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What is a physical example of entanglement occurring naturally? I am looking for an instance that I would see in nature outside the lab. I am looking for this example to be concrete and physical.

I just want to know of an example, so one would be sufficient. I'm not seeking a list.

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$\begingroup$

What is a physical example of entanglement occurring naturally? I am looking for an instance that I would see in nature outside the lab. I am looking for this example to be concrete and physical.

I just want to know of an example, so one would be sufficient. I'm not seeking a list.

Any $S=0$ combination formed by two $S=1/2$ particles is entangled. This spin state is $$ |S=0,S_z=0\rangle = \frac{1}{\sqrt{2}} \left( |\uparrow\downarrow\rangle - |\downarrow\uparrow\rangle \right)\;, $$ which can not be written as a direct product state (and is therefore entangled).

For example, parahelium. (N.b., the ground state of Helium is parahelium, since the spatial part of the wave function is symmetric in the ground state. Therefore, most of the Helium in the universe could be expected to have an entangled spin state and therefore this entangled state is ubiquitous outside of the lab.)

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    $\begingroup$ exactly what I was looking for - thanks ! $\endgroup$
    – Relativisticcucumber
    Commented Jun 18 at 2:16

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