We present a full demonstration of unambiguous state discrimination between
nonorthogonal quantum states. Using a free-space interferometer we have re
alized the optimum quantum measurement scheme for two nonorthogonal states
of light, known as the Ivanovic-Dieks-Peres (IDP) measurement. We have gain
ed access to all three possible outcomes of this measurement. All aspects o
f this generalized measurement scheme, including its superiority over a sta
ndard von Neumann measurement, have been demonstrated within 1.5% of the ID
P predictions.