FIG. 2. Experimental setup. We prepare two copies of a Bell state |φ+〉 by a laser-emitted ultraviolet pulse split into three beams pumping SPDC sources. The scheme guarantees that both copies are emitted by different sources. Conversely, in a two source setting, the fourfold coincidences in the BSMs could be generated by two photon pairs emitted from a single source, invalidating the experiment. The four terms of the mixture are obtained by rotating QWP1,2. Purity and overlap measurements are implemented via BSM schemes. A multichannel unit counts the sixfold coincidences (one detector fire in each output mode).
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