Detailed balance and entropy for atoms in squeezed vacuum

It is shown that the principle of detailed balance is not obeyed in both systems of a single 2-level atom and the two-atom Dicke model interacting with a broad band squeezed vacuum field. Examination of the Boltzmann-Gibbs entropy shows that the stability of either system depends on the initial conditions. In particular, the two-atom system is unstable even for initially ground state condition, unlike the thermal field driven case.