Scanning probe microscopy is one of the most versatile windows into the nanoworld, providing imaging access to a variety of sample properties, depending on the probe employed.Tunneling probes map electronic properties of samples 1 , magnetic and photonic probes image their magnetic and dielectric structure 2,3 while sharp tips probe mechanical properties like surface topography, friction or stiffness 4 . Most of these observables, however, are accessible only under limited circumstances. For instance, electronic properties are measurable only on conducting samples while atomic-resolution force microscopy requires careful preparation of samples in ultrahigh vacuum 5,6 or liquid environments 7 .Here we demonstrate a scanning probe imaging method that extends the range of accessible quantities to label-free imaging of chemical species operating on arbitrary samples -including insulating materials -under ambient conditions. Moreover, it provides three-dimensional depth information, thus revealing subsurface features. We achieve these results by recording nuclear magnetic resonance signals from a sample surface with a recently introduced scanning probe, a single nitrogen-vacancy center in diamond. We demonstrate NMR imaging with 10 nm resolution and achieve chemically specific contrast by separating fluorine from hydrogen rich regions.Our result opens the door to scanning probe imaging of the chemical composition and atomic structure of arbitrary samples. A method with these abilities will find widespread application in material science even on biological specimens down to the level of single macromolecules.The development of a scanning probe sensor able to image nuclear spins has been a long and outstanding goal of nanoscience, proposed shortly after the invention of scanning probe microscopy itself 8 . To date, this goal is most closely met by magnetic resonance force microscopy (MRFM), an extension of atomic-force-microscopy with sensitivity to spins, which has successfully imaged nanoscale distributions of nuclear spins in three dimensions 9 .However, its operation is experimentally challenging, requiring low (sub-Kelvin) temperature and long (weeks/image) acquisition times, which has so far precluded its adoption as a routine technique. To surmount these problems, single electron spins with optical readout capability have been proposed as an alternative local probe for spin distributions 10 . This complementary approach has become a realistic prospect since recent research has established the nitrogenvacancy center, a color defect in diamond 11 , as a candidate system for this scheme 12,13 . This center serves as an atomic-sized magnetic field sensor, which has proven sufficiently sensitive to detect the field of single nuclear spins in its diamond lattice environment [14][15][16] as well as ensembles of 10-10 4 spins in a nanometer-sized sample volume on the diamond surface [17][18][19] .Here we employ a single NV center as a scanning probe to image distributions of nuclear spins in an external sample. All our ...

We present a novel spectroscopy protocol based on optimal control of a single quantum system. It enables measurements with quantum-limited sensitivity ( √ denoting the system's coherence time) but has an orders of magnitude larger dynamic range than pulsed spectroscopy methods previously employed for this task. We employ this protocol to image nanoscale magnetic fields with a single scanning NV center in diamond. Here, our scheme enables quantitative imaging of a strongly inhomogeneous field in a single scan without closed-loop control, which has previously been necessary to achieve this goal.Optimal control of quantum systems is an experimental technique that has evolved over the two past decades [1-3] as a generalization of related techniques like composite pulses [4] or adiabatic control [5]. It implements unitary operations ("quantum gates") of very high fidelity by irradiating a quantum system with numerically optimized excitation pulses. Amplitude and phase of this pulse are an arbitrary function of time, which is tailored such as to result in a specific unitary operation.Numerical optimization can generate pulses that achieve near-perfect operation (i.e. high fidelity) over a wide range of experimental parameters, such as excitation power or detuning, rather than a single specific set. This is in contrast to simple (e.g. rectangular) pulses and arises from the fact that optimization has access to the much larger space of arbitrary amplitude and phase profiles. Thanks to these additional degrees of freedom, the resulting pulse can satisfy a larger number of constraints. In practice, this has been used to generate "robust" pulses which are immune against fluctuations of the excitation power, or pulses that implement a specific operation within a large bandwidth of different system frequencies [3], as they may arise e.g. by inhomogeneous broadening.Here we show that optimal control can be used to achieve an opposite goal, a pulse that is maximally sensitive to fluctuations of one experimental parameter (in our case the static magnetic field) while it preserves robustness against fluctuations of all other parameters and, in particular, a large operating bandwidth. With these properties, such a pulse enables sensitive spectroscopy of a system even in the presence of large unknown frequency offsets. The concept is illustrated in more detail in Fig. 1(a-b). Sensitive spectroscopy classically relies on sharp selective excitation ( Fig. 1(a)), realized for instance by a long low-power excitation pulse (Rabi spectroscopy) or a suitable pulse sequence (Ramsey spectroscopy). We extend these schemes by designing an optimal control pulse which generates a grating of equally sharp excitation lines, evenly spaced over a large bandwidth (Fig. 1(b)). With this protocol, small changes of the system's resonance frequency can be tracked without tuning the excitation pulse to the system's frequency.

