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From Charles G. Wohl's Quantum Mechanics 1, Page 9 (2014)

This Mach-Zehnder interferometer allows us to understand that we cannot know which path a photon will take while still getting interference.

"About half the time a photon enters the system, we get a click at C4; but then we get no click at either C1 or C2. The photon took the upper path but was absorbed—we can’t both record the arrival of the photon at C4 and have it continue undisturbed on its way. The other half of the time, we get a click at either C1 or C2. The photon took the lower path, but now it is equally likely to get to C1 and C2—the upper path is now blocked by C4, and the interference has been destroyed by our looking to see which path was taken. But now we have the same kind of question we asked about a neutron in the double-slit experiment: Since the photon only ever appears at one place or another, how can it somehow take—or even know about—two paths, as the interference (when C4 is absent) seems to require? Is light particles or is it waves?"