Optimal covariant measurements: the case of a compact symmetry group and   phase observables
  We study various optimality criteria for quantum observables. Observables are represented as covariant positive operator valued measures and we consider the case when the symmetry group is compact. Phase observables are examined as an example. 
Claudio Carmeli, Teiko Heinosaari, Juha-Pekka Pellonp\"a\"a,   Alessandro Toigo