  We present precise, sub-wavelength optical intensity measurement using a
single trapped $^{87}$Rb atom as a sensor. The intensity is measured by the
scalar ac Stark shift it produces on the $F=1 \rightarrow F'=2$ 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.
