Other Tools and Concepts
Quantum theory abounds with matrices of different kinds, and often it is not easy to keep track of all this. In this chapter, we introduce some further operators and their matrices, namely the time evolution, rotation, and projection operators. After a digression on pure and mixed quantum states, the density matrix is introduced in an elementary manner and used to derive again some previous results. It is followed by a section on coherences in quantum physics, and a discussion on Dirac’s bra-ket notation.