www.qutools.com quED Manual 21
Figure 9: As the BBO is actually made up of two, the exact position of the overlap region (pump,
coupler 1 and 2) determines the ratio of horizontally and vertically polarized photon pairs. Only
if the overlap region is symmetric relative to the interface of the two crystals HH and VV pairs
are collected with the same efficiency. You can easily tune the position with the horizontal
pointing of the couplers.
Figure 10: For a good entanglement quality, it is crucial to collect the down-conversion pairs
symmetric around the pump beam (green lines,
). From the pre-alignment you might
arrive with a situation as indicated with the red lines. While the absolute value of is not
crucial for the entanglement, follow the instructions in section 4.1 to make the coupling
symmetric.
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