It occurs to me that I've probably been confusing "phase" in a number of contexts. In particular:(adsbygoogle = window.adsbygoogle || []).push({});

1) If we write a photon's polarization as [itex]\psi = |x\rangle + e^{i\theta}|y\rangle[/itex], then we can call [itex]\theta[/itex] a "phase."

2) When a photon bounces off a mirror, it picks up a relative phase of [itex]i[/itex]. If I understand correctly, this is the same "phase" as above. It is also the "phase" referred to in the two-slit experiment.

3) Inside a Mach-Zehnder interferometer, after a beam splitter, we represent the state as something like [itex]\psi = |0\rangle + i|1\rangle[/itex]. If I understand, the phase shift happens because of (2). But it's not clear to me in what sense "which-path" information has aphase.

And I should add:

4) We can perform the two-slit experiment with electrons and get a similar result. How can we explain it with "phase" (like we did in 2) if there's no polarization basis in which "phase" is meaningful?

Can anyone help me clarify?

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# I What is photon phase?

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