We report the realization of a quantum circuit in which an ensemble of electronic spins is coupled to a frequency tunable superconducting resonator. The spins are nitrogen-vacancy centers in a diamond crystal. The achievement of strong coupling is manifested by the appearance of a vacuum Rabi splitting in the transmission spectrum of the resonator when its frequency is tuned through the nitrogen-vacancy center electron spin resonance.

We present a study of the charge state conversion of single nitrogen-vacancy (NV) defects hosted in nanodiamonds (NDs). We first show that the proportion of negatively-charged NV − defects, with respect to its neutral counterpart NV 0 , decreases with the size of the ND. We then propose a simple model based on a layer of electron traps located at the ND surface which is in good agreement with the recorded statistics. By using thermal oxidation to remove the shell of amorphous carbon around the NDs, we demonstrate a significant increase of the proportion of NV − defects in 10-nm NDs. These results are invaluable for further understanding, control and use of the unique properties of negatively-charged NV defects in diamond.

International audienceReceived 15 September 2011; revised 14 November 2011; published 19 December 2011 We have investigated the early stages of graphitization on detonation nanodiamond during sequential annealing treatments under vacuum using x-ray photoelectron spectroscopy. Two different temperature-dependent regimes were observed. Below 900 °C, the nanodiamond surface reconstructs into graphitic domain but does not alter the diamond core. Above 900 °C, graphitization, i.e., carbon hybridization changes from sp3 to sp2, occurs from the nanodiamond surface toward the diamond core. Graphitization is observed at much lower temperatures on nanodiamonds than on bulk diamond due to the high concentration of structural defects on their surface. These results indicate that low-temperature annealing under vacuum is an efficient method to uncouple surface and bulk graphitization. Hybrid nanocarbons formed in these conditions, constituted of a diamond core with a thin graphitic outer shell, may have interesting catalytic and chemical properties