Coherent states and the classical-quantum limit considered from the point of view of entanglement,
Résumé
We consider the quasi-classical situation in which a quantum system interacts with another quantum system or with a quantum environment without getting entangled with it. We show how this regime is intimately linked to three paradigms commonly used in classical, pre-quantum physics to describe particles (that is, the material point, the test particle and the diluted particle (droplet model)). This entanglement-free regime also provides a simplified insight on what is called in the decoherence approach "islands of classicality", that is, preferred bases that would be selected through evolution by a Darwinist mechanism which aims at optimizing information. We show how, under very general conditions, coherent states are natural candidates for classical pointer states. This occurs essentially because, when a (supposedly bosonic) system coherently exchanges only one quantum at a time with its (supposedly bosonic) environment, coherent states of the system do not get entangled with the environment, due to the bosonic symmetry.