Rydberg atoms and cavities: observation of decoherence in a quantum measurement

We present the observation of the decoherence phenomenon, observed on a simple system, modelling an ideal quantum measurement. The experiment is based on the interaction of a circular Rydberg atom with a microwave field that contains a few photons stored in a superconducting cavity. It creates a quantum superposition involving field states with macroscopically distinct phases. The coherence of the state is tested by a second atom. We observe the transformation of an initially coherent superposition into a statistical mixture. This work represents an experimental contribution to the fundamental issue of the nonexistence of quantum superpositions at a macroscopic scale. Our results are in very good agreement with theory and support the interpretation of decoherence as being the result of the coupling between a system and its environment. (author)