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In physics, and more specifically, quantum mechanics, the Afshar experiment is an optical experiment, devised by Shahriar S. Afshar in 2004, that is claimed by its proponents to have disproved the Niels Bohr's principle of complementarity. Since this principle is a central feature of quantum mechanics, the proper interpretation of the experiment has engendered some controversy in the physics community, and in response papers, generally Afshar's interpretation has not been accepted. The controversy has been mostly limited to blogs, physics colloquia, academic conferences, and arXiv e-print archives; and although Afshar's papers have been published in the reputable American Institute of Physics and SPIE conference proceedings, as of 2006, neither a description of the experiment, nor any discussion of its theoretical interpretation, has been published in a refereed physics journal.

Overview

The principle of complementarity states that two complementary physical observables cannot both be measured for any given quantum particle. For example, a particle's position and momentum cannot be observed at the same time: this is Werner Heisenberg's uncertainty principle.

One of Afshar's assertions is that, in his experiment, one can check for interference fringes of a photon stream (a momentum measurement) while at the same time observing the photon's path (a position measurement). Afshar's experiment attempts to do this using a variant of the classic Thomas Young double-slit experiment. Such interferometer experiments typically have two "arms" or paths a photon may take. Afshar attempts to both preserve interference, and to determine "which way" or "Welcher Weg" the photon went. Many of the claims associated with this experiment cut across several conventional ideas in quantum mechanics.

History

Shahriar S. Afshar's experimental work was done initially at the Institute for Radiation-Induced Mass Studies (IRIMS) in 2001 and later reproduced at Harvard University in 2003, while he was a Research Scholar there. He presented his results in a seminar talk in March 2004 entitled Waving Copenhagen Good-bye: Were the founders of Quantum Mechanics wrong? . The experiment was subsequently featured as the cover story in the July 24, 2004 edition of New Scientist. , and published in Proc. SPIE 5866, 229-244 in July of 2005 . Afshar's claim that his experiment invalidates the complementarity principle would have far-reaching implications for the understanding of quantum mechanics, potentially challenging the Copenhagen interpretation and according to John Cramer, the many-worlds interpretation of quantum mechanics. Cramer also asserts that Afshar's results support his transactional interpretation of quantum mechanics.

As of late May, 2005, Afshar has presented his work at various university seminars and in late March of 2005, at the American Physical Society meeting in Los Angeles . His paper has been published by the International Society for Optical Engineering (SPIE) in July, 2005 . Afshar's results have also been reported in the New Scientist as cited above and in other popular science magazines. 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. Among those writing were Alistair Rae (Centre for Photonic Systems, Cambridge University), David Dunstan (Head of the Physics Department, University of London) and Alwyn Eades (Director of the Microscopy Center, Materials Science Department, Lehigh University) who viewed Afshar's interpretation with skepticism.

Experimental setup and Afshar's interpretation

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 is received by a separate photo-detector (Fig. 1). In this setup, a photon that goes through pinhole number one impinges only on detector number one, and similarly, if it goes through pinhole two. Therefore, if observed at the image plane, the setup is such that the light behaves as a stream of particles and can be assigned to a particular pinhole.

When the light acts as a wave, because of interference one can observe that there are regions that the photons avoid, called dark fringes. Afshar now places a grid of thin wires just before the lens (Fig. 2). These wires are placed in previously measured positions of the dark fringes of an interference pattern which is produced by the dual pinhole setup when observed directly. If one of the pinholes is blocked, the interference pattern can no longer be formed, and some of the light will be blocked by the wires. Consequently, one would expect that the image quality is reduced, as is indeed observed by Afshar. Afshar then claims that he can check for the wave characteristics of the light in the same experiment, by the presence of the grid.

At this point, Afshar compares the results of what is seen at the photo-detectors when one pinhole is closed with what is seen at the photo-detectors when both pinholes are open. When one pinhole is closed, the grid of wires causes some diffraction in the light, and blocks a certain amount of light received by the corresponding photo-detector. When both pinholes were open, however, the effect of the wires is minimized, so that the results are comparable to the case in which there are no wires placed in front of the lens (Fig.3).

Afshar's conclusion is that the light exhibits a wave-like behavior when going through the wires, since the light goes through the spaces between the wires when both slits were open, but also exhibits a particle-like behavior after going through the lens, with photons going to a given photo-detector.

This behavior, Afshar argues, contradicts the principle of complementarity, since it shows both complementary wave and particle characteristics in the same experiment, for the same photons. Afshar asserts this experiment has also been conducted with single photons and the results are identical to the high flux experiment, although these results were not available at the time of the talk at Harvard.

Theory

In the context of the traditional double-slit experiment, Bohr's principle of complementarity can be mathematically formalized by using the Englert-Greenberger relation expressing a limitation on fringe visibility V and path distinguishability D. We have :

${\displaystyle D^{2}+V^{2}\leq 1\,}$

This complementarity relation was first proven by Jaeger, Shimony, and Vaidman (see G. Jaeger, A. Shimony, and L. Vaidman, Phys. Rev. A, Vol. 51, 54 (1995)), and over one year afterwards also by Englert (see B. Englert, Phys. Rev. Lett., Vol. 77, 2154 (1996)).

Afshar claims that his experiment violates this limitation (and hence the principle of complementarity), however it is unclear how the Englert-Greenberger relation extends to more complicated experimental setups such as Afshar's.

Critiques

Though Afshar's work is still the subject of ongoing interpretation and discussion, a significant portion of the scientific community is of the opinion that Afshar's experiment does not refute complementarity. The following is a partial list of critiques of Afshar's work. Afshar's rebuttals are available on his Q&A archive and FAQ .

• Bill Unruh, Professor of Physics at University of British Columbia, .

• Lubos Motl, Assistant Professor of Physics, Harvard University

• Ruth Kastner, Committee on the History and Philosophy of Science, University of Maryland, College Park, ,

• Aurelien Drezet, University of Graz Institut of experimental physics, Austria,

• Ole Steuernagel, School of Physics, Astronomy and Mathematics, University of Hertfordshire,

References

• Shahriar S. Afshar, "Sharp complementary wave and particle behaviours in the same welcher weg experiment", (2003) IRIMS www.irims.org/quant-ph/030503/; Proc. SPIE 5866 (2005) 229-244, ; AIP Cof. Proc. 810, (2006) 294-299. (Crossed beam experiment).

• John G. Cramer, "A Farewell to Copenhagen?" (2005), Analog Science Fiction and Fact. (A non-technical discussion in a popular forum)

• Marcus Chown, "Afshar's Quantum Bombshell", "Quantum rebel" (July 24, 2004) New Scientist magazine; "A great leap forward", (October 6, 2004) The Independent.
• Ira Flatow, "Einstein, Bohr and the Nature of Light", , (July 30, 2004) Science Friday radio prgram, NPR.

• Foundational quantum physics
• Quantum measurement
• Physics experiments