A Primary temperature standard with cold Rydberg atoms

Traceable characterization of blackbody radiation (BBR) is a major hurdle in self-consistent realization of the SI. Here we demonstrate an atomic realization of the Kelvin by measuring the BBR-induced state transfer between Rydberg states of alkali atoms. The rate of these transitions relates to the BBR photon density through SI constants and calculable atomic properties, thus constituting a primary temperature standard. We prepare a cold cloud of ⁸⁵Rb in its Rydberg state, allow BBR to transfer population, and read out the state populations using selective field ionization. We use the time evolution of the populations to calculate the BBR field at the relevant wavelengths, primarily at 130 GHz. We find a statistical sensitivity to the fractional temperature uncertainty of 0.09 Hz^−1/2, corresponding to 26 K⋅Hz^−1/2 at room temperature. This represents a calibration-free SI-traceable temperature measurement, for which we calculate a systematic fractional temperature uncertainty of 0.006, corresponding to 2 K at room temperature when used as a primary temperature standard.