  Recent studies of the decoherence induced by the quantum nature of the laser
field driving a two-state atom [J. Gea-Banacloche, Phys. Rev. A 65, 022308
(2002); S. J. van Enk and H. J. Kimble, Quantum Inf. and Comp. 2, 1 (2002)]
have been questioned by Itano [W. M. Itano, Phys. Rev. A 68, 046301 (2003)] and
the proposal made that all decoherence is due to spontaneous emission. We
analyze the problem within the formalism of cascaded open quantum systems. Our
conclusions agree with the Itano proposal. We show that the decoherence,
nevertheless, may be divided into two parts--that due to forwards scattering
and to scattering out of the laser mode. Previous authors attribute the former
to the quantum nature of the laser field.
