  Sensitive probing of temperature variations on nanometer scales represents an
outstanding challenge in many areas of modern science and technology. In
particular, a thermometer capable of sub-degree temperature resolution as well
as integration within a living system could provide a powerful new tool for
many areas of biological research, including temperature-induced control of
gene expression and cell-selective treatment of disease. Here, we demonstrate a
new approach to nanoscale thermometry that utilizes coherent manipulation of
the electronic spin associated with nitrogen-vacancy (NV) color centers in
diamond. We show the ability to detect temperature variations down to 1.8 mK
(sensitivity of 9 mK/sqrt(Hz)) in an ultra-pure bulk diamond sample. Using NV
centers in diamond nanocrystals (nanodiamonds), we directly measure the local
thermal environment at length scales down to 200 nm. Finally, by introducing
both nanodiamonds and gold nanoparticles into a single human embryonic
fibroblast, we demonstrate temperature-gradient control and mapping at the
sub-cellular level, enabling unique potential applications in life sciences.
