Decoherence and dissipation of atomic Schrodinger cats

We investigate the time development of a superposition of macroscopically d istinct quantum states (Schrodinger cats) in an ensemble of two-level atoms . The system is interacting with a thermal environment of a macroscopic num ber of photon modes. The final equilibrium state of the atomic subsystem is diagonal in the energy eigenstates, and is determined by the Boltzmann dis tribution. The time scale of decoherence is, however, generally much shorte r than that of dissipation. The initial fast regime of the time evolution a ssociated with the decoherence is directed towards a classical state which is different from the thermal equilibrium. For general initial conditions t he distance between the actual state of the system and this classical state is decreasing fast, suggesting an appropriate measure of decoherence.