Quantum Bayes rule - art. no. 014305

We state a quantum Version of the Bayes rule for statistical inference and give a simple general derivation within the framework of generalized measur ements. The rule can be applied to measurements on N copies of a system if the initial state of the N copies is exchangeable. As an illustration, we a pply the rule to N qubits. Finally, we show that quantum state estimates de rived via the principle of maximum entropy are fundamentally different from those obtained via the quantum Bayes rule.