| Abstract: | In quantum mechanics, the role of an observer is fundamentally different from that of a classical observer. The quantum mechanical observer necessarily plays an active role in the dynamics of the system that it is observing. This apparent difficulty may be turned into a tool to drive an initially trivial system into a complicated quantum many-body state simply by observing it. I will present two remarkable examples of states induced by measurement. In the first, we examine the role of a moving density measuring device interacting with a system of fermions, and in particular, show that it would leave behind a wake of purely quantum origin. In the second example, inspired by topological Floquet insulators, we will see how a suitably chosen set of density measurements, repeated periodically, will induce robust chiral edge motion of fermions. These examples show how quantum mechanical observation can be added as a versatile tool to the arsenal of quantum engineering in condensed matter systems. |