# How does a photon released from particle A interacting with particle B, entangle them?

What does a photon carry from its parent particle that when it interacts with another particle, it entangles both the particles?

• It depends what you mean by "entanglement" A photon carries energy/momentum and spin orientation. It can transfer this information to a particle it interacts witth, and the state of the second particle can give information about the state and energy level of the parent particle, for example. – anna v Feb 27 '19 at 6:31
• @annav suppose the entanglement found between the two particles after one shared photon with the other is spin-entanglement. – Viswa Vijeta Feb 27 '19 at 7:55
• why don't you read this question and answers physics.stackexchange.com/q/457922 . – anna v Feb 27 '19 at 8:47

## 1 Answer

First of all, the photon needs to be entangled with the particle that emitted it. That is not difficult to accomplish. One can for instance use the conservation of momentum to obtain an entangled state: $$|\psi\rangle = \sum_n |\mathbf{p}_n\rangle_{phot} |\mathbf{p}-\mathbf{p}_n\rangle _A\alpha_n ,$$ where $$\sum_n |\alpha_n|^2 = 1$$ and $$\mathbf{p}$$ is the combined momentum of both particle A and the photon. When the photon now interacts with particle B, it will transfer an amount of momentum that is correlated with that of particle A. Hence, the two particles become entangled.

• You appear to be talking about conservation of momentum. – R.W. Bird Aug 4 '20 at 14:50
• Yes, conservation of momentum is one of the ways one can get entanglement – flippiefanus Aug 5 '20 at 3:48
• What's "spooky" about that? – R.W. Bird Aug 5 '20 at 13:24
• What do you mean? Why does it have to be spooky? Please explain in a clearly phrased question what you have a problem with. – flippiefanus Aug 6 '20 at 3:34
• As I recall, Einstein referred to entanglement as spooky action at a distance. – R.W. Bird Aug 6 '20 at 13:36