QND experiments in quantum electrodynamics with Rydberg atoms

In the fourth lesson, we described the non-destructive counting of microwave photons in a cavity with a very high quality factor. This QND method, which achieves radiation quantum resolution, exploits the concepts of Cavity Quantum Electrodynamics (CQED), whose principles we began by recalling. We have briefly mentioned QED experiments carried out in the optical domain, to distinguish them from the microwave studies that will be of particular interest to us here. The QND counting method uses the remarkable properties of Rydberg atoms in circular states coupled to a superconducting microwave cavity to detect photons. We've devoted most of this lesson to the atoms, and left the description of the cavity for the next lesson. A remarkable aspect of the correspondence principle is that the properties of circular states (whose quantum numbers are all large) can be understood from a quasi-classical description, introducing quantum concepts (quantization of atomic orbits and the radiated field) only minimally, like the description in Bohr's old quantum theory. We then classically described the radiation of these circular states, their susceptibility to electric fields and their coupling to the cavity. Finally, we analyzed the methods for preparing and detecting circular Rydberg atoms and their velocity selection.