Asking photons where they have been

Quantum mechanics does not provide a clear answer to the question: What was the past of a photon which went through an interferometer?

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!

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