Afshar experiment

The Afshar experiment is a variation of the double-slit experiment in quantum mechanics, devised and carried out by Shahriar Afshar in 2004. In the experiment, light generated by a laser passes through two closely spaced pinholes, and is refocused by a lens so that the image of each pinhole falls on a separate single-photon detector. In addition, a grid of thin wires is placed just before the lens on the dark fringes of an interference pattern.

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Afshar claimed that the experiment gives information about which path a photon takes through the apparatus, while simultaneously allowing interference between the paths to be observed. According to Afshar, this violates the complementarity principle of quantum mechanics.

The experiment has been analyzed and repeated by a number of investigators. There are several theories that explain the effect without violating complementarity. John G. Cramer claims the experiment provides evidence for the transactional interpretation of quantum mechanics over other interpretations.