"Quantum sensing" describes the use of a quantum system, quantum properties or quantum phenomena to perform a measurement of a physical quantity. Historical examples of quantum sensors include magnetometers based on superconducting quantum interference devices and atomic vapors, or atomic clocks. More recently, quantum sensing has become a distinct and rapidly growing branch of research within the area of quantum science and technology, with the most common platforms being spin qubits, trapped ions and flux qubits. The field is expected to provide new opportunities -especially with regard to high sensitivity and precision -in applied physics and other areas of science. In this review, we provide an introduction to the basic principles, methods and concepts of quantum sensing from the viewpoint of the interested experimentalist.

Crystal defects in diamond have emerged as unique objects for a variety of applications, both because they are very stable and because they have interesting optical properties. Embedded in nanocrystals, they can serve, for example, as robust single-photon sources or as fluorescent biomarkers of unlimited photostability and low cytotoxicity. The most fascinating aspect, however, is the ability of some crystal defects, most prominently the nitrogen-vacancy (NV) center, to locally detect and measure a number of physical quantities, such as magnetic and electric fields. This metrology capacity is based on the quantum mechanical interactions of the defect's spin state. In this review, we introduce the new and rapidly evolving field of nanoscale sensing based on single NV centers in diamond. We give a concise overview of the basic properties of diamond, from synthesis to electronic and magnetic properties of embedded NV centers. We describe in detail how single NV centers can be harnessed for nanoscale sensing, including the physical quantities that may be detected, expected sensitivities, and the most common measurement protocols. We conclude by highlighting a number of the diverse and exciting applications that may be enabled by these novel sensors, ranging from measurements of ion concentrations and membrane potentials to nanoscale thermometry and single-spin nuclear magnetic resonance.

We have combined ultrasensitive magnetic resonance force microscopy (MRFM) with 3D image reconstruction to achieve magnetic resonance imaging (MRI) with resolution <10 nm. The image reconstruction converts measured magnetic force data into a 3D map of nuclear spin density, taking advantage of the unique characteristics of the "resonant slice" that is projected outward from a nanoscale magnetic tip. The basic principles are demonstrated by imaging the 1 H spin density within individual tobacco mosaic virus particles sitting on a nanometer-thick layer of adsorbed hydrocarbons. This result, which represents a 100 millionfold improvement in volume resolution over conventional MRI, demonstrates the potential of MRFM as a tool for 3D, elementally selective imaging on the nanometer scale.MRFM ͉ MRI ͉ nuclear magnetic resonance ͉ molecular structure imaging M agnetic resonance imaging (MRI) is well-known in medicine and in the neurosciences as a powerful tool for acquiring 3D morphological and functional information with resolution in the millimeter-to-submillimeter range (1, 2). Unfortunately, despite considerable effort, attempts to push the spatial resolution of conventional MRI into the realm of highresolution microscopy have been stymied by fundamental limitations, especially detection sensitivity (3, 4). Consequently, the highest resolution MRI microscopes today remain limited to voxel volumes Ͼ40 m 3 (5-8). The central issue is that MRI is based on the manipulation and detection of nuclear magnetism, and nuclear magnetism is a relatively weak physical effect. It appears that conventional coil-based inductive detection techniques simply cannot provide adequate signal-to-noise ratio for detecting voxel volumes below the micrometer size. This sensitivity constraint is unfortunate because MRI has much to offer the world of microscopy with its unique contrast modalities, its elemental selectivity, and its avoidance of radiation damage.Despite the many challenges, there is strong motivation to extend MRI to finer resolution, especially if the nanometer scale can be reached. At the nanometer scale, one might hope to directly and nondestructively image the 3D structure of individual macromolecules and molecular complexes (9). Such a powerful molecular imaging capability could be of particular interest to structural biologists trying to unravel the structure and interactions of proteins, especially for those proteins that cannot be crystallized for X-ray analysis, or are too large for conventional NMR spectroscopy. Nanoscale MRI, with its capacity for true 3D, subsurface imaging, its potential for generating contrast by selective isotopic labeling and its nondestructive nature, would be a welcome complement to the characteristics of electron microscopy. The key to pushing MRI to the nanoscale is detection sensitivity.Recently, a significant breakthrough in magnetic resonance detection sensitivity has been achieved by using magnetic resonance force microscopy (MRFM) (9-13), resulting in single spin detection for electrons ...

We cool the fundamental mechanical mode of an ultrasoft silicon cantilever from a base temperature of 2.2 K down to 2.9+/-0.3 mK using active optomechanical feedback. The lowest observed mode temperature is consistent with limits determined by the properties of the cantilever and by the measurement noise. For high feedback gain, the driven cantilever motion is found to suppress or "squash" the optical interferometer intensity noise below the shot noise level.

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.