Individual paramagnetic defect centers in diamond nanocrystals have been investigated by low-temperature high-resolution optical spectroscopy. Narrow fluorescence excitation spectral lines have been found, indicating transitions between individual spin sublevels. Spectral diffusion is explained by cross relaxation among spin sublevels and by the presence of excited electrons in the conduction band of diamond. The relaxation times are in the millisecond range. The system may be useful for quantum information processing with individual electron spins.

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Single NV centers in diamond coupled by hyperfine interaction (hfi) to neighboring 13 C nuclear spins are now widely used in emerging quantum technologies as elements of quantum memory adjusted to a nitrogen-vacancy (NV) center electron spin qubit. For nuclear spins with low flip-flop rate, single shot readout was demonstrated under ambient conditions. Here we report on a systematic search for such stable NV− 13 C systems using density functional theory to simulate the hfi and spatial characteristics of all possible NV− 13 C complexes in the H-terminated cluster C 510 [NV] -H 252 hosting the NV center. Along with the expected stable 'NV-axial− 13 C' systems wherein the 13 C nuclear spin is located on the NV axis, we found for the first time new families of positions for the 13 C nuclear spin exhibiting negligible hfi-induced flipping rates due to near-symmetric local spin density distribution. Spatially, these positions are located in the diamond bilayer passing through the vacancy of the NV center and being perpendicular to the NV axis. Analysis of available publications showed that, apparently, some of the predicted non-axial near-stable NV− 13 C systems have already been observed experimentally. A special experiment performed on one of these systems confirmed the prediction made.