In this appendix we shall describe in more detail how the experiment of Brune et al. mentioned in Section 6.4.1 provided evidence of the phenomenon of decoherence in an entirely controlled manner. In addition to its intrinsic interest, this experiment is a prime example of actual experiments which allow the fundamentals of quantum mechanics to be tested with a precision undreamed of by its founders, and the study of this experiment constitutes a beautiful exercise in quantum physics. It will also allow us to give a small sample of the current ideas on the notion of measurement in quantum mechanics. We shall first return to the interference experiment of Section 1.4.4, this time discussing it within the framework of an elementary theory of measurement. Then we shall examine the realization of Ramsey fringes using Rydberg atoms, and show how the interaction of these atoms with an electromagnetic field progressively blurs these fringes when we try to answer the “which of the two trajectories?” question. Finally, we shall show how the use of a pair of atoms allows decoherence to be tested.

An elementary model of measurement

Let us return to the discussion of the Young's slit experiment with the trajectories labeled as in Fig. 1.13, enlarging on it with the introduction of a mathematical formulation. Let c1(x) [c2(x)] be the complex probability amplitude for an atom to be localized at a point x on the screen after having passed through slit 1 [2]. The (arbitrary) normalization is fixed by |c11(x)|2 = |c2(x)|2 = 1.