[wwarner]

I'm not an expert, but I thought the paper was more novel than that, in that it challenges my picture of entanglement. The paper demontrates entangled behavior with photons with no common history. They are "identical" in their conserved properties, rather than "entangled" which to me implies that their conserved properties sum to the values that existed before the event that produced the entangled pair.

To me, the surprise is that identical pairs always take the same path.

[abdullahkhalids]

This might challenge your picture of entanglement, but definitely not that of a physicist. Otherwise, I can't make head or tail of your statements.

[wwarner]

I'll try to restate. I was under the impression that entanglement implied what the paper calls "common history", such as when an excited calcium atom decays to its ground state and produces two entangled photons. I also understood entanglement to be particles with some opposite properties, so that the sum of say the angular momentum of the system before and after the decay remains constant. 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.

[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!