Millikelvin Spatial Thermometry of Trapped Ions

We demonstrate millikelvin thermometry of laser cooled trapped ions with high-resolution imaging. This equilibrium approach is independent of the cooling dynamics and has lower systematic error than Doppler thermometry, with \pm5 mK accuracy and \pm1 mK precision. We used it to observe highly anisotropic dynamics of a single ion, finding temperatures of < 60 mK and > 15 K simultaneously along different directions. This thermometry technique can offer new insights into quantum systems sympathetically cooled by ions, including atoms, molecules, nanomechanical oscillators, and electric circuits.