6.1 Composite systems
In this chapter we will discuss the quantum theory of composite systems, systems composed of two or more distinct subsystems. When our theory is extended to these, we encounter the remarkable phenomenon of quantum entanglement. As we shall see, the statistical correlations between entangled quantum systems may be very different from the correlations that are possible between separated classical systems.
Suppose the composite quantum system AB is composed of two distinct subsystems A and B. There are certainly situations in which the subsystems A and B can individually be assigned definite quantum state vectors ...