Microwave saturation spectroscopy of nitrogen-vacancy ensembles in   diamond
  Negatively-charged nitrogen-vacancy (NV$^-$) centers in diamond have generated much recent interest for their use in sensing. The sensitivity improves when the NV ground-state microwave transitions are narrow, but these transitions suffer from inhomogeneous broadening, especially in high-density NV ensembles. To better understand and remove the sources of broadening, we demonstrate room-temperature spectral "hole burning" of the NV ground-state transitions. We find that hole burning removes the broadening caused by magnetic fields from $^{13}$C nuclei and demonstrate that it can be used for magnetic-field-insensitive thermometry. 
P. Kehayias and M. Mr\'ozek and V.M. Acosta and A. Jarmola and D.S.   Rudnicki and R. Folman and W. Gawlik and D. Budker
physics.optics cond-mat.mes-hall physics.atom-ph