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 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.