I might be a little out of my league, but I'm trying to work through the DLCZ protocol for quantum communication (http://arxiv.org/abs/quant-ph/0105105). I've read the paper and gone through the introductory parts of this thesis: http://thesis.library.caltech.edu/2059/, but I'm still unclear on a very fundamental point: What does the beamsplitter in FIG 1 of the arxiv paper do? Specifically, it boils down to the following question: Why does a click in D1 or D2 signal entanglement of the atomic ensembles? Thanks so much for your help! I'm really curious to figure this out and I realize that this probably stems from my lack of understanding in how a beam splitter works. Every reference I've read just states that a click means entanglement was successful.