The third lesson showed how the decoherence of a physical system can be affected by altering the spectrum of its environment or the symmetry of its coupling with that environment. The situation considered was that of cavity quantum electrodynamics, a research theme that has already served to illustrate many aspects of previous lectures. The principle of the experiments is to modify the boundary conditions of the field bathing an atom or a set of atoms by confining the system in a box with reflecting walls, an electromagnetic cavity. This can either prevent the atoms from emitting (by suppressing the modes resonating with the atoms), or cause it to emit faster than in free space (by increasing the density of resonant modes). Examples of these two limiting cases are given in Chapter 4. A large number of experiments have been carried out in the microwave field on highly excited atoms, known as Rydberg atoms. The physics of these atoms has been briefly reviewed.
Cavity quantum electrodynamics (CQED) also makes it possible to couple a set of atoms symmetrically to the same field mode. This leads to constructive or destructive interference effects, and to faster or slower emission than that of a single atom. The case of accelerated emission, called superradiance, is not the one that concerns us in the context of a study of decoherence decay, but we felt it was sufficiently interesting in itself to be mentioned in this lecture (chapter 5). Most of the experiments described in this lesson correspond to the so-called "perturbative" regime of cavity quantum electrodynamics, where the atom-cavity coupling is weak enough for the field to be considered as an "environment" of the atomic system, inducing irreversible evolution on it. We have distinguished this regime from that of "strong coupling", in which the environment interacts coherently with the atoms. It is the strong-coupling regime that is realized in the cavity quantum information experiments described in previous years' lectures.
