Abstract
The fifth lesson began with a reminder of the basics of cavity electrodynamics, a subject developed at the Collège by Serge Haroche using a jet of Rydberg atoms passing through a superconducting cavity formed by two convex microwave mirrors facing each other. In quantum circuits, the analogous system consists of an artificial Josephson atom coupled to a cavity formed by a transmission line. This type of system provides a very pure illustration of the nature of the quantum interaction between light and matter. In particular, in the first part of the lesson, we were interested in demonstrating, in quantum circuits, the quantized character of the electromagnetic field, as revealed by the shifting levels of the atom coupled to the field. In this way, we have calculated the frequency shifts of the atom's levels due to the presence of a single photon in the cavity in the transmon regime described in the third lesson. This very simple analytical calculation treats the atom's anharmonicity as a disturbance. It is interesting to calculate the cavity frequency shift resulting from the atom's excitation in the same way, and to show that it is identical to the first.
In the second part of the lesson, we explained the link between this level-shift effect of cavity electrodynamics and the principle of dynamic cooling, which can also be observed in artificial atoms by subjecting the cavity to microwave irradiation at the frequency corresponding to the exchange process between an atomic excitation and a cavity photon.
