Some modified two-slit interference experiments were carried out showing an apparent paradox in wave–particle duality. In a typical such experiment, the screen, where the interference pattern is supposed to be formed, is replaced by a converging lens. The converging lens forms the images of the two slits at two spatially separated detectors. It was claimed that each of these two detectors give information about which slit a photon came from, even though they come from the region of interference. These experiments (...) generated a lot of debate. The various refutations pointed out that the controversial claims involved some questionable assumptions. However the refutations were largely philosophical in nature, and one may like to substantiate those with arguments which are testable, at least in principle. Here such an experiment is theoretically analyzed by introducing path markers which are two orthogonal polarization states of the photon. Analyzing the polarization at the two detectors shows that the photons which give rise to interference, and reach a particular detector, always come from both the slits. This provides clarity in understanding such experiments by making use of testable quantum correlations. (shrink)

Shahriar S. Afshar claimed that his 2007 modified version of the double-slit experiment violates complementarity. He makes two modifications to the standard double-slit experiment. First, he adds a wire grid that is placed in between the slits and the screen at locations of interference minima. The second modification is to place a converging lens just after the wire grid. The idea is that the wire grid implies the existence of interference minima, while the lens can simultaneously obtain which-way information. More (...) recently, Cramer argued that the experiment bolstered the Transactional Interpretation of Quantum mechanics. His argument scrutinizes Bohr’s complementarity in favor of TIQM. We analyze this experiment by simulation using the path integral formulation of quantum mechanics and find that it agrees with the wave-particle duality relation given by Englert, Greenberg and Yasin. We conclude that the use of Afshar’s experiment to provide a testbed for quantum mechanical interpretations is limited. (shrink)