Primary blackbody radiometry using cold Rydberg atoms*

Rydberg states of alkali atoms are highly sensitive to electromagnetic radiation in the GHz-to-THz regime because their transitions have large electric dipole moments. Because of this, environmental blackbody radiation (BBR) can excite transitions between Rydberg states at μs timescales. The rate of these transitions relates to the BBR photon density through SI constants and calculable atomic properties, and thus constitutes a primary temperature standard. Here, we track the BBR-induced transfer of a prepared ⁸⁵Rb Rydberg state to its neighbors, reading out the state population using selective field ionization. We use the time evolution of these state populations to characterize 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.