How do you get photos to become entangled? And is this even possible, or just a theory?
Can you explain how photons can get entangled by SPDC, as well as the CHSH inequality? I cannot understand the Wikipedia explanation.Photons are usually entangled using SPDC (Spontaneous Parametric Down conversion), in which basically a photon of higher energy is split into 2 photons of lower energies using a non-linear crystal. The photons are entangled under certain conditions. You can check wikipedia about this.
There have been lot of experiments to violate Bell/CHSH inequality starting from the one by Alain Aspect. All of these prove the presence of entanglement by separating the particles over larger and larger distance. So it is not just a theory (at least for most of the scientific community although there are still some who doubt it).