  The number of atoms trapped within the mode of an optical cavity is
determined in real time by monitoring the transmission of a weak probe beam.
Continuous observation of atom number is accomplished in the strong coupling
regime of cavity quantum electrodynamics and functions in concert with a
cooling scheme for radial atomic motion. The probe transmission exhibits sudden
steps from one plateau to the next in response to the time evolution of the
intracavity atom number, from N >= 3 to N = 2 to 1 to 0, with some trapping
events lasting over 1 second.
