Measurement of ultra-low 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 $7.5(3.4)\times 10^{-20},$\text{V}$^{2}$\text{m}$^{-2} $\text{Hz}$^{-1}$, which corresponds to a scaled noise spectral density below $8.8(4.0)\times 10^{-12},$\text{V}$^{2}$\text{m}$^{-2}$, results which are more than two orders of magnitude smaller than those reported by other ion trap experiments.