Supporting Information. Detailed description of the experimental setup and the data acquisition. Overview on sample and T2 dephasing fitting function. Dependence of the transfer function on the NV axis orientation. SQUID susceptometry measurements. Study of the fit parameters. Measurements of the collapses and revivals of the 13 C nuclei at different fields.Significance of simultaneous acquisition of T1 and T2 for the fit.

Extension of the ECRH operational space in ASDEX Upgrade 2 Abstract. ASDEX Upgrade is operated with tungsten-coated plasma-facing components since several years. H-mode operation with good confinement has been demonstrated. Nevertheless purely NBI-heated H-modes with reduced gas puff, moderate heating power or/and increased triangularity tend to accumulate tungsten, followed by a radiative collapse. Under these conditions, central electron heating with ECRH, usually in X2 polarisation, changes the impurity transport in the plasma centre, reducing the central tungsten concentration and, in many cases, stabilizing the plasma. In order to extend the applicability of central ECRH to a wider range of magnetic field and plasma current additional ECRH schemes with reduced single pass absorption have been implemented: X3 heating allows to reduce the magnetic field by 30 %, such that the first H-modes with an ITER-like value of the safety factor of q 95 = 3 could be run in the tungsten-coated device. O2 heating increases the cutoff density by a factor of 2 allowing to address higher currents and triangularities. For both schemes scenarios have been developed to cope with the associated reduced absorption. In case of central X3 heating, the X2 resonance lies close to the pedestal top at the high-field side of the plasma, serving as a beam dump. For O2, holographic mirrors have been developed which guarantee a second pass through the plasma centre. The beam position on these reflectors is controlled by fast thermocouples. Stray-radiation protection has been implemented using sniffer-probes.

The multi-frequency Electron Cyclotron Heating (ECRH) system at the ASDEX Upgrade tokamak employs depressed collector gyrotrons, step-tunable in the range 105-140 GHz. The system is equipped with a fast steerable launcher allowing for remote steering of the ECRH RF beam during the plasma discharge. The gyrotrons and the mirrors are fully integrated in the discharge control system. The polarization can be controlled in a feed-forward mode. 3 Sniffer probes for millimeter wave stray radiation detection have been installed. Key words:Electron cyclotron resonance heating, step-tunable gyrotron, fast steerable launcher. IntroductionElectron cyclotron resonance heating (ECRH) and current drive (ECCD) experiments at the ASDEX Upgrade tokamak first started in 1996 with a single frequency (140 GHz) system that comprises 4 gyrotrons with 0.5 MW / 2 sec each (optionally 0.7 MW / 1 sec) and 4 independent transmission lines and launchers [1]. A new multi-frequency ECRH system is currently under construction that employs depressed collector gyrotrons, step-tunable in the range 105-140 GHz with 1 MW output power at 140 GHz and slightly reduced power (~800 kW) at lower frequencies [2]. The pulse length of the system is 10 s, corresponding to the maximum flat top time of ASDEX Upgrade plasma discharges. In its final stage it will consist of 4 gyrotrons, where the first 3 gyrotrons are two-frequency gyrotrons, operating at 105 and 140 GHz. For the fourth gyrotron a broadband output window is under development that will allow operation also at intermediate frequencies. The transmission line consists of a quasi-optical Matching Optics Unit (MOU) and of non-evacuated corrugated HE 11 waveguides with an inner diameter of 87 mm and a total length of approximately 70 m. The launchers of the new system have a poloidal fast steering capability that allows for a change of the deposition location during the discharge without changing the toroidal magnetic field. The ultimate goal is to have a very flexible system for localized plasma heating and current drive that allows for feedback control of neoclassical tearing modes, pressure profile and transport [3].Since 2007 ASDEX Upgrade operates with fully tungsten covered plasma facing components. Central heating with ECRH plays a key role in suppressing tungsten accumulation in the plasma center. Up to now the standard operation mode is the extraordinary mode at the second harmonic, X2-mode, because of its full single pass absorption. In order to extend the applicability of central ECRH to a wider range of magnetic field and plasma current, schemes with reduced single-pass absorption have been implemented: X3-mode heating allows to reduce the magnetic field by 30 %, such that the first H-modes with an ITER-like safety factor of q 95 =3 could be run. Heating with the second harmonic ordinary mode, O2-mode, increases the plasma cutoff density for the ECRH by a factor of 2 and therefore allows to access higher plasma currents and triangularities [4].