  In the framework of an interference setup in which only two outcomes are
possible (such as in the case of a Mach-Zehnder interferometer), we discuss in
a simple and pedagogical way the difference between a standard, unitary quantum
mechanical evolution and the existence of a real collapse of the wave function.
Moreover, we also present the Elitzur-Vaidman bomb, the delayed choice
experiment, and the effect of decoherence. In the end, we propose two simple
experiments to visualize decoherence and to test the role of an entagled
particle.
