  We consider the non-equilibrium dynamics of a gas of impenetrable bosons
released from a harmonic trapping potential to a circle. The many body dynamics
is solved analytically and the time dependence of all the physically relevant
correlations is described. We prove that, for large times and in the
thermodynamic limit, the reduced density matrix of any subsystem converges to a
generalized Gibbs ensemble as a consequence of the integrability of the model.
We discuss the approach to the stationary behavior at late times. We also
describe the time-dependence of the entanglement entropy which attains a very
simple form in the stationary state.
