Abstract
The third and fourth lessons discussed the link between entanglement, decoherence and complementarity by describing and analyzing simple quantum interference and measurement situations. The concept of complementarity expresses the fact that the corpuscular or wave aspects of physical systems are manifested according to whether or not the experimental apparatus makes it possible to determine the "path" followed by the system. If information on this path can be found in the environment, then the corpuscular aspect dominates and quantum interference cannot be observed. In this case, the system under study is necessarily entangled with its environment. This phenomenon generally occurs when the systems under study are large enough, and is known as decoherence. A simple, fully calculable model of decoherence has been presented (coupling a harmonic oscillator to a reservoir of elementary oscillators).
