Figure 1
Illustration: Carin Cain, after Ref. [1]

Figure 1: Parametric down conversion generates pairs of photons in an entangled, orthogonal-polarization state. In their experiment, Rådmark et al. focus on events where three indistinguishable pairs are produced when a UV pulse passes through a non-linear crystal. The photons in each pair are emitted in different directions, so the “six-pack” can be separated into two sets of three (green beads). The photons are then separated from each other using beam splitters (purple rectangles) and sent into polarization analyzers – beam splitters (blue rectangles) that transmit or reflect the photon depending on its polarization - and finally, single-photon detectors. The black rectangles marked “U” depict unitary polarization transformations used to analyze the generated state and verify its robustness against collective noise.