I read that Jones matrices can descibe optical devices with polarized light. They are 2 by 2 matrices.A basis can be Linear polarization along x or along y for a photon moving along z. I can use when there is a device with one input channel. Consider now a Mach Zehnder Interferometer (MZI). there is a beam splitter with 2 input channels. I saw papers which use Jones like matrices but where the input polarizations H and V were replaced by Port1 and port2 and the output giving the detectors. A MZI needs 4 input parameters LH, LV, BH, BV (L B for left bottom) I did not see descriptions of MZI using 4*4 matrices. Have you a link? I found Mueller 4*4 matrices but they speak of something else.