Course 2 Mesoscopic state superpositions and decoherence in quantum optics

Abstract

This chapter presents the main ideas and methods of quantum optics. It shows how mesoscopic states could be prepared and studied using the conceptually simple methods of quantum optics. It analyzes simple models of decoherence and describes the experiments which illustrate the fundamental connexion between environment induced decoherence, entanglement and complementarity. It describe beam splitters and particle interference effects followed by Schrodinger cats in Cavity Quantum Electrodynamics experiments. The chapter also discusses mesoscopic state superpositions of atoms in Bose Einstein condensates. Schrodinger cat states in quantum or atomic optics are characterized by their extreme fragility and sensitivity to decoherence, which occurs at a rate essentially proportional to the number of particles in the system. This puts severe limits to the size of these cat states. They have to be built within a finite time, to let the processes responsible for the preparation of the superposition to take place. This preparation time must be shorter than the decoherence time of the final cat state and this sets, in practice, an absolute limit to the number of particles in the system. In Cavity Quantum Electrodynamics (CQED), the maximum number of photons involved in cat states could not exceed a few hundred, even if the technology of cavities were considerably improved.