Measurement and Control of Single Nitrogen-Vacancy Center Spins above   600 K
  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. 
D. M. Toyli, D. J. Christle, A. Alkauskas, B. B. Buckley, C. G. Van de   Walle, and D. D. Awschalom
cond-mat.mes-hall quant-ph