BACKType-II spontaneous parametric down-conversion process
established itself as the standard method for the generation of
polarization-entangled photon pairs. In this process photons of an intense pump
beam spontaneously convert in a nonlinear crystal with low probability into
pairs of mutually orthogonally polarized photons, conventionally called signal
and idler. Energy and momentum conservation in the nonlinear interaction ensures
that the emitted photons exhibit nonclassical wavelength and emission direction
correlations. When signal and idler photons have the same wavelength, referred
to as degenerate case, the photons emerge from the crystal symmetrically to the
pump beam along two cones, which intersect for certain orientations of the
crystal optic axis. The polarization of each photon collected along the
intersection lines is undefined, as it cannot be assigned to one of the two
orthogonally polarized cones, but is perfectly anti-correlated with the
polarization of the other
one. Therefore, the polarization of these photons is
entangled
In cooperation with M. Stipčević from IRB Zagreb and P. Troyek from LMU Munich, Setup for generation of Entangled photon pairs was built: