  One of the central principles of quantum mechanics is that if there are
multiple paths that lead to the same event, and there is no way to distinguish
between them, interference occurs. It is usually assumed that distinguishing
information in the preparation, evolution or measurement of a system is
sufficient to destroy interference. For example, determining which slit a
particle takes in Young's double slit experiment or using distinguishable
photons in the two-photon Hong-Ou-Mandel effect allow discrimination of the
paths leading to detection events, so in both cases interference vanishes.
Remarkably for more than three independent quantum particles,
distinguishability of the prepared states is not a sufficient condition for
multiparticle interference to disappear. Here we experimentally demonstrate
this for four photons prepared in pairwise distinguishable states, thus
fundamentally challenging intuition of multiparticle interference.
