About the Hilbert-space formalism of quantum theory, and the role of measurements in quantum information theory, as well as introducing the quantum dramas of Alice and Bob.
Eventually we have to talk about quantum measurements, since, at some point, someone has to look at a measuring device and register the outcome of whatever quantum circuits we’ve been designing. It turns out that this is a bit more tricky than one might think. Quantum measurement is not a passive acquisition of information: if you measure, you disturb. Even though it is a physical process, like any other quantum evolution, it is traditionally described by a different set of mathematical tools.