Absorption of blackbody radiation by Rydberg atoms in a resonant cavity is shown to be a collective process in which the atoms behave as a Bose gas. The equilibrium energy of this gas was measured and found equal to twice the photon energy (factor of 2 accounting for atomic transition degeneracy). It is shown that this experiment exhibits the Brownian motion of the atomic-system Bloch vector.

• Received 3 May 1982

DOI:https://doi.org/10.1103/PhysRevLett.49.117