The theory of quantum mechanics applies to closed systems. In such ideal
situations, a single atom can exist, for example, in a superposition of
being in two different positions at the same time. Real systems, in
contrast, always interact with their environment, with the consequence that
macroscopic quantum superpositions like Schrodinger's cat are not observed.
Moreover, macroscopic superpositions decay so quickly that the dynamics of
decoherence can not even be observed. However, mesoscopic systems offer
the possibility of observing the decoherence of such quantum superpositions
states of the motion of a single trapped atoms. Decoherence is induced by
coupling the atoms to engineered reservoirs, where the coupling an state of
the environment are under the experimenter's control. We exhibit this with
three experiments, finding that the decoherence scales exponentially with
the square of the size of the superposition.