  We study the quantum stability of the dynamics of ions in a Paul trap. We
revisit the results of Wang et al. [Phys. Rev. A 52, 1419 (1995)], which showed
that quantum trajectories did not have the same region of stability as their
classical counterpart, contrary to what is obtained from a Floquet analysis of
the motion in the periodic trapping field. Using numerical simulations of the
full wave-packet dynamics, we confirm that the classical trapping criterion are
fully applicable to quantum motion, when considering both the expectation value
of the position of the wave packet and its width.
