Highly symmetric POVMs and their informational power

We discuss the dependence of the Shannon entropy of rank-1 normalized finite POVMs on the choice of the input state, looking for the states that minimize this quantity. To distinguish the class of measurements where the problem can be solved analytically, we introduce the notion of highly symmetric POVMs and classify them in dimension two (for qubits). In this case we prove that the entropy is minimal (and hence the relative entropy is maximal), when the input state is orthogonal to one of the states constituting a POVM. The method used in the proof, employing the Michel theory of critical points for group action, the Hermite interpolation and the structure of invariant polynomials for unitary-antiunitary groups can also be applied in higher dimensions and for other entropy-like functions.