  Detection of radiation signals is at the heart of precision metrology and
sensing. In this article we show how the fluctuations in photon counting
signals can be exploited to optimally extract information about the physical
parameters that govern the dynamics of the emitter. For a simple two-level
emitter subject to photon counting, we show that the Fisher information and the
Cram\'er- Rao sensitivity bound based on the full detection record can be
evaluated from the waiting time distribution in the fluorescence signal which
can, in turn, be calculated for both perfect and imperfect detectors by a
quantum trajectory analysis. We provide an optimal estimator achieving that
bound.
