. Dispersive readout provides a microwave interface to solid state spins, translating a spin signal into a microwave phase shift. We estimate that its sensitivity can outperform optical readout schemes, owing to the high accuracy achievable in a measurement of phase. The scheme is moreover applicable to optically inactive spin defects and it is non-destructive, which renders it insensitive to several systematic errors of optical readout and enables the use of quantum feedback.

Cellular micromotion-a tiny movement of cell membranes on the nm-µm scale-has been proposed as a pathway for inter-cellular signal transduction and as a label-free proxy signal to neural activity. Here we harness several recent approaches of signal processing to detect such micromotion in video recordings of unlabeled cells. Our survey includes spectral filtering of the video signal, matched filtering, as well as 1D and 3D convolutional neural networks acting on pixel-wise time-domain data and a whole recording respectively.

Scanning probe microscopy (SPM) is traditionally based on very sharp tips, where the small size of the apex is critical for resolution. This paradigm is about to shift, since a novel generation of planar probes (such as color centers in diamond, superconducting sensors, and single electron transistors) promises to image small electric and magnetic fields with hitherto inaccessible sensitivity. To date, much effort has been put into fabricating these planar sensors on tip-like structures. This compromises performance and poses a considerable engineering challenge, which is mastered by only a few laboratories. Here we present a radically simplified, tipless, approach, a technique for scanning an extended planar sensor parallel to a planar sample at a distance of few tens of nanometers. It is based on a combination of far-field optical techniques to measure both tilt and distance between probe and sample with sub-mrad and sub-nm precision, respectively. Employing these measurements as a feedback signal, we demonstrate near-field optical imaging of plasmonic modes in silver nanowires by a single NV center. Our scheme simultaneously improves the sensor quality and enlarges the range of available sensors beyond the limitations of existing tip-based schemes.

We investigate the correlation between surface roughness and corresponding T2 times of near-surface nitrogenvacancy centers (~10 nm/5 keV implantation energy) in diamond. For this purpose we compare five different polishing techniques, including both purely mechanical as well as chemical mechanical approaches, two different substrate sources (IIa Technologies and Element Six) and two different surface terminations (Oand Htermination) during nitrogen-vacancy forming. All coherence times are measured and compared before and after an oxygen surface treatment at 520 °C. We find that the coherence times of shallow nitrogen-vacancy centers are surprisingly independent of surface

Nitrogen–vacancy defects in diamond can be used to visualize electric fields in an operating semiconductor device.