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Do we always have to create new particles to get to be entangled? or can we already entangle existing particles?

the only thing I've found is from Wikipedia.

Methods of creating entanglement Entanglement is usually created by direct interactions between subatomic particles. These interactions can take numerous forms. One of the most commonly used methods is spontaneous parametric down-conversion to generate a pair of photons entangled in polarization. Other methods include the use of a fiber coupler to confine and mix photons, photons emitted from decay cascade of the bi-exciton in a quantum dot, the use of the Hong–Ou–Mandel effect, etc. In the earliest tests of Bell's theorem, the entangled particles were generated using atomic cascades.

It is also possible to create entanglement between quantum systems that never directly interacted, through the use of entanglement swapping. Two independently prepared, identical particles may also be entangled if their wave functions merely spatially overlap, at least partially.

so is entanglement swapping the only way. or is there more?

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There is no requirement that particles have to be newly created to be entangled. Essentially any time particles interact with each other they become partially entangled. Your quote shows that even this is not a requirement- two particles can become entangled without ever directly interacting.

The reason you so often hear about particle creation when talking about particle entanglement is practical. The easy methods to produce pairs of maximally entangled particles tend to involve creating new particles that have to be entangled in order to follow conservation laws.

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  • $\begingroup$ Thank you! this helps alot $\endgroup$
    – RhinoPak
    Sep 3, 2021 at 3:13

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