  Quantum phase transitions, transitions between many-body ground states, are
of extensive interest in research ranging from condensed matter physics to
cosmology. Key features of the phase transitions include a stage with rapidly
growing new order, called inflation in cosmology, followed by the formation of
topological defects. How inflation is initiated and evolves into topological
defects remains a hot debate topic. Ultracold atomic gas offers a pristine and
tunable platform to investigate quantum critical dynamics. We report the
observation of coherent inflationary dynamics across a quantum critical point
in driven Bose-Einstein condensates. The inflation manifests in the exponential
growth of density waves and populations in well-resolved momentum states. After
the inflation stage, extended coherent dynamics is evident in both real and
momentum space. We present an intuitive description of the quantum critical
dynamics in our system and demonstrate the essential role of phase fluctuations
in the formation of topological defects.
