  The validity of the superposition principle and of Born's rule are
well-accepted tenants of quantum mechanics. Surprisingly, it has recently been
predicted that the intensity pattern formed in a three-slit experiment is
seemingly in contradiction with the predictions of the most conventional form
of the superposition principle when exotic looped trajectories are taken into
account. However, the probability of observing such paths is typically very
small and thus rendering them extremely difficult to measure. In this work, we
confirm the validity of Born's rule and present the first experimental
observation of these exotic trajectories as additional paths for the light by
directly measuring their contribution to the formation of optical interference
fringes. We accomplish this by enhancing the electromagnetic near-fields in the
vicinity of the slits through the excitation of surface plasmons. This process
effectively increases the probability of occurrence of these exotic
trajectories, demonstrating that they are related to the near-field component
of the photon's wavefunction.
