Measurement of Ultralow Heating Rates of a Single Antiproton in a Cryogenic Penning Trap

We report on the first detailed study of motional heating in a cryogenic Penning trap using a single antiproton. Employing the continuous Stern-Gerlach effect we observe cyclotron quantum transition rates of 6(1) quanta/h and an electric-field noise spectral density below 7.5(3.4)×10^-20 V² m^-2 Hz^-1, which corresponds to a scaled noise spectral density below 8.8(4.0)×10^-12 V² m^-2, results which are more than 2 orders of magnitude smaller than those reported by other ion-trap experiments.