Abstract

A down-conversion source of truly polarization-entangled photons has been developed [1], much brighter and more stable than all previous sources. In type-II phase-matching, the correlated photons are emitted with orthogonal polarizations. In contrast to the more familiar case of type-I phase matching, the cones of the emitted light are not concentric with the pump beam direction. Instead, the cone of photons that are extraordinary-polarized inside the birefringent nonlinear crystal (BBO, in our experiments) lies predominantly between the pump and the crystal optic axis, while the cone of the ordinary-polarized photons lies opposite (see Fig 1a). As a result, if the crystal is tilted slightly away from the collinear condition (where the two cones are tangent exactly along the pump beam direction), there arise two directions in which the photons belong to both the ordinary and the extraordinary cones (see Fig 1b).

© 1996 Optical Society of America