For the case of an electron passing through a double slit, there will be an interference pattern if it is indistinguishable which slit the electron passes through. If we put a light source behind the slits to measure which slit the electron passes through, the interference pattern is destroyed.(adsbygoogle = window.adsbygoogle || []).push({});

So by the same logic, since there is interference in an interferometer, it must be indistinguishable which of the two paths the light takes or a photon takes.

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# Are the two paths in an interferometer indistinguishable?

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