Laser Cooling and Qubit Measurements on a Forbidden Transition in Neutral Cs Atoms

We experimentally demonstrate background-free, hyperfine-level-selective measurements of individual Cs atoms by simultaneous cooling to 5.3 μK and imaging on the 6s_{1/2}→5d_{5/2} electric-quadrupole transition. We achieve hyperfine-resolved detection with fidelity 0.9993(4) and atom retention of 0.9954(5), limited primarily by vacuum lifetime. Performing state measurements in a 3D cooling configuration enables repeated low loss measurements. A theoretical analysis of an extension of the demonstrated approach, based on quenching of the excited state with an auxiliary field, identifies parameters for hyperfine-resolved measurements with a projected fidelity of ∼0.9995 in ∼60 μs.