  We propose and theoretically analyze the use of coherent population trapping
of a single diamond nitrogen vacancy (NV) center for continuous real-time
sensing. The formation of the dark state in coherent population trapping
prevents optical emissions from the NV center. Fluctuating magnetic fields,
however, can kick the NV center out of the dark state, leading to a sequence of
single-photon emissions. A time series of the photon counts detected can be
used for magnetic field estimations, even when the average photon count per
update time interval is much smaller than 1. For a theoretical demonstration,
the nuclear spin bath in a diamond lattice is used as a model fluctuating
magnetic environment. For fluctuations with known statistical properties, such
as an Ornstein-Uhlenbeck process, Bayesian inference-based estimators can lead
to an estimation variance that approaches the classical Cramer-Rao lower bound
and can provide dynamical information on a timescale that is comparable to the
inverse of the average photon counting rate. Real-time sensing using coherent
population trapping adds a new and powerful tool to the emerging technology of
quantum sensing.
