  A well-isolated system often shows relaxation to a quasi-stationary state
before reaching thermal equilibrium. Such a prethermalization has attracted
considerable interest recently in association with closely related fundamental
problems of relaxation and thermalization of isolated quantum systems.
Motivated by the recent experiment in ultracold atoms, we study the dynamics of
a one-dimensional Bose gas which is split into two subsystems, and find that
individual subsystems relax to Gibbs states, yet the entire system does not due
to quantum entanglement. In view of recent experimental realization on a small
well-defined number of ultracold atoms, our prediction based on exact few-body
calculations is amenable to experimental test.
