QUROPE Quantum Information Processing and Communication in Europe

An optical microcavity with small mode radius is used to measure the local density of a cold atom cloud. Atom densities below 1 per cavity mode volume are measured with signals near the photon shot-noise limit. Atom detection is fast and efficient, reaching fidelities in excess of 97% after 10 us and 99.9% after 30 us. Notably, the fluctuations of the detected photon counts are smaller than expected for Poissonian distributions of atoms probed with Poissonian light fields. This noise suppression is attributed to multi-atom effects on the collective atomic dipole interacting with the cavity field. Our measurements confirm a decade-old theory of atomic beams in cavity quantum electrodynamics.