Rotary echo tests of coherence in Rydberg-atom excitation*

Rotary echoes are employed to study excitation dynamics in many-body Rydberg systems. In this method, a phase reversal of a narrow-band excitation field, applied at a variable time during the excitation pulse, results in echo signals the visibility of which reveal the degree of coherence of the excitation process. Rotary echoes are measured for several $n$D$_{5/2}$ Rydberg levels of rubidium with principal quantum numbers near $n=43$, where the strength of electrostatic Rydberg-atom interactions is modulated by a F\"orster resonance. The Rydberg-atom interactions are shown to diminish the echo visibility, in agreement with recent theoretical work. The equivalence of echo signals with spectroscopic data is demonstrated.