  We experimentally demonstrate protective measurements by weakly coupling the
polarization of a single-photon-level field to a measurement pointer that
corresponds to the arrival time of the photon. By using an optical loop, we
implement a variable, controlled number (1-9) of protection and measurement
stages. We demonstrate the measurement of expectation values of photon
polarization by measuring arrival times while simultaneously protecting the
polarization state. No knowledge of the initial photon state is required or
available in our experiment, demonstrating that protective measurements provide
a genuine information gain that cannot simply be reduced to a priori
information about the protection procedure.
