Diamond has gained a reputation as a uniquely versatile material, yet one that is intricate to grow and process. Resonating nanostructures made of single-crystal diamond are expected to possess excellent mechanical properties, including high-quality factors and low dissipation. Here we demonstrate batch fabrication and mechanical measurements of single-crystal diamond cantilevers with thickness down to 85 nm, thickness uniformity better than 20 nm and lateral dimensions up to 240 mm. Quality factors exceeding one million are found at room temperature, surpassing those of state-of-the-art single-crystal silicon cantilevers of similar dimensions by roughly an order of magnitude. The corresponding thermal force noise for the best cantilevers is B5 Á 10 À 19 N Hz À 1/2 at millikelvin temperatures. Single-crystal diamond could thus directly improve existing force and mass sensors by a simple substitution of resonator material. Presented methods are easily adapted for fabrication of nanoelectromechanical systems, optomechanical resonators or nanophotonic devices that may lead to new applications in classical and quantum science.

Quantum sensing takes advantage of well-controlled quantum systems for performing measurements with high sensitivity and precision. We have implemented a concept for quantum sensing with arbitrary frequency resolution, independent of the qubit probe and limited only by the stability of an external synchronization clock. Our concept makes use of quantum lock-in detection to continuously probe a signal of interest. Using the electronic spin of a single nitrogen-vacancy center in diamond, we demonstrate detection of oscillating magnetic fields with a frequency resolution of 70 microhertz over a megahertz bandwidth. The continuous sampling further guarantees an enhanced sensitivity, reaching a signal-to-noise ratio in excess of 10 for a 170-nanotesla test signal measured during a 1-hour interval. Our technique has applications in magnetic resonance spectroscopy, quantum simulation, and sensitive signal detection.

We present a new method for automatic detection of peaks in noisy periodic and quasi-periodic signals. The new method, called automatic multiscale-based peak detection (AMPD), is based on the calculation and analysis of the local maxima scalogram, a matrix comprising the scale-dependent occurrences of local maxima. The usefulness of the proposed method is shown by applying the AMPD algorithm to simulated and real-world signals

Multipulse sequences based on Carr-Purcell decoupling are frequently used for narrow-band signal detection in single-spin magnetometry. We have analyzed the behavior of multipulse sensing sequences under real-world conditions, including finite pulse durations and the presence of detunings. We find that these nonidealities introduce harmonics to the filter function, allowing additional frequencies to pass the filter. In particular, we find that the XY family of sequences can generate signals at the 2f ac , 4f ac , and 8f ac harmonics and their odd subharmonics, where f ac is the ac signal frequency. Consideration of the harmonic response is especially important for diamond-based nuclear-spin sensing where the nuclear magnetic resonance frequency is used to identify the nuclear-spin species, as it leads to ambiguities when several isotopes are present.

Nuclear magnetic resonance (NMR) spectroscopy is a powerful technique for analyzing the structure and function of molecules, and for performing three-dimensional imaging of the spin density. At the heart of NMR spectrometers is the detection of electromagnetic radiation, in the form of a free induction decay (FID) signal 1 , generated by nuclei precessing around an applied magnetic field. While conventional NMR requires signals from 10 12 or more nuclei, recent advances in sensitive magnetometry 2,3 have dramatically lowered this number to a level where few or even individual nuclear spins can be detected [4][5][6][7][8] . It is natural to ask whether continuous FID detection can still be applied at the single spin level, or whether quantum back-action modifies or even suppresses the NMR response. Here we report on tracking of single nuclear spin precession using periodic weak measurements 9-12 . Our experimental system consists of 13 C nuclear spins in diamond that are weakly interacting with the electronic spin of a nearby nitrogen-vacancy center, acting as an optically readable meter qubit. We observe and minimize two important effects of quantum back-action: measurementinduced decoherence 13 and frequency synchronization with the sampling clock 14,15 . We use periodic weak measurements to demonstrate sensitive, high-resolution NMR spectroscopy of multiple nuclear spins with a priori unknown frequencies. Our method may provide the optimum route for performing single-molecule NMR 16-18 at atomic resolution.Measurement back-action, an important feature of quantum measurements 19,20 , can usually be neglected in NMR because the spin-detector coupling is extremely weak. One prominent exception is radiation damping 21 , where the collective coupling of the nuclear ensemble gives rise to a damping of the magnetic resonance by the electric detection circuit. As nuclear ensembles become smaller, eventually consisting of only few or even a single nuclear spin, the close coupling to the detector is expected to modify 22,23 or inhibit 24 the free evolution of the spin. Recent work on ensembles of cold atoms 25 and trapped ions 13 reported simultaneous tracking of spin angle and amplitude through the use of weak, quantumnon-demolition measurements, indicating an avenue for mitigating back-action. Here, we show that it is possible to track the precession of a single nuclear spin and to extract the two central pieces of information in NMR: the free precession frequency and the dephasing time.To probe the coherent precession of a single nuclear spin we implemented the measurement system depicted in Fig. 1a. Our system consists of a 13 C nucleus (spin I = 1/2) isolated in the nearly spin-free lattice of a diamond crystal. The nuclear spin undergoes a free precession around the Z axis with an angular velocity given by the Larmor frequency ω 0 = γ n B 0 , where B 0 is the local magnetic field and γ n the nuclear gyromagnetic ratio. To detect the nuclear precession, we periodically couple the nuclear spin to the electronic spin ...