We use an optical cavity to detect single atoms magnetically trapped on an atom chip. We implement the detection using both fluorescence into the cavity and atom-induced reduction in cavity transmission. In fluorescence, we register 2.0(2) photon counts per atom, which allows us to detect single atoms with 75% efficiency in $250\mu \mathrm{s}$. In absorption, we measure transmission attenuation of 3.3(3)% per atom, which allows us to count small numbers of atoms with a resolution of about 1 atom.

• Received 23 March 2006

DOI:https://doi.org/10.1103/PhysRevLett.97.023002