Fig. 2
Scheme of a MZI operating with HG modes. A Mach-Zehnder Interferometer together with a Dove prism in one of its arms operates as a HG mode sorter. At the output of the last BS, the X mode emerges on the v path and the Y mode on the h path. This modes can be either redirected to detectors, as in Figure 1, to project onto Bell states, or the Y mode can be redirected into a external path (dashed line), introducing a delay τ between modes, that is, |1X〉 and |1Yτ〉. Thus, such MZIs can be used as ’polarizing’ beam splitters (thinking of X and Y as polarizations) for Bell-state detection as well as OFD devices