Single-photon detection of microwave blackbody radiations in a low-temperature resonant-cavity with high Rydberg atoms

Blackbody radiations in a microwave resonant cavity have been measured in a single-photon counting mode at low temperature from 67 mK to 1 K with Rydberg atoms. Specifically the 111p3/2p3/2 states excited from the initially prepared 111s1/2s1/2 states of 85Rb by absorbing 2527 MHz blackbody photons have been selectively detected with a newly developed selective-field-ionization scheme. Fraction of the number of 111p3/2p3/2 states to the total incident Rydberg atoms was observed to decrease abruptly with decreasing temperature. A simple theoretical prediction in over-damped regime reproduces well the experimental results. This observation confirms in a realistic situation that the Rydberg atoms provide a very sensitive low-noise single-quantum detector for microwave radiations.