Linear optics implementation of general two-photon projective   measurement
  We will present a method of implementation of general projective measurement of two-photon polarization state with the use of linear optics elements only. The scheme presented succeeds with a probability of at least 1/16. For some specific measurements, (e.g. parity measurement) this probability reaches 1/4. 
Andrzej Grudka and Antoni Wojcik