  We investigate optimal discrimination between two projective quantum
measurements on a single qubit. We consider scenario where the measurement that
should be identified can be performed twice and we show that adaptive
discrimination strategy, entangled probe states, and feed-forward all help to
increase the probability of correct identification of the measurement. We also
experimentally demonstrate the studied discrimination strategies and test their
performance. The employed experimental setup involves projective measurements
on polarization states of single photons and preparation of required probe
two-photon polarization states by the process of spontaneous parametric
down-conversion and passive linear optics.
