  A recent discussion of quantum limitations to the fidelity with which
superpositions of internal atomic energy levels can be generated by an applied,
quantized, laser pulse is shown to be based on unrealistic physical
assumptions. This discussion assumed the validity of Jaynes-Cummings dynamics
for an atom interacting with a laser field in free space, that is, when the
atom is not surrounded by a resonant cavity. If the laser field is a multimode
quantum coherent state, and the Rabi frequency is much greater than the
spontaneous decay rate, then the total atomic decoherence rate is on the order
of the spontaneous decay rate. With the use of a unitary transformation of the
field states due to Mollow, it can be shown that the atomic decoherence rate is
the same as if the laser field were treated classically, without any additional
contribution due to the quantum nature of the laser field.
