  The ultimate consequence of quantum many-body physics is that even the air we
breathe is governed by strictly unitary time evolution. The reason that we
perceive it nonetheless as a completely classical high temperature gas is due
to the incapacity of our measurement machines to keep track of the dense
many-body entanglement of the gas molecules. The question thus arises whether
there are instances where the quantum time evolution of a macroscopic system is
qualitatively different from the equivalent classical system? Here we study
this question through the expansion of noninteracting atomic clouds. While in
many cases the full quantum dynamics is indeed indistinguishable from classical
ballistic motion, we do find a notable exception. The subtle quantum
correlations in a Bose gas approaching the condensation temperature appear to
affect the expansion of the cloud, as if the system has turned into a diffusive
collision-full classical system.
