Quantum processes from the eye of ''Laplace demon'' If an imaginary being, like the Laplace demon were to observe quantum processes, are all the states in his description of the quantum process pure states?
 A: A non-pure state describes "classical uncertainty" as to what is the actual quantum state, so under the assumption of a demon having complete knowledge of the quantum state of the universe, I would say that the demon should describe the state as pure.
Moreover, if one admits that it is meaningful to think of the "quantum state" of the whole universe, which is a closed system by definition, then arguably its evolution is completely unitary and deterministic, and therefore the global state remains pure at all times.
According to this view, having complete knowledge of the initial state of the universe, it is in principle possible (assuming unbounded computational power, perfect knowledge of all the laws of nature, etc. etc.) to compute the state of the universe at all times.
This point of view makes sense if one thinks of the measurement processes as arising from decoherence of a quantum system with its environment, in which case if one considers the evolution of the whole universe, which by definition is not embedded in any environment, then there is no collapse/decoherence in the process.
It is important to remark that this view is not shared by everyone. For example, there are people studying the possibility that the collapse of the wavefunction is an intrinsic phenomenon, whose description requires modifications to the Schroedinger equation.
If this is true, then non-deterministic collapse happens regardless of the system being closed or not, and even with complete knowledge of the initial state it wouldn't be possible to predict the evolution of the state.
