I saw many of you saying in their posts that non-linear crystals like barium borate are the only means of producing entangled photons. And because they are expensive, only some of you can afford them. But I browsed the international science magazines and found this: https://physics.aps.org/articles/v7/25 "Photons are often entangled at their creation. For example, pairs of identical photons can be generated by “down-converting” single pump photons in a nonlinear crystal. Unfortunately, this process is inefficient because the down-conversion has a low probability. One alternative is to entangle photons coming from different sources. This can be done by sending two photons into a beam splitter and taking advantage of the quantum phenomenon of two-photon interference: If an observer behind the beam splitter cannot tell which path each individual photon took, their probabilistic outcomes can add up or cancel out, leaving those photons that exit the beam splitter through different ports in an entangled state. This effect has been the workhorse of the quantum photonics community for decades." http://www.livescience.com/19975-spooky-quantum-entanglement.html "The way you entangle them is to send them onto a half-silvered mirror," Zeilinger told LiveScience. "It reflects half of the photons, and transmits half. If you send two photons, one to the right and one to the left, then each of the two photons have forgotten where they come from. They lose their identities and become entangled." As you can see, entanglement can be done by a beam splitter for two photons, because this beam splitter creates an identical superposition of trajectories for both photons. We do not know if the photon coming from the left is transmitted or if the photon coming from the right is reflected, and their trajectories coincide.