  In this paper, we investigate decoherence of Knill, Laflamme, and Milburn's
nonlinear sign-shift gate that is implemented with the Jaynes-Cummings model.
Introducing a stochastic variable as an external electric field, we let it
couple with a dipole moment of a two-level atom. We examine this model using a
semiclassical theory. Results of the Monte Carlo simulations under the
semiclassical approximation correspond well with those obtained with the
quantum mechanical perturbation theory for the stochastic process. In the
results of the simulations, we observe both the $\mbox{T}_{1}$ and
$\mbox{T}_{2}$ decays. This paper is a sequel to the reference [H.~Azuma, Prog.
Theor. Phys. {\bf 126}, 369 (2011)].
