Cavity-enhanced room-temperature magnetometry using absorption by   nitrogen-vacancy centers in diamond
  We demonstrate a cavity-enhanced room-temperature magnetic field sensor based on nitrogen-vacancy centers in diamond. Magnetic resonance is detected using absorption of light resonant with the 1042 nm spin-singlet transition. The diamond is placed in an external optical cavity to enhance the absorption, and significant absorption is observed even at room temperature. We demonstrate a magnetic field sensitivity of 2.5 nT/sqrt(Hz), and project a photon shot-noise-limited sensitivity of 70 pT/sqrt(Hz) for a few mW of infrared light, and a quantum projection-noise-limited sensitivity of 250 fT/sqrt(Hz) for the sensing volume of 90 um x 90 um 200 um. 
K. Jensen, N. Leefer, A. Jarmola, Y. Dumeige, V. M. Acosta, P.   Kehayias, B. Patton, and D. Budker
quant-ph