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 $N\ge 3$ to $N=2\to 1\to 0$ atoms, with some trapping events lasting over 1 s.

• Received 16 March 2004

DOI:https://doi.org/10.1103/PhysRevLett.93.143601