Complementarity and which-path information

Summary form only given. Complementarity is one of the most basic principles of quantum mechanics. A quantum mechanical object, be it a photon or a massive particle, shows position (particle-like) or momentum (wave-like) attributes exclusively depending on the experimental situation. Over the years, Young´s two-slit experiment has been emphasized as a good example of complementarity. We report two two-slit experiments in which complementarity is enforced by availability of which-path information at the measurement stage of the quantum, which is different from the traditional understanding. A type-II BBO crystal is pumped by a CW UV laser generating spontaneous parametric down-conversion (SPDC). Entangled signal and idler photons are then sent in different directions. A double slit is placed in the idler beam and the idler detectors are scanned in the transverse direction.