  We demonstrate real-time magnetometry by detecting fluorescence from a
nitrogen vacancy center in the setting of coherent population trapping and by
estimating magnetic field from the time series of the observed photon counts,
which are correlated with the underlying field. The proof-of-principle
experiment uses an external time-varying magnetic field that follows an
Ornstein-Uhlenbeck (OU) process. By taking into consideration the statistical
properties of the OU process, a Bayesian inference-based estimator can
effectively update dynamical information of the field in real time with the
detection of just a single photon.
