  Advancing our understanding of non-equilibrium phenomena in quantum many-body
systems remains among the greatest challenges in physics. Here, we report on
the experimental observation of a paradigmatic many-body problem, namely the
non-equilibrium dynamics of a quantum impurity immersed in a bosonic
environment. We use an interferometric technique to prepare coherent
superposition states of atoms in a Bose-Einstein condensate with a small
impurity-state component, and monitor the evolution of such quantum
superpositions into polaronic quasiparticles. These results offer a systematic
picture of polaron formation from weak to strong impurity interactions. They
reveal three distinct regimes of evolution with dynamical transitions that
provide a link between few-body processes and many-body dynamics. Our
measurements reveal universal dynamical behavior in interacting many-body
systems and demonstrate new pathways to study non-equilibrium quantum
phenomena.
