Coherent coupling between single quantum objects is at the heart of modern quantum physics. When coupling is strong enough to prevail over decoherence, it can be used for the engineering of correlated quantum states. Especially for solid-

Nitrogen-vacancy colour centres in diamond can undergo strong, spin-sensitive optical transitions under ambient conditions, which makes them attractive for applications in quantum optics 1 , nanoscale magnetometry 2,3 and biolabelling 4 . Although nitrogen-vacancy centres have been observed in aggregated detonation nanodiamonds 5 and milled nanodiamonds 6 , they have not been observed in very small isolated nanodiamonds 7 . Here, we report the first direct observation of nitrogen-vacancy centres in discrete 5-nm nanodiamonds at room temperature, including evidence for intermittency in the luminescence (blinking) from the nanodiamonds. We also show that it is possible to control this blinking by modifying the surface of the nanodiamonds.Detonation nanodiamond is routinely produced on an industrial scale, and the raw material can be disintegrated into a stable 5-nm monodisperse colloid 8 . The combination of inert core and chemically reactive surface, which can host a variety of moieties, is appealing for chemists, biologists and material scientists 9,10 . Quantum magnetometry 2,3 is an example of an emerging technology that will directly benefit from the availability of nanocrystals with welldefined sizes in the 5-nm range, because the sensitivity to single spins is inversely proportional to the cube of the distance between the sensor (that is, the nitrogen-vacancy (NV) centre) and the spin being detected.Producing and detecting NV colour centres in isolated 5-nm detonation nanodiamond has been controversial, and there has been some scepticism regarding their stability as a useful emitter in a discrete crystal. For example, theoretical calculations of the crystal energy budget favour the location of nitrogen on the surface rather than in the core, which seems to explain the limited observation of NV centres in chemical vapour deposition and high-pressure high-temperature grains of less than 40 nm in size 11,12 , and favours the prediction that nanodiamonds smaller than 10 nm in size do not contain NV centres 7,13 . Although sub-10-nm nanodiamonds with NV centres have been produced using a top-down approach (milling luminescent high-pressure hightemperature microdiamonds into 7-nm particles 6,14 ), the question of NV stability in isolated detonation nanodiamonds persists.In aggregated detonation nanodiamonds (agglomerates and agglutinates 8 ), high-sensitivity, time-gated luminescence and electronic paramagnetic resonance spectroscopy have been used to extract a weak NV signal from a strong luminescence background 5 . The experiments highlight the eclipsing nature of the graphitic surface layers in nanodiamond aggregates-NV centres were simply not visible through the broadband luminescence from the surface and grain boundary material. To distinguish the NV spectral signature from the large grain boundary luminescence overhead, diamond synthesis yielding discrete sub-10-nm detonation nanodiamonds is vital. Here, we use a robust deaggregation and dispersion method, which diminishes the crystal-crystal interaction to...

Lifetime limited optical excitation lines of single nitrogen vacancy (NV) defect centers in diamond have been observed at liquid helium temperature. They display unprecedented spectral stability over many seconds and excitation cycles. Spectral tuning of the spin selective optical resonances was performed via the application of an external electric field (i.e. the Stark shift). A rich variety of Stark shifts were observed including linear as well as quadratic components. The ability to tune the excitation lines of single NV centers has potential applications in quantum information processing. 1Coupling between light and single spins in solids has attracted widespread attention particularly for applications in quantum computing and quantum communications 1 . The nitrogen-vacancy defect (NV) optical center in diamond is a particularly attractive solid state system for such applications. Its strong optical transition allows photoluminescence-based detection of single defect centers 2 . The potential of the NV center as a single photon source has been well recognized over the past few years 3,4 . Furthermore, because of its paramagnetic spin ground state, there are applications for quantum memory and quantum repeater systems 5 .In particular the long spin decoherence time (0.35ms), optical control of spin states 6-8 and the robustness of the spin coherence have enabled the demonstration of basic building block for quantum computing even at room temperature 9 .Recently it was demonstrated that the permanent magnetic dipole moment of the NV center can be exploited to couple defects for a separation distance of a few nm. Whilst this demonstrates the capability for the generation of correlated quantum states in defect center clusters, coupling based on this technique will be difficult to scale to many qubit systems.Other coupling schemes have recently been proposed which use instead their optical transition dipole moments and in some cases envisage coupling of the NV center to cavities. At the core of many such schemes is the underlying assumption that the optical transition can be tuned in resonance either with another NV center or with a cavity via an external applied field. Therefore, the ability to tune the frequency of spin-selective optical transitions of single NV centers is of crucial importance for any scalable architecture based on diamond NV centers.Externally controlled magnetic and electric fields are among the most prominent parameters that can be used for such control. Electric fields in particular allow for wide tuning of eigenstates. The electric field induced shift of the optical resonance lines has been observed for single atoms, ions in the gas phase 10 and single molecules 11 and quantum dots 12, 14 in the solid state. By contrast, for color centers in diamond, only a few bulk studies on electric field induced spectral line shifts have been carried out 15 . Usually these studies are difficult because the magnitude of the Stark effect is of the order of the inhomogeneous linewidth. Moreover, ...

Nitrogen-vacancy (NV -) color centers in diamond were created by implantation of 7 keV 15 N (I = ½) ions into type IIa diamond. Optically detected magnetic resonance was employed to measure the hyperfine coupling of the NV -centers. The hyperfine spectrum from 15 NV -arising from implanted 15 N can be distinguished from 14 NVcenters created by native 14 N (I = 1) sites. Analysis indicates 1 in 40 implanted 15 N atoms give rise to an optically observable 15 NV -center. This report ultimately demonstrates a mechanism by which the yield of NV -center formation by nitrogen implantation can be measured.