I have been reading the article review for Optical Quantum Memory (http://arxiv.org/abs/1002.4659) when I came across this section about DLCZ protocol. I don't understand about Beam splitter. For what I can interpret, the beam splitter overlaps both photons which emitted simultaneously by two atomic ensembles. My question is how does the beam splitter mixes two photons together so they become entangled?
P.S. My English is not good. I am sorry.