Measurement in QM

This set of OneTab shared tabs collects some recent blog activity on the measurement process in Quantum Mechanics. This reminds me of the fact that once as a graduate student (in the ’70s), on hearing once too often that the measurement process was a mystery because unitary evolution cannot take a pure state into a mixed state, I thought I had something useful to say on the matter but was pointed to the discussion in von Neumann’s book and later elaborations by Jauch and Hepp that seemed to deal with the issue on the same lines (namely by modelling the measured and measuring systems together as a tensor product of a pure state of the former and a mixed state of the latter which could evolve unitarily in such a way that the marginal state in the measured system does evolve from pure to mixed).

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