QUROPE Quantum Information Processing and Communication in Europe

We introduce lossless state detection of trapped neutral atoms based on cavity-enhanced fluorescence. In an experiment with a single 87-Rb atom, a hyperfine-state detection fidelity of 99.4% is achieved in 85 microseconds. The quantum bit is interrogated many hundred times without loss of the atom while a result is obtained in every read-out attempt. The fidelity proves robust against atomic frequency shifts induced by the trapping potential. Our scheme does not require strong coupling between atom and cavity and can be generalized to other systems with an optically accessible quantum bit.