Optimal discrimination of mixed states: the quantum Chernoff bound
  This paper has been withdrawn by the authors, due to a flaw in the proof of Theorem 1. This preprint is superseded by quant-ph/0610027, where a correct proof can be found. Thanks to Rainer Siegmund-Schultze for spotting the error. 
J. Calsamiglia, Ll. Masanes, R. Munoz-Tapia, A. Acin, E. Bagan