Geodesics and the best measurement for distinguishing quantum states

Abstract

From statistical distinguishability of probability distributions, one can define distinguishability of quantum states. A corresponding measurement to perform, optimal in a definite sense, for distinguishing between two given states rho_A and rho_B, has been derived by Fuchs and Caves. We show that the Bures-Uhlmann geodesic through the two states singles out this measurement. The geodesic `bounces' at the boundary of the set of quantum states. Whenever the geodesic hits the boundary, the state orthogonal to that boundary state is one of the basis states for the measurement.