These lecture notes review microwave cavity experiments in which Rydberg atoms interact one by one with superconducting cavities. These experiments, performed at Ecole Normale Supérieure (ENS), illustrate the concepts of complementarity, entanglement and decoherence and demonstrate basic steps of quantum logic operations. The recent development of super high-Q cavities storing quantum fields for times larger than a tenth of a second has made possible the Quantum Non-Demolition (QND) counting of photons and the first observation of quantum jumps of light. By combining QND counting procedures with homodyne methods, non-classical states of trapped fields have been reconstructed, among which Schrödinger cat states of light. A time-resolved procedure yields snapshots of the evolving decoherence of these states. In future developments, we plan to implement feedback methods to maintain quantum coherence over long time periods and to study the non-local properties of mesoscopic fields stored in two cavities.