  Here we propose an implementation of all possible Positive Operator Value
Measures (POVMs) of two-photon polarization states. POVMs are the most general
class of quantum measurements. Our setup requires linear optics, Bell State
measurements and an entangled three-photon ancilla state, which can be prepared
separately and in advance (or 'off-line'). As an example we give the detailed
settings for a simultaneous measurement of all four Bell States for an
arbitrary two-photon polarization state, which is impossible with linear optics
alone.
