Maximum likelihood estimation for a group of physical transformations
  The maximum likelihood strategy to the estimation of group parameters allows to derive in a general fashion optimal measurements, optimal signal states, and their relations with other information theoretical quantities. These results provide a deep insight into the general structure underlying optimal quantum estimation strategies. The entanglement between representation spaces and multiplicity spaces of the group action appear to be the unique kind of entanglement which is really useful for the optimal estimation of group parameters. 
G. Chiribella, G. M. D'Ariano, P. Perinotti, M. F. Sacchi