  We experimentally demonstrate that when three single photons transmit through
two polarization channels, in a well-defined pre- and postselected ensemble,
there are no two photons in the same polarization channel by weak-strength
measurement, a counter-intuitive quantum counting effect called quantum
pigeonhole paradox. We further show that this effect breaks down in
second-order measurement. These results indicate the existence of quantum
pigeonhole paradox and its operating regime.
