Compact and freely movable single atom detector

A compact fluorescence detector has been developed which allows to count slow, single Na atoms. By using glass fibers for the exciting and the fluorescence light we obtained free movability of the detector. The detection efficiency has been analyzed and optimized by an extensive investigation of all processes concerning the laser-induced fluorescence, especially the random walk of the atom, saturation effects, and optical pumping. Single atoms are identified by registration of photon bursts which stand out from the Poisson distributed background. An absolute calibration of the detection efficiency has been achieved by a time-resolved fluorescence measurement in combination with a corresponding Monte–Carlo simulation. Atomic fluxes well below 1 atom/s can be determined with an uncertainty of less than ±10%. Assuming a continuous flux of 1 atom/s with 25 m/s typically about 50% of these atoms are counted. The detector is an excellent tool for differential scattering experiments with slow atomic beams.