About quantum channels, which are to density operators what unitaries are to state vectors: mathematical models for physically realisable transformations. Also about many eponymous constructions, such as the Stinespring and the Kraus representations, the Jamiołkowski isomorphism, and the Choi matrix.
Quantum evolution of any isolated system is unitary, but its constituent parts may evolve in a more complicated way.
We have already discussed how entanglement forces us to describe quantum states of open quantum systems (i.e. those which are only part of some larger system) in terms of density operators. In this chapter we will describe how open systems evolve. The question we are asking here is: what are the most general physically admissible transformations of density operators? That is, if state vectors evolve according to unitary operations, and we generalise state vectors to density operators, then what is the “good” corresponding generalisation of unitary operations?