Improved discrimination of unitaries by entangled probes
  We consider the problem of discriminating among a set of unitaries by means of measurements performed on the state undergoing the transformation. We show that use of entangled probes improves the discrimination in the two following cases: i) for a set of unitaries that are the UIR of a group and, ii) for any pair of transformations provided that multiple uses of the channel are allowed. 
G. Mauro D'Ariano, Paoloplacido Lo Presti, Matteo G. A. Paris