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Jamister

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- What are the basic assumptions of QM about the density matrix?

**Summary:**What are the basic assumptions of QM about the density matrix?

The subject of density matrix in quantum mechanics is very unclear to me.

In the books I read (for example Sakurai),they don't tell what are the basic assumptions and how you derive from them the results of the density matrix.

Is the basic assumption of the Copenhagen interpretation* of QM that there exists a wave function of the system, and we just don't know it, because of that we use the density matrix as an statistical method? And if so, how is it formally derived from the laws of QM.

Is the density matrix formulation is necessary to understand results from real experiments, or is it possible to do it without it.Copenhagen interpretation* = regular QM