I learnt about pure state, mixed state, reduce density matrix, etc. from this now famous paper http://philsci-archive.pitt.edu/5439/1/Decoherence_Essay_arXiv_version.pdf (thanks to Bill) I'd like to know something. Atty said somewhere: "A pure state means that we have prepared many copies of the system of one spin, and each copy of the system is in the same pure state. For example, every copy of the single spin is pointing up" How do you apply this to an electron in an atom. How do you prepare it such that even if you prepared many copies of the system, it is always same state. Can you do that to the electron that is without any definite state (such as no position eigenstates chosen)? Or must it always have result like the electron always end up in one position eigenstate.. but how do you prepare many copies of the electron with always one particular position eigenstate chosen among infinity? Or can't this be done.. how then do you prepare an electron(s) in atom in pure state?