Dynamical disappearance of superposition states in the thermodynamic limit

It is shown that a macroscopic superposition state of radiation, strongly interacting with an ensemble of two-level atoms, is removed, generating a coherent state describing a classical radiation field, when the thermodynamic limit is taken on the unitary evolution obtained by the Schrödinger equation. Decoherence appears as a dynamical effect, in agreement with a recent proposal (Frasca 2001 Phys. Lett. A 283 271). To prove that this effect is quite general, we show that this same behaviour appears when a superposition of two Fock number states is also considered. Higher order corrections are computed, showing that this result tends to become exact in the thermodynamic limit. It appears as a genuine example of intrinsic collapse of the wavefunction.