Afshar experiment

The Afshar experiment is an optics experiment, devised and carried out by Shahriar Afshar at Harvard University in 2004, which is a variation of the double slit experiment in quantum mechanics. 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.[1] Afshar claimed that the experiment violates the principle of complementarity of quantum mechanics,[2] 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.[3] The experiment has been repeated by a number of investigators and its results have been confirmed, but its interpretation is controversial, and some disagree that it violates complementarity, while also disagreeing amongst themselves as to why.[4][5][6][7][8]


Afshar's experiment uses a variant of Thomas Young's classic double-slit experiment to create interference patterns to investigate complementarity. Such interferometer experiments typically have two "arms" or paths a photon may take.[1] 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 observing each photon's path (a measurement of the particle nature of the photons).[1][9]


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.[10] The results were presented at a Harvard seminar in March 2004,[11] and published as conference proceeding by the International Society for Optical Engineering (SPIE).[1] The experiment was featured as the cover story in the July 24, 2004 edition of New Scientist.[10][12] 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.[13] Afshar presented his work also at the American Physical Society meeting in Los Angeles, in late March 2005.[14] His peer-reviewed paper was published in Foundations of Physics in January 2007.[3]

Afshar claims that his experiment invalidates the complementarity principle and has far-reaching implications for the understanding of quantum mechanics, challenging the Copenhagen interpretation. According to Cramer, Afshar's results support Cramer's own transactional interpretation of quantum mechanics and challenge the many-worlds interpretation of quantum mechanics.[15] This claim has not been published in a peer reviewed journal.

Experimental setup

Fig.1 Experiment without obstructing wire grid
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

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). A photon that goes through pinhole number one impinges only on detector number one, and similarly, if it goes through pinhole two it impinges only on detector number two, which is why we see the pinholes separately in the image plane close to the mirrors before the photon-detectors.

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 some of the light will be blocked by the wires. Consequently, the image quality is reduced.

When one pinhole is closed, the grid of wires causes appreciable diffraction in the light, and blocks a certain amount of light received 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, which the photons avoid. The effect is not dependent on the light intensity (photon flux).


Afshar's conclusion is that 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, when both slits were open, but also exhibits particle-like behavior after going through the lens, with photons going to a given photo-detector. Afshar argues that this behavior contradicts the principle of complementarity, since it shows both complementary wave and particle characteristics in the same experiment for the same photons.

Peer Reviews: Support and 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.

The most recent work claims that Afshar's core claim, that the Englert–Greenberger duality relation is violated, is contested. They 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."[6] Below is a synopsis of the papers by critics highlighting their main arguments, and the disagreements they have amongst themselves:

Some researchers claim that, while the fringe visibility is high, no which-way information ever exists:

Other researchers agree that the fringe visibility is high and that the which-way information is not simultaneously measured, but they believe that the which-way information does exist under some circumstances.

Another group does not question the which-way information, but rather contends that the measured fringe visibility is actually quite low:

Others question Afshar's interpretation and offer alternatives:

In the history of quantum mechanics, much has been written about the double-slit experiment, and much debate as to its interpretation has ensued. Indeed, to explain the interference patterns for subatomic particles, explanations have been given not only in terms of the principle of complementarity and wave-particle duality but also in terms of quantum consciousness and parallel universes. In this paper, the topic will be discussed from the perspective of spin-coupling in the hope of further clarification. We will also suggest that this explanation allows for a realist interpretation of the Afshar Experiment.

Specific support

There also is support for the Afshar interpretation from John Cramer:

See also


  1. 1 2 3 4 S. S. Afshar (2005). "Violation of the principle of complementarity, and its implications". Proceedings of SPIE. The Nature of Light: What Is a Photon?. 5866: 229–244. arXiv:quant-ph/0701027Freely accessible. Bibcode:2005SPIE.5866..229A. doi:10.1117/12.638774.
  2. J. Zheng; C. Zheng (2011). "Variant simulation system using quaternion structures". Journal of Modern Optics. 59 (5): 484. Bibcode:2012JMOp...59..484Z. doi:10.1080/09500340.2011.636152.
  3. 1 2 S. S. Afshar; E. Flores; K. F. McDonald; E. Knoesel (2007). "Paradox in wave-particle duality". Foundations of Physics. 37 (2): 295–305. arXiv:quant-ph/0702188Freely accessible. Bibcode:2007FoPh...37..295A. doi:10.1007/s10701-006-9102-8.
  4. 1 2 R. Kastner (2005). "Why the Afshar experiment does not refute complementarity?". Studies in History and Philosophy of Modern Physics. 36 (4): 649–658. doi:10.1016/j.shpsb.2005.04.006.
  5. 1 2 O. Steuernagel (2007). "Afshar's experiment does not show a violation of complementarity". Foundations of Physics. 37 (9): 1370. arXiv:quant-ph/0512123Freely accessible. Bibcode:2007FoPh...37.1370S. doi:10.1007/s10701-007-9153-5.
  6. 1 2 V. Jacques; et al. (2008). "Illustration of quantum complementarity using single photons interfering on a grating". New Journal of Physics. 10 (12): 123009. arXiv:0807.5079Freely accessible. Bibcode:2008NJPh...10l3009J. doi:10.1088/1367-2630/10/12/123009.
  7. D. D. Georgiev (2007). "Single photon experiments and quantum complementarity" (PDF). Progress in Physics. 2: 97–103.
  8. D. D. Georgiev (2012). "Quantum histories and quantum complementarity". ISRN Mathematical Physics. 2012: 327278. doi:10.5402/2012/327278.
  9. S. S. Afshar (2006). "Violation of Bohr's complementarity: One slit or both?". AIP Conference Proceedings. 810: 294–299. arXiv:quant-ph/0701039Freely accessible. Bibcode:2006AIPC..810..294A. doi:10.1063/1.2158731.
  10. 1 2 Chown, Marcus (2004). "Quantum rebel wins over doubters". New Scientist. 183 (2457): 30–35.(subscription required)
  11. S. S. Afshar (2004). "Waving Copenhagen Good-bye: Were the founders of Quantum Mechanics wrong?". Harvard seminar announcement. Retrieved 2013-12-01.
  12. Afshar's Quantum Bomshell Science Friday
  13. J. G. Cramer (2004). "Bohr is still wrong". New Scientist. 183 (2461): 26.
  14. S. S. Afshar (2005). "Experimental Evidence for Violation of Bohr's Principle of Complementarity". APS Meeting, March 21–25, Los Angeles, California: 33009. Bibcode:2005APS..MARP33009A.
  15. J. G. Cramer (2005). "A farewell to Copenhagen?". Analog Science Fiction and Fact.
  16. R. E. Kastner (2006). "The Afshar Experiment and Complementarity". APS Meeting, March 13–17, Baltimore, Maryland: 40011. Bibcode:2006APS..MARD40011K.
  17. W. Unruh (2004). "Shahriar Afshar – Quantum Rebel?".
  18. L. Motl (2004). "Violation of complementarity?".
  19. Aurelien Drezet (2005). "Complementarity and Afshar's experiment". arXiv:quant-ph/0508091Freely accessible.
  20. Aurelien Drezet (2011). "Wave particle duality and the Afshar experiment" (PDF). Progress in Physics. 1: 57–67. arXiv:1008.4261v1Freely accessible.

Further reading

External links

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