Using pulsed photoionization the coherent spin manipulation and echo formation of ensembles of NV -centers in diamond are detected electrically realizing contrasts of up to 17 %. The underlying spin-dependent ionization dynamics are investigated experimentally and compared to Monte-Carlo simulations. This allows the identification of the conditions optimizing contrast and sensitivity which compare favorably with respect to optical detection.The nitrogen vacancy center NV -in diamond is a promising candidate for quantum applications, since its coherence time at room temperature is in the range of ms [1] and its spin can be read out by optical fluorescence detection [2]. These features have enabled the use of NV -centers, e.g., as a quantum sensor for magnetic fields [3,4] and temperature [5] , for scanning-probe spin imaging [6] and structure determination of single biological molecules [7]. Despite its apparent simplicity, however, optical spin readout has drawbacks: it is highly inefficient, requiring several 100 repetitions for a single spin readout, and cumbersome to implement in many applications. Electric readout of spin in a suitable diamond semiconductor device appears as an attractive way to surmount these limitations. It could enable access to NV -centers in dense arrays, with a spacing limited by the few-nm-small feature size of electron beam lithography [8] rather than the optical wavelength. It might, moreover, provide a way to read out other spin defects [9-11], potentially including optically inactive ones.Two methods for electric readout of NV -centers have been demonstrated. The method in Ref. 12 uses nonradiative energy transfer to graphene and detects the spin signal in the current through the graphene sheet generated by this transfer. In contrast, the method presented in Ref. 13 uses the charge carriers generated directly in the diamond host crystal by photoionization of the NV -centers (photocurrent detection of magnetic resonance, PDMR). Both methods, however, have until now only been used with continuous wave (cw) spin manipulation and have therefore remained limited to NV -detection. Here we demonstrate a scheme based on both pulsed spin manipulation and pulsed photoionization to truely read out the spin state of NV -centers electrically after coherent control, using Rabi oscillations and echo experiments as examples. We employ this scheme to establish a quantitative model of photoionization, simulate the readout efficiency and predict, that under optimized conditions pulsed electric readout could outperform optical fluorescence detection.The spin-dependent photoionization cycle can be understood as an effective four-photon process, whose spin dependence relies on the NV -center's inter-system crossing (ISC) which is also key to the classic optical readout ( Fig. 1 a)) [13]. A first photon (green arrows) triggers shelving (black arrow) of NV -centers in spin state |2 (corresponding to the m S = ±1 spin quantum numbers of NV -) into the long-lived metastable singlet state |5 by this ISC. Si...

Quantum sensors-qubits sensitive to external fields-have become powerful detectors for various small acoustic and electromagnetic fields. A major key to their success have been dynamical decoupling protocols which enhance sensitivity to weak oscillating (AC) signals. Currently, those methods are limited to signal frequencies below a few MHz. Here we harness a quantum-optical effect, the Mollow triplet splitting of a strongly driven two-level system, to overcome this limitation. We microscopically understand this effect as a pulsed dynamical decoupling protocol and find that it enables sensitive detection of fields close to the driven transition. Employing a nitrogen-vacancy center, we detect GHz microwave fields with a signal strength (Rabi frequency) below the current detection limit, which is set by the center's spectral linewidth 1=T Ã 2 . Pushing detection sensitivity to the much lower 1/T 2 limit, this scheme could enable various applications, most prominently coherent coupling to single phonons and microwave photons.

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 (color centers in diamond 1 , superconducting sensors 2 and single electron transistors 3 ) 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 4,5 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.The rise of nanotechnology has been largely fueled by scanning probe microscopy, most prominently atomic force and scanning tunneling microscopy, which have provided a tool to image topography, friction and electronic structure with a resolution down to the atomic level 6,7 . Today, the imaging of quantities other than forces and electric transport properties stands out as a frontier in this field of research. Scanning electron transistors (SETs) are being used to image weak electric stray fields 3,8 while weak magnetic fields are accessible to scanning Hall bars 9 , nitrogenvacancy (NV) centers in diamond 1,10-12 and scanning superconducting interference devices (SQUIDs) 13-15 .These two latter sensors have, more recently, even imaged surface temperature 16 and thermal conductivity 17 .Fig 1 -Planar scanning probe microscope. a, An extended planar sensor (blue) can be brought into 10 nm-scale proximity with a planar sample (orange) if tilt is controlled to the sub-mrad level. NV: nitrogen-vacancy center, SQUID: superconducting sensor, SET: single electron transistor. b, Optical measurement of tilt by interference reflection microscopy. Reflections in a multilayer interfere, forming Newton's fringes. c, Optical measurement of the sensor-sample gap Δ by a combination of total internal reflection microscopy (TIRM) and Brewster angle microscopy (BAM). A sample scatters light from an evanescent field at the sensor surface, changing the intensity and/or polarization of the reflected beam. Effects of both b and c are imaged via a microscope looking from above.Most of these novel techniques share a central challenge: the sensors are extended planar devices -circuits fabricated on a substrate or crystals -but scanning probe positioning requires a sharp tip to provide forc...