  Many-body correlations govern a variety of important quantum phenomena such
as the emergence of superconductivity and magnetism. Understanding quantum
many-body systems is thus one of the central goals of modern sciences. Here we
demonstrate an experimental approach towards this goal by utilizing an
ultracold Rydberg gas generated with a broadband picosecond laser pulse. We
follow the ultrafast evolution of its electronic coherence by time-domain
Ramsey interferometry with attosecond precision. The observed electronic
coherence shows an ultrafast oscillation with a period of 1 femtosecond, whose
phase shift on the attosecond timescale is consistent with many-body
correlations among Rydberg atoms beyond mean-field approximations. This
coherent and ultrafast many-body dynamics is actively controlled by tuning the
orbital size and population of the Rydberg state, as well as the mean atomic
distance. Our approach will offer a versatile platform to observe and
manipulate non-equilibrium dynamics of quantum many-body systems on the
ultrafast timescale.
