For mesoscopic ensembles containing 100 or more atoms we measure the total
atom number and the number of atoms in a specific hyperfine state with
single-atom resolution. The measurement detects the atom-induced shift of the
resonance frequency of an optical cavity containing the ensemble. This work
extends the range of cavity-based detection with single-atom resolution by more
than an order of magnitude in atom number, and provides the readout capability
necessary for Heisenberg-limited interferometry with atomic ensembles.Comment: 5 pages, 4 pdf figure
