Afