then, that a system $S$ is composed of two subsystems $A$ and $B$. Of course, in general, the partition of $S$ into subsystems $A$ and $B$ is arbitrary. Now, given that $S$ is an isolated quantum system, it invariably possesses a well-defined quantum state. However, if such state is an entangled state between $A$ and $B$, it is impossible to assign well-defined states to such subsystems. That is, the quantum formalism entails that, in such situations, the subsystems simply do not possess well-defined states. There is, however, a way to encode *some* of the information regarding a subsystem in a density matrix. In order to do so, one first constructs the pure density matrix associated with the whole system $S$ and then takes the partial trace of such matrix over the rest of the system. That is, if $| \Psi \rangle$ is the state of $S$, one defines $\rho_A = Tr_B(\rho)$ with $\rho = | \Psi \rangle \langle \Psi|$. If, on one hand, the state of $S$ is separable, the subsystem $A$ possesses a pure state and $\rho_A$ is just the corresponding pure density matrix. If, on the other hand, the state of $S$ is entangled, $A$ does not possess 