A differential photon-rate meter for real-time peak tracking in optically detected magnetic resonance at low photon-count rates

The optically detected magnetic resonance of the NV^- center in diamond provides a mechanism for precise, nanoscale magnetometry. As an alternative to measuring and fitting complete resonance spectra, resonance peak locking and tracking methods provide real-time and continuous measurements of the magnetic field. But the weak photoluminescence from small ensembles of NV^- center including single NV^- centers poses a problem for peak tracking. The emitted light requires single-photon detection which produces a narrow (≈ 20 ns) voltage pulse per detected photon. The discrete voltage pulses are not amenable for demodulation by regular lockin amplifiers. Here, we address active feedback control and real-time field tracking with photon detection rates in the range of 4 X 10³ s^-1 to 1 X 10⁶ s^-1, which are typical of single NV^- centers, and we present a custom differential rate meter with phase sensitive detection. Without compromising signal-to-noise, this real-time data processing scheme provides all the typical functionalities of a lock-in amplifier needed for real-time peak locking and tracking. We demonstrate continuous field measurements at sweep rates exceeding 50 μT/s. This scheme covers a broad magnetic field range, limited by the frequency range of the microwave generator.