  The fastest possible collective response of a quantum many-body system is
related to its excitations at the highest possible energy. In condensed-matter
systems, the corresponding timescale is typically set by the Fermi energy.
Taking advantage of fast and precise control of interactions between ultracold
atoms, we report on the observation of ultrafast dynamics of impurities coupled
to an atomic Fermi sea. Our interferometric measurements track the
non-perturbative quantum evolution of a fermionic many-body system, revealing
in real time the formation dynamics of quasiparticles and the quantum
interference between attractive and repulsive states throughout the full depth
of the Fermi sea. Ultrafast time-domain methods to manipulate and investigate
strongly interacting quantum gases open up new windows on the dynamics of
quantum matter under extreme non-equilibrium conditions.
