Main Question or Discussion Point
Are systems ever in a pure quantum mechanical state? If they are, is it possible to know the precise pure QM state? The example I am thinking of is the spin of an electron. If we measure the spin about the "z-axis" and find the result to be "up" then we say the electron is in the pure state with spin up about the z-axis. However, given that the measurement device itself is described by QM I'm thinking we don't know exactly how it is oriented (or maybe more correctly it doesn't even have an exact orientation), all we really know is that it's measuring about an axis very close to the z-axis, with some statistical spread, right? So the resulting state of the electron is like a mixed state, except due to a fundamental/unavoidable uncertainty? I haven't thought this through yet in other situations. Any references that explain this further would be much appreciated.