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We present a protocol to achieve double quantum magnetometry at large static magnetic fields. This is a regime where sensitive sample parameters, such as the chemical shift, get enhanced facilitating their characterization. In particular, our method delivers two-tone stroboscopic radiation patterns with modulated Rabi frequencies to achieve larger spectral signals. Furthermore, it does not introduce inhomogeneous broadening in the sample spectrum preventing signal misinterpretation. Moreover, our protocol is designed to work under realistic conditions such as the presence of moderate microwave power and errors on the radiation fields. Albeit we particularise to nitrogen vacancy centers, our protocol is general, thus applicable to distinct quantum sensors.Introduction-The detection of magnetic signals emitted by spin ensembles is central in nanoscale nuclear magnetic resonance (nanoscale NMR) [1]. Here, the hyperfine quantum state of a sensor such as the nitrogen vacancy (NV) center in diamond [2,3] gets modified owing to the interaction with a nuclear or electronic target, leading to quantum sensing or quantum detection [4,6]. To this end, the NV center quantum state is initialised and measured with a laser field, while it can be readily controlled with microwave (MW) radiation [2,3]. In addition, the NV center presents quantum coherence, as well as a long decay time of the order of milliseconds, even at room temperature [5]. These are capacities that make NV centers in diamond ideal candidates for experiments at physiological conditions [6]. The coupling of the NV center with a target signal is typically conducted trough dynamical decoupling (DD) techniques [7] that, in addition, are able to remove noisy contributions over the sensor. Typically, continuous and pulsed (or stroboscopic) DD techniques are considered. The former relies on the Hartmann-Hahn resonance condition [8], while certain stroboscopic DD radiation patterns, as those of the XY family [9-15], show a superior level of robustness against errors on the control fields. First experiments with NV centers were able to detect a classical electromagnetic field [16], while individual 13 C nuclear spin emitters embedded in the diamond lattice have been identified with unprecedented resolution [17][18][19][20][21][22][23][24]. In addition, nanoscale NMR of small volume samples of the order of picolitres, as well as of single molecules located external to the diamond lattice, have been achieved with single NV centers [25][26][27][28][29][30] and with NV ensembles [31,32].Several developments have been carried out to overcome the poor spectral resolution achievable with NV quantum sensors at room temperature. Among them, we have the combination of NV-based quantum detection with the presence of a quantum memory [33], or by synchronising NV measurements with a classical clock [32,34,35]. These hybrid techniques have allowed the detection of coherent target signals with a frequency resolution of the order of few hertz with NV centers. Here, it is important to r...

Nuclear magnetic resonance (NMR) schemes can be applied to micron-, and nanometer-sized samples by the aid of quantum sensors such as nitrogen vacancy (NV) color centers in diamond. These minute devices allow for magnetometry of nuclear spin ensembles with high spatial and frequency resolution at ambient conditions, thus having a clear impact in different areas such as chemistry, biology, medicine, and material sciences. In practice, NV quantum sensors are driven by microwave (MW) control fields with a twofold objective: On the one hand, MW fields bridge the energy gap between NV and nearby nuclei which enables a coherent and selective coupling among them while, on the other hand, MW fields remove environmental noise on the NV leading to enhanced interrogation time. In this work we review distinct MW radiation patterns, or dynamical decoupling techniques, for nanoscale NMR applications. perspective

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We present a protocol that detects molecular conformational changes with two nitroxide electron-spin labels and a nitrogen-vacancy (NV) center in diamond. More specifically, we demonstrate that the NV can detect energy shifts induced by the coupling between electron-spin labels. The protocol relies on the judicious application of microwave and radiofrequency pulses in a range of parameters that ensures stable nitroxide resonances. Furthermore, we demonstrate that our scheme is optimized by using nitroxides with distinct nitrogen isotopes. We use detailed numerical simulations and Bayesian inference techniques to demonstrate that our method enables the detection of conformational changes under realistic conditions including strong NV dephasing rates as a consequence of the diamond surface proximity and nitroxide thermalization mechanisms. Finally, we show that random molecular tumbling can be exploited to extract the inter-label distance.