9 Open systems
9.1 Open system dynamics
Any quantum system Q in our laboratory is a subsystem of something bigger. Nevertheless, we can often ignore the rest of the world and consider only states, dynamics, and measurements pertaining to Q itself. When can we do this? That is the question that concerns us in this chapter.
Consider two quantum systems, a system of interest Q and another, external system E. We will suppose that Q can have any initial state, but E starts out in a fixed pure state that we will call |0 .1 Thus, Q and E initially have ...