A probe composed of a single calcium ion has produced one of the most precise measurements of the three-dimensional structure of a light-wave field. Researchers at Max Planck Institut in Garching, Germany, isolated the atom in a radio-frequency trap and surrounded it with an optical cavity containing a standing wave with a 397-nm wavelength. The ion, sensitive to radiation with the standing wave's resonant line, fluoresced, and the scientists observed it via a lens and a photomultiplier tube.They controlled the location of the ion within the standing wave by positioning the cavity mirrors by means of piezoelectric transducers and moving the light field. The resulting measurements of the wave's modes and Hermite-Gauss functions produced spatial resolutions as high as 60 nm.