Cavity-Based Single Atom Preparation and High-Fidelity Hyperfine State Readout

We prepare and detect the hyperfine state of a single 87Rb atom coupled to a fiber-based high-finesse cavity on an atom chip. The atom is extracted from a Bose-Einstein condensate and trapped at the maximum of the cavity field, resulting in a reproducibly strong atom-cavity coupling. We use the cavity reflection and transmission signal to infer the atomic hyperfine state with a fidelity exceeding 99.92% in a readout time of 100 μs. The atom is still trapped after detection.