Locking and Tracking Magnetic Resonance Spectra of NV- Center for Real-time Magnetometry using a Differential Photon-Rate Meter

We describe a real-time data processing and frequency control method to track peaks in optically detected magnetic resonance of nitrogen-vacancy centers in diamond. This procedure allows us to measure magnetic field continuously with sensitivity ~6 uT/Hz^{1/2} and to track resonances at sweep rates exceeding 110 uT/s. We use a custom-built differential photon rate meter and active feedback control to "lock" the microwave excitation frequency of the magnetic resonance. Our scheme covers a broad magnetic field range, limited by the frequency range of the microwave generator. This measurement procedure automates the processing of voltage pulse outputs from the photon counters, and it provides sensitivity comparable to traditional photon counting methods