Didn't they originally use polarizing filters to measure photonic phase?

If it were possible to measure the phase of a photon after a beam splitter in a nondestructive way, shouldn't it be possible to determine whether measuring one causes state collapse in the other?

This says that photonic entanglement is polarization, and that photonic phase can be inferred from second order of intensity, IIUC:

"Bridging coherence optics and classical mechanics: A generic light polarization-entanglement complementary relation" (2023) https://journals.aps.org/prresearch/abstract/10.1103/PhysRev...

Shouldn't it then be possible to nondestructively measure photons and thus entanglement?

> If it were possible to measure the phase of a photon after a beam splitter in a nondestructive way

"Non-destructive measurement" is an oxymoron. It's not a real measurement if it doesn't destroy the coherence of entanglement. Weak measurements do destroy some entanglement, just not "all" of it.