Entanglement in cascaded-crystal parametric down-conversion

We use spontaneous parametric down-conversion in a cascade of crystals, dri ven by a single monochromatic cw pump laser. to study the interference of e ntangled photon pairs. By changing the distance between the crystals, the o bserved quantum interference pattern varies continuously from that associat ed with a longer single crystal to that associated with independent emissio ns from two distinct crystals. Postselection via spectral filtering suppres ses this phenomenon. These findings are expected to advance the field of qu antum-state engineering.