Precise, super-resolving intensity measurement by quantum jump spectroscopy of a single neutral atom

We present precise, sub-wavelength optical intensity measurement using a single trapped ^87Rb atom as a sensor. The intensity is measured by the scalar ac Stark shift it produces on the F=1 → F'=2 hyperfine transition of the D_2 line, chosen for its F' = F+1 structure and very small tensor polarizability. To boost signal and reduce measurement-induced perturbations, we use a quantum jump spectroscopy technique in which a single absorbed photon on a transition of interest induces the scattering of hundreds of photons on a bright closed transition. The method greatly reduces systematic effects associated with the atomic state, optical polarization, probe power, and atom heating, and gives the atomic temperature as a second spectroscopic observable. We demonstrate the method by measuring the intensity at the focus of an optical tweezer.