  We study the spin and orbital dynamics of single nitrogen-vacancy (NV)
centers in diamond between room temperature and 700 K. We find that the ability
to optically address and coherently control single spins above room temperature
is limited by nonradiative processes that quench the NV center's
fluorescence-based spin readout between 550 and 700 K. Combined with electronic
structure calculations, our measurements indicate that the energy difference
between the 3E and 1A1 electronic states is approximately 0.8 eV. We also
demonstrate that the inhomogeneous spin lifetime (T2*) is temperature
independent up to at least 625 K, suggesting that single NV centers could be
applied as nanoscale thermometers over a broad temperature range.
