  We extend the approach of Ref. [Yu-xi Liu, L. F. Wei, and F. Nori, Europhys.
Lett. 67, 941 (2004)] for preparing superposition states of a cavity field
interacting with a superconducting charge qubit. We study effects of the
nonlinearity on the creation of such states. We show that the main contribution
of nonlinear effects is to shorten the time necessary to build the
superposition.
