Spectroscopy of composite solid-state spin environments for improved   metrology with spin ensembles
  For precision coherent measurements with ensembles of quantum spins the relevant Figure-of-Merit (FOM) is the product of polarized spin density and coherence lifetime, which is generally limited by the dynamics of the spin environment. Here, we apply a coherent spectroscopic technique to characterize the dynamics of the composite solid-state spin environment of Nitrogen-Vacancy (NV) centers in room temperature diamond. For samples of very different NV densities and impurity spin concentrations, we show that NV FOM values can be almost an order of magnitude larger than previously achieved in other room-temperature solid-state spin systems, and within an order of magnitude of the state-of-the-art atomic system. We also identify a new mechanism for suppression of electronic spin bath dynamics in the presence of a nuclear spin bath of sufficient concentration. This suppression could inform efforts to further increase the FOM for solid-state spin ensemble metrology and collective quantum information processing. 
N. Bar-Gill, L. M. Pham, C. Belthangady, D. Le Sage, P. Cappellaro, J.   R. Maze, M. D. Lukin, A. Yacoby, R. Walsworth
cond-mat.mes-hall quant-ph