Information-Disturbance Tradeoff in Quantum State Discrimination
  When discriminating between two pure quantum states, there exists a quantitative tradeoff between the information retrieved by the measurement and the disturbance caused on the unknown state. We derive the optimal tradeoff and provide the corresponding quantum measurement. Such an optimal measurement smoothly interpolates between the two limiting cases of maximal information extraction and no measurement at all. 
Francesco Buscemi and Massimiliano F. Sacchi