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...

Spins in solids are cornerstone elements of quantum spintronics. Leading contenders such as defects in diamond or individual phosphorus dopants in silicon have shown spectacular progress, but either lack established nanotechnology or an efficient spin/photon interface. Silicon carbide (SiC) combines the strength of both systems: it has a large bandgap with deep defects and benefits from mature fabrication techniques. Here, we report the characterization of photoluminescence and optical spin polarization from single silicon vacancies in SiC, and demonstrate that single spins can be addressed at room temperature. We show coherent control of a single defect spin and find long spin coherence times under ambient conditions. Our study provides evidence that SiC is a promising system for atomic-scale spintronics and quantum technology.

Projective measurement of single electron and nuclear spins has evolved from a gedanken experiment to a problem relevant for applications in atomic-scale technologies like quantum computing. Although several approaches allow for detection of a spin of single atoms and molecules, multiple repetitions of the experiment that are usually required for achieving a detectable signal obscure the intrinsic quantum nature of the spin's behavior. We demonstrated single-shot, projective measurement of a single nuclear spin in diamond using a quantum nondemolition measurement scheme, which allows real-time observation of an individual nuclear spin's state in a room-temperature solid. Such an ideal measurement is crucial for realization of, for example, quantum error correction protocols in a quantum register.

The detection of single nuclear spins would be useful for fields ranging from basic science to quantum information technology. However, although sensing based on diamond defects and other methods have shown high sensitivity, they have not been capable of detecting single nuclear spins, and defect-based techniques further require strong defect-spin coupling. Here, we present the detection and identification of single and remote (13)C nuclear spins embedded in nuclear spin baths surrounding a single electron spin of a nitrogen-vacancy centre in diamond. We are able to amplify and detect the weak magnetic field noise (∼10 nT) from a single nuclear spin located ∼3 nm from the centre using dynamical decoupling control, and achieve a detectable hyperfine coupling strength as weak as ∼300 Hz. We also confirm the quantum nature of the coupling, and measure the spin-defect distance and the vector components of the nuclear field. The technique marks a step towards imaging, detecting and controlling nuclear spins in single molecules.

We report on the realization of an all-optical transistor by mapping gate and source photons into strongly interacting Rydberg excitations with different principal quantum numbers in an ultracold atomic ensemble. We obtain a record switch contrast of 40 % for a coherent gate input with mean photon number one and demonstrate attenuation of source transmission by over 10 photons with a single gate photon. We use our optical transistor to demonstrate the nondestructive detection of a single Rydberg atom with a fidelity of 0.72(4).In analogy to their electronic counterparts, all-optical switches and transistors are required as basic building blocks for both classical and quantum optical information processing [1,2]. Reaching the fundamental limit of such devices, where a single gate photon modifies the transmission or phase accumulation of multiple source photons, requires strong effective interaction between individual photons. Engineering sufficiently strong optical nonlinearities to facilitate photon-photon interaction is one of the key goals of modern optics and immense progress towards this goal has been made in a variety of systems in recent years. Most prominent so far are cavity QED experiments, which use high finesse resonators to enhance interaction between light and atoms [3][4][5][6][7] or artificial atoms [8][9][10][11]. In particular, a high-contrast, highgain optical transistor operated by a single photon stored in an atomic ensemble inside a cavity has recently been demonstrated [12]. Cavity-free approaches include single dye molecules [13] and atoms coupled to hollow-core [14,15] and tapered nano-fibers [16].A novel free-space approach to realize single-photon nonlinearities is to map strong interaction between Rydberg atoms [17] onto slowly travelling photons [18,19] using electromagnetically induced transparency (EIT) [20]. This has already been used to demonstrate highly efficient single-photon generation [21], attractive interaction between single photons [22], entanglement generation between light and atomic excitations [23], and most recently single-photon all-optical switching [24].However, demonstration of amplification, that is, controlling many photons with a single one, has so far only been achieved in a cavity QED setup [12]. Gain G 1 is one of the key properties of the electric transistor that lies at the heart of its countless applications. In this work, we demonstrate an all-optical transistor with gain G > 10. We employ two-color Rydberg EIT in a freespace ensemble to independently couple gate and source photons to different Rydberg states [25]. For a coherent gate pulse containing on average one photon, we observe a switch contrast of C coh = 0.39(4), defined aswhere N with gate s,out and N no gate s,out denote the mean numbers of transmitted source photons with and without gate photons. From this we extrapolate that a singlephoton Fock state gate pulse causes a switch contrast C sp = 0.53(2). The measured switch contrast is robust against the number of incoming source photons, enab...