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2 Bayes – Laplace and Jeffrey ’ s rules as entropic projections

We prove that the standard quantum mechanical description of a quantum state change due to measurement, given by Lüders’ rules, is a special case of the constrained maximisation of a quantum relative entropy functional. This result is a quantum analogue of the derivation of the Bayes–Laplace rule as a special case of the constrained maximisation of relative entropy. The proof is provided for the Umegaki relative entropy of density operators over a Hilbert space as well as for the Araki relative entropy of normal states over a W ∗-algebra. We also introduce a quantum analogue of Jeffrey’s rule, derive it in the same way as above, and discuss the meaning of these results for quantum bayesianism.