  Rydberg atoms excited by cold blackbody radiation are shown to display
long-lived quantum coherences on timescales of tens of picoseconds. By solving
non-Markovian equations of motion with no free parameters we obtain the time
evolution of the density matrix, and demonstrate that the blackbody-induced
temporal coherences manifest as slowly decaying (100 ps) quantum beats in
time-resolved fluorescence. An analytic model shows the dependence of the
coherent dynamics on the energy splitting between atomic eigenstates,
transition dipole moments, and coherence time of the radiation. Experimental
detection of the fluorescence signal from a trapped ensemble of $10^8$ Rydberg
atom is discussed, but shown to be technically challenging at present,
requiring CMB amplification somewhat beyond current practice.
