"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.

The ability to sensitively detect individual charges under ambient conditions would benefit a wide range of applications across disciplines. However, most current techniques are limited to low-temperature methods such as single-electron transistors 1,2 , single-electron electrostatic force microscopy 3 and scanning tunnelling microscopy 4 . Here we introduce a quantum-metrology technique demonstrating precision three-dimensional electric-field measurement using a single nitrogen-vacancy defect centre spin in diamond. An a.c. electric-field sensitivity reaching 202 ± 6 V cm −1 Hz −1/2 has been achieved. This corresponds to the electric field produced by a single elementary charge located at a distance of ∼150 nm from our spin sensor with averaging for one second. The analysis of the electronic structure of the defect centre reveals how an applied magnetic field influences the electric-field-sensing properties. We also demonstrate that diamond-defect-centre spins can be switched between electric-and magnetic-field sensing modes and identify suitable parameter ranges for both detector schemes. By combining magnetic-and electric-field sensitivity, nanoscale detection and ambient operation, our study should open up new frontiers in imaging and sensing applications ranging from materials science to bioimaging.Sensitive imaging or detection of charges is an outstanding task in a variety of applications. The development, for example, of single-electron transistors (SET; ref. 5) has pushed charge sensing to an unprecedented sensitivity of 10 −6 electron charge, and is being used in low-temperature detection and scanning applications 2 , and in sensors in quantum devices 6 . Inspired by the development of tunnelling microscopy a variety of scanning probes have been devised to measure surface electrical properties, such as scanning capacitance microscopy (SCM; ref. 7), scanning Kelvin probe (ref. 8) and electric field-sensitive atomic force microscopy (EFM; ref. 9). Indeed, the last method has shown the remarkable ability to detect the presence of individual charges 3 .We report on a fundamentally new method that uses the spin of single defect centres in diamond to sense electric-field-dependent shifts in energy levels. Sensitive electric-field detection is based on the remarkable properties of the NV centre 10 . The most notable of these are: the detection of fluorescence from single defects to provide an atom-sized local probe 11 , outstandingly long spin dephasing times 12 , as well as the controlled positioning of single centres 13,14 . These properties have led to a variety of applications of the NV centre, ranging from quantum science 15 and precision magnetic-field sensing [16][17][18][19][20][21][22][23][24] to biolabelling 25,26 . It is the aim of the present work to explore the interplay between the Zeeman shift, local strain effects and the Stark shift of the ground state spin manifold and use the improved understanding of this interplay for the sensing of electric fields. We will show that the decoupling of the...

Application of nuclear magnetic resonance (NMR) spectroscopy to nanoscale samples has remained an elusive goal, achieved only with great experimental effort at subkelvin temperatures. We demonstrated detection of NMR signals from a (5-nanometer)(3) voxel of various fluid and solid organic samples under ambient conditions. We used an atomic-size magnetic field sensor, a single nitrogen-vacancy defect center, embedded ~7 nanometers under the surface of a bulk diamond to record NMR spectra of various samples placed on the diamond surface. Its detection volume consisted of only 10(4) nuclear spins with a net magnetization of only 10(2) statistically polarized spins.

We investigate the effect of surface termination on the charge state of nitrogen vacancy centers, which have been ion-implanted few nanometers below the surface of diamond. We find that, when changing the surface termination from oxygen to hydrogen, previously stable NV − centers convert into NV 0 and, subsequently, into an unknown non-fluorescent state. This effect is found to depend strongly on the implantation dose. Simulations of the electronic band structure confirm the dissappearance of NV − in the vicinity of the hydrogen-terminated surface. The band bending, which induces a p-type surface conductive layer leads to a depletion of electrons in the nitrogenvacancies close to the surface. Therefore, hydrogen surface termination provides a chemical way for the control of the charge state of nitrogen-vacancy centers in diamond. Furthermore, it opens the way to an electrostatic control of the charge state with the use of an external gate electrode.