Fig. 2

Experimental setup: For parametric down conversion a 2 mm long BBO crystal was used. The crystal was cut for collinear type I phase matching around 405 nm. The path delay between the two arms of the Mach-Zehnder-interferometer for observing the fringes and determining the visibility was realized by a motorized high resolution translation stage (Δl). The beam selection from different positions of the light cone structure as schemed in inset b was realized by a beam tilt and shift arrangement which also allowed for the perfect overlay of the selected two modes at the second beam splitter BS₂. The detectors are single mode fiber coupled avalanche photo diodes SPCM-AQRH-13 from Perkin Elmer with lenses in front for determining the TEM₀₀ modes of the idler- and the two signal-fields