  Strong dipole-exchange interactions due to spontaneously produced contaminant
states can trigger rapid dephasing in many-body Rydberg ensembles [E.
Goldschmidt et al., PRL 116, 113001 (2016)]. Such broadening has serious
implications for many proposals to coherently use Rydberg interactions,
particularly Rydberg dressing proposals. The dephasing arises as a runaway
process where the production of the first contaminant atoms facilitates the
creation of more contaminant atoms. Here we study the time dependence of this
process with stroboscopic approaches. Using a pump-probe technique, we create
an excess "pump" Rydberg population and probe its effect with a different
"probe" Rydberg transition. We observe a reduced resonant pumping rate and an
enhancement of the excitation on both sides of the transition as atoms are
added to the pump state. We also observe a timescale for population growth
significantly shorter than predicted by homogeneous mean-field models, as
expected from a clustered growth mechanism where high-order correlations
dominate the dynamics. These results support earlier works and confirm that the
time scale for the onset of dephasing is reduced by a factor which scales as
the inverse of the atom number. In addition, we discuss several approaches to
minimize these effects of spontaneous broadening, including stroboscopic
techniques and operating at cryogenic temperatures. It is challenging to avoid
the unwanted broadening effects, but under some conditions they can be
mitigated.
