GEOMETRIC PHASES IN 2-PHOTON INTERFERENCE EXPERIMENTS

Photon pairs produced by type-I and type-II parametric down-conversion are passed through a Michelson interferometer. Two quarter-wave plate s in one arm of the interferometer allow variation of the polarization state of the photons. We investigate experimentally the geometric pha se or Pancharatnam phase acquired by single photons and photon pairs d ependent on the solid angle that is subtended by the circuit that repr esents the varying state of polarization on the Poincare sphere. It is found that the geometric phase acquired by the pair depends on the in itial polarization state of the two photons. If both photons are in eq ual states of linear polarization, we observe a doubling of the geomet ric phase compared to single photons; in the case of orthogonal states of linear polarization, the geometric phase is completely canceled. O ur results show the role of Pancharatnam's phase in nonclassical two-p hoton interference phenomena and the interplay between the geometric p hase and the dynamical phase in these phenomena.