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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.

The fluorescence and motional dynamics of single diamond nanocrystals in buffer solution and in living cells is investigated. Stable hydrosols of nanodiamonds in buffer solutions are investigated by fluorescence correlation spectroscopy. Measurement of the effective hydrodynamic radius yields particles of 48 nm diameter, which is in excellent agreement with atomic force microscopy measurements made on the same particles. Fluorescence correlation spectroscopy measurements indicate that nanocrystals easily form aggregates when the buffer pH is changed. This tendency is reduced when the surface of the diamonds is covered with surfactants. Upon incubation, cells spontaneously take up nanocrystals that uniformly distribute in cells. Most of the particles get immobilized within a few minutes. The binding of streptavidin to biotinylated aggregates of 4 nm diameter nanodiamonds is demonstrated.

A single defect center in diamond periodically excited by a laser is shown to provide a simple realization for a system obeying a fluctuation theorem for nonthermal noise. The distribution of these fluctuations is distinctly non-Gaussian, which has also been verified by numerical calculation. For driving protocols symmetric under time reversal a more restricted form of the theorem holds, which is also known from entropy fluctuations caused by thermal noise.