Accepted Tuesday Nov 03, 2009
We present theory and experiment for the task of discriminating two nonorthogonal states, given multiple copies. We implement several local measurement schemes, on both pure states and states mixed by depolarizing noise. We find that schemes which, without noise, are optimal (or have optimal scaling), perform worse with noise than simply repeating the optimal single-copy measurement. Applying optimal control theory, we derive the globally-optimal local measurement strategy, which outperforms all other local schemes, and experimentally implement it for various levels of noise.