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 while at the private, Boston-based Institute for Radiation-Induced Mass Studies (IRIMS). [1] The results were presented at a Harvard seminar in March 2004. [2] Afshar claimed that the experiment gives information about which of two paths a photon takes through the apparatus while simultaneously allowing interference between the two paths to be observed, by showing that a grid of wires, placed at the nodes of the interference pattern, does not alter the beams. [3] Afshar claimed that the experiment violates the principle of complementarity of quantum mechanics, [4] which states roughly that the particle and wave aspects of quantum objects cannot be observed at the same time, and specifically the Englert–Greenberger duality relation. [5] The experiment has been repeated by a number of investigators but its interpretation is controversial and there are several theories that explain the effect without violating complementarity. [6] [7] [8] [9] [10]



Afshar's experiment uses a variant of Thomas Young's classic double-slit experiment to create interference patterns to investigate complementarity. One of Afshar's assertions is that, in his experiment, it is possible to check for interference fringes of a photon stream (a measurement of the wave nature of the photons) while at the same time determining each photon's "which-path" information (a measurement of the particle nature of the photons). [3] [11] In his experiment, pinhole A is correlated to detector 1 when pinhole B is closed, and pinhole B is correlated to detector 2 when pinhole A is closed. Afshar's claim for the violation of the principle of complementarity depends crucially on his assertion that these correlations remain, and thus which-path information is preserved, when both pinholes are open, and cites Wheeler [12] in support. [5]


Shahriar S. Afshar's experimental work was done initially at the Institute for Radiation-Induced Mass Studies (IRIMS) in Boston in 2001 and later reproduced at Harvard University in 2003, while he was a research scholar there. [1] The results were presented at a Harvard seminar in March 2004, [2] and published as conference proceeding by The International Society for Optical Engineering (SPIE). [3] The experiment was featured as the cover story in the July 24, 2004 edition of New Scientist . [1] [13] The New Scientist feature article itself generated many responses, including various letters to the editor that appeared in the August 7 and August 14, 2004 issues, arguing against the conclusions being drawn by Afshar, with John G. Cramer's response. [14] Afshar presented his work also at the American Physical Society meeting in Los Angeles, in late March 2005. [15] His peer-reviewed paper was published in Foundations of Physics in January 2007. [5]

Experimental setup

Fig.1 Experiment without obstructing wire grid Afshar-experiment.png
Fig.1 Experiment without obstructing wire grid
Fig.2 Experiment with obstructing wire grid and one pinhole covered Afshar-experiment-wire.png
Fig.2 Experiment with obstructing wire grid and one pinhole covered
Fig.3 Experiment with wire grid and both pinholes open. The wires lie in the dark fringes and thus block very little light Afshar-experiment-1.png
Fig.3 Experiment with wire grid and both pinholes open. The wires lie in the dark fringes and thus block very little light

The experiment uses a setup similar to that for the double-slit experiment. In Afshar's variant, light generated by a laser passes through two closely spaced circular pinholes (not slits). After the dual pinholes, a lens refocuses the light so that the image of each pinhole falls on separate photon-detectors (Fig. 1). With Pinhole 2 closed, a photon that goes through Pinhole 1 impinges only on Photon Detector 1. Similarly, with Pinhole 1 closed, a photon that goes through Pinhole 2 impinges only on Photon Detector 2. With both pinholes open, Afshar claims, citing Wheeler [12] in support, that Pinhole 1 remains correlated to Photon Detector 1 (and vice versa for Pinhole 2 to Photon Detector 2), and therefore that which-way information is preserved when both pinholes are open. [5]

When the light acts as a wave, because of quantum interference one can observe that there are regions that the photons avoid, called dark fringes. A grid of thin wires is placed just before the lens (Fig. 2) so that the wires lie in the dark fringes of an interference pattern which is produced by the dual pinhole setup. If one of the pinholes is blocked, the interference pattern will no longer be formed, and the grid of wires causes appreciable diffraction in the light and blocks some of it from detection by the corresponding photon detector. However, when both pinholes are open, the effect of the wires is negligible, comparable to the case in which there are no wires placed in front of the lens (Fig.3), because the wires lie in the dark fringes of an interference pattern. The effect is not dependent on the light intensity (photon flux).

To establish violation of the principle of complementarity, Afshar considers the case in which both pinholes are open and asserts both high visibility V of interference as well as high distinguishability D (corresponding to which-path information), so that V2 + D2 > 1. [5] His claim depends heavily on whether which-path information is preserved when both pinholes are open.

Afshar's interpretation

Afshar's conclusion is that, when both pinholes are open, the light exhibits wave-like behavior when going past the wires, since the light goes through the spaces between the wires but avoids the wires themselves, but also exhibits particle-like behavior after going through the lens, with photons going to a correlated photo-detector. Afshar argues that this behavior contradicts the principle of complementarity to the extent that it shows both wave and particle characteristics in the same experiment for the same photons.


Specific criticism

A number of scientists have published criticisms of Afshar's interpretation of his results, some of which reject the claims of a violation of complementarity, while differing in the way they explain how complementarity copes with the experiment. Afshar has responded to these critics in his academic talks, his blog, and other forums. For example, one paper contests Afshar's core claim, that the Englert–Greenberger duality relation is violated. The researchers re-ran the experiment, using a different method for measuring the visibility of the interference pattern than that used by Afshar, and found no violation of complementarity, concluding "This result demonstrates that the experiment can be perfectly explained by the Copenhagen interpretation of quantum mechanics." [8]

Below is a synopsis of papers by several critics highlighting their main arguments and the disagreements they have amongst themselves:

Specific support

See also

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Further reading