Recognizing how some states are built, from mixed states to singlets
  It is well known that different preparations of a mixed state cannot be distinguished by a measurement of that state. Yet we show that some other experiments let us make this distinction despite a very general belief that this would not be possible. Issues in quantum cryptography that prompted this work are only briefly mentioned in this letter. 
Charles Tresser, Enrique Pujals
quant-ph