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We describe a real-time method to obtain normalized differential count rate signals from modulated systems with photon detection at single-photon count rates. The method is demonstrated with a real-time peak-locking and frequency control to track magnetic field using optically detected magnetic resonance of nitrogen-vacancy centers in diamond. This procedure allows us to measure the magnetic field continuously with a sensitivity of 4.1 µT/Hz1/2 and to track magnetic field sweep rates up to 50 µT/s. The differential rate meter automates the processing of voltage pulse outputs from the photon detector and provides noise levels on par with traditional photon counting methods using digital counters.
Ambal et al. (Fri,) studied this question.