Multipulse Double-Quantum Magnetometry With Near-Surface Nitrogen   Vacancy Centers
  We discuss multipulse magnetometry that exploits all three magnetic sublevels of the S=1 nitrogen-vacancy center in diamond to achieve enhanced magnetic field sensitivity. Based on dual frequency microwave pulsing, the scheme works in arbitrary magnetic bias fields and is twice as sensitive to ac magnetic fields as conventional two-level magnetometry. We derive the spin evolution operator for dual frequency microwave excitation and show its effectiveness for double-quantum state swaps. Using multipulse sequences of up to 128 pulses under optimized conditions, we show enhancement of the SNR by up to a factor of 2 in detecting NMR statistical signals, with a 4 times enhancement theoretically possible. 
H. J. Mamin, M. H. Sherwood, M. Kim, C. T. Rettner, K. Ohno, D. D.   Awschalom, D. Rugar
cond-mat.mes-hall quant-ph