A recent paper [G. M. Palma and P. L. Knight, Phys. Rev. A 39, 1962 (1989)] has stated that the two-atom Dicke model exhibits phase-dependent population decay. In this Comment we show that population decay is invariant with respect to the input phase angle of squeezing in the two-atom Dicke model.
We show that atoms interacting with a squeezed radiation field reservoir can exhibit phase-dependent population decay and relax into a highly correlated pure state, in striking difference to decay in a heat bath. We use a Heisenberg operator treatment of radiative transitions in a broadband squeezed light field described as a quantum white-noise field with correlated fluctuations to describe the modified decay of a single atom, and of two atoms cooperatively decaying in a squeezed vacuum. We compare this decay with that generated by a broadband thermal field which lacks the essential field-mode correlations characteristic of squeezing.