Are you able to create an experimental set-up to that of http://img.photobucket.com/albums/v608/steviet/scan0001.jpg, where the wave plates on the input paths can be adjusted to various angles. (At the output of BS2 are two polarising beam splitters.)
What I'm hoping to do is to be able to define the input of the photons on BOTH input paths, where there will be a photon coming in on EACH input path. The photons are assumed to hit both beam splitters at the same time.
The input of the input photons could be a definite polarisation (incl. +H or -H, +V or -V), or a superposition (e.g. the input photon on path 1 is |H>|V> - |V>|H>, i.e. entangled with another photon - but not the photon entering on path 2).