A polarization gadget with two quarter wave plates: Application to   Mueller Polarimetry
  We show that there are number of ways to transform an arbitrary polarization state to another with just two quarter wave plates (QWP). We have verified this geometrically using the trajectories of the initial and final polarization states corresponding to all the fast axis orientations of a QWP on the Poincare sphere. The exact analytical expression for the locus of polarization states has also been given that describes the trajectory. An analytical treatment of the equations obtained through matrix operations corresponding to the transformation supports the geometrical representation. This knowledge can be used to obtain the Mueller matrix by just using quarter wave plates which has been shown experimentally by exploiting projections of the output states on the input states. 
Salla Gangi Reddy, Shashi Prabhakar, Chithrabhanu Perumangattu, R. P.   Singh, R. Simon
physics.optics