Coherent dynamics of Rydberg atoms in cosmic microwave background radiation

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.