  We propose and demonstrate a linear optical device which deterministically
performs optimal quantum measurement or minimum disturbance measurement on a
single-photon polarization qubit with the help of an ancillary path qubit
introduced to the same photon. We show theoretically and experimentally that
this device satisfies the minimum disturbance measurement condition by
investigating the relation between the information gain (estimation fidelity)
and the state disturbance due to measurement (operation fidelity). Our
implementation of minimum disturbance measurement is postselection-free in the
sense that all detection events are counted toward evaluation of the estimation
fidelity and the operation fidelity, i.e., there is no need for coincidence
postselection of the detection events.
