As quantum mechanics ventures into the world of applications and engineering, materials science faces the necessity to design matter to quantum grade purity. For such materials, quantum effects define their physical behaviour and open completely new (quantum) perspectives for applications. Carbon-based materials are particularly good examples, highlighted by the fascinating quantum properties of, for example, nanotubes or graphene. Here, we demonstrate the synthesis and application of ultrapure isotopically controlled single-crystal chemical vapour deposition (CVD) diamond with a remarkably low concentration of paramagnetic impurities. The content of nuclear spins associated with the (13)C isotope was depleted to 0.3% and the concentration of other paramagnetic defects was measured to be <10(13) cm(-3). Being placed in such a spin-free lattice, single electron spins show the longest room-temperature spin dephasing times ever observed in solid-state systems (T2=1.8 ms). This benchmark will potentially allow observation of coherent coupling between spins separated by a few tens of nanometres, making it a versatile material for room-temperature quantum information processing devices. We also show that single electron spins in the same isotopically engineered CVD diamond can be used to detect external magnetic fields with a sensitivity reaching 4 nT Hz(-1/2) and subnanometre spatial resolution.

Magnetic resonance imaging and optical microscopy are key technologies in the life sciences. For microbiological studies, especially of the inner workings of single cells, optical microscopy is normally used because it easily achieves resolution close to the optical wavelength. But in conventional microscopy, diffraction limits the resolution to about half the wavelength. Recently, it was shown that this limit can be partly overcome by nonlinear imaging techniques 1,2 , but there is still a barrier to reaching the molecular scale. In contrast, in magnetic resonance imaging the spatial resolution is not determined by diffraction; rather, it is limited by magnetic field sensitivity, and so can in principle go well below the optical wavelength. The sensitivity of magnetic resonance imaging has recently been improved enough to image single cells 3,4 , and magnetic resonance force microscopy 5 has succeeded in detecting single electrons 6 and small nuclear spin ensembles 7 . However, this technique currently requires cryogenic temperatures, which limit most potential biological applications 8 . Alternatively, single-electron spin states can be detected optically 9,10 , even at room temperature in some systems [11][12][13][14] . Here we show how magneto-optical spin detection can be used to determine the location of a spin associated with a single nitrogen-vacancy centre in diamond with nanometre resolution under ambient conditions. By placing these nitrogen-vacancy spins in functionalized diamond nanocrystals, biologically specific magnetofluorescent spin markers can be produced. Significantly, we show that this nanometre-scale resolution can be achieved without any probes located closer than typical cell dimensions. Furthermore, we demonstrate the use of a single diamond spin as a scanning probe magnetometer to map nanoscale magnetic field variations. The potential impact of single-spin imaging at room temperature is far-reaching. It could lead to the capability to probe biologically relevant spins in living cells.The nitrogen-vacancy centre in diamond is a unique solid state system that allows ultrasensitive and rapid detection of single electronic spin states under ambient conditions 12 . The nitrogen-vacancy defect is a naturally occurring impurity that is responsible for the pink colouration of diamond crystals when present in high concentration. It was demonstrated that this colour centre can be produced in diamond nanocrystals by electron irradiation. Fluorescing nitrogen-vacancy diamond nanocrystals can be used as markers for bioimaging applications 15 . Such markers have attracted widespread interest because of their unprecedented photostability and non-toxicity 16,17 . It was recognized recently that the magnetic properties of such fluorescent labels can in principle be used for novel microscopy 18,19 . Here we demonstrate the realization of a magneto-optic microscope using nitrogen-vacancy diamond as the magnetic fluorescent label that moreover does not bleach or blink. Figure 1c and d show the fluorescenc...

Understanding and controlling the complex environment of solid-state quantum bits is a central challenge in spintronics and quantum information science. Coherent manipulation of an individual electron spin associated with a nitrogen-vacancy center in diamond was used to gain insight into its local environment. We show that this environment is effectively separated into a set of individual proximal 13C nuclear spins, which are coupled coherently to the electron spin, and the remainder of the 13C nuclear spins, which cause the loss of coherence. The proximal nuclear spins can be addressed and coupled individually because of quantum back-action from the electron, which modifies their energy levels and magnetic moments, effectively distinguishing them from the rest of the nuclei. These results open the door to coherent manipulation of individual isolated nuclear spins in a solid-state environment even at room temperature.

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...