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 111(p3/2) states excited from the initially prepared 111(s1/2) states of Rb-85 by absorbing 2527 MHz blackbody photons have been selectively detected with a newly developed selective-field-ionization scheme. Fraction of the number of 111(p3/2) 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. (c) 2005 Elsevier B.V. All rights reserved.