  We demonstrate an all-optical thermometer based on an ensemble of
silicon-vacancy centers (SiVs) in diamond by utilizing a temperature dependent
shift of the SiV optical zero-phonon line transition frequency,
$\Delta\lambda/\Delta T= 6.8\,\mathrm{GHz/K}$. Using SiVs in bulk diamond, we
achieve $70\,\mathrm{mK}$ precision at room temperature with a sensitivity of
$360\,\mathrm{mK/\sqrt{Hz}}$. Finally, we use SiVs in $200\,\mathrm{nm}$
nanodiamonds as local temperature probes with $521\,\mathrm{ mK/\sqrt{Hz}}$
sensitivity. These results open up new possibilities for nanoscale thermometry
in biology, chemistry, and physics, paving the way for control of complex
nanoscale systems.
