Afshar experiment

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The Afshar experiment is a variation of the double-slit experiment in quantum mechanics, devised and carried out by Shahriar Afshar in 2004.^[1][2] 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.^[3]

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.^[4][5] According to Afshar, this violates the complementarity principle of quantum mechanics.^[3][6]

The experiment has been analyzed and repeated by a number of investigators.^[7] There are several theories that explain the effect without violating complementarity.^{[8][9][10][11]} John G. Cramer claims the experiment provides evidence for the transactional interpretation of quantum mechanics over other interpretations.