[abdullahkhalids]

> So it was news to me that (a) photons without a common history, and (b) photons indistinguishable in all degrees of freedom could be entangled.

Oh no. It is definitely possible to entangle two photons from two different sources. Say two hydrogen atoms emit a photon each; then you just need a non-linear crystal to entangle them, or use linear crystals and post-select good results. Here is a recent example for the latter for quantum repeaters [1], though this has been done for decades now. And making two different photon sources indistinguishable (or controllably-distinguishable) is like the first thing you do in a quantum optics lab.

> I also understood entanglement...

That's not a correct way to think of entanglement. Two particles are (maximally) entangled if

(1) you do the same measurement on the two particles, the results are (perfectly) correlated.

(2) if you do orthogonal measurements on the two particles, the results are (perfectly) uncorrelated.

And all this happens in a way that no local classical theory can explain.

[1] https://arxiv.org/pdf/1908.05351.pdf

[wwarner]

Thanks, ya done me a solid!