  Quantum mechanics does not provide a clear answer to the question: What was
the past of a photon which went through an interferometer? Various welcher weg
measurements, delayed-choice which-path experiments and weak-measurements of
photons in interferometers presented the past of a photon as a trajectory or a
set of trajectories. We have carried out experimental weak measurements of the
paths of photons going through a nested Mach-Zehnder interferometer which show
a different picture: the past of a photon is not a set of continuous
trajectories. The photons tell us that they have been in the parts of the
interferometer which they could not have possibly reached! Our results lead to
rejection of a "common sense" approach to the past of a quantum particle. On
the other hand, they have a simple explanation within the framework of the
two-state vector formalism of quantum theory.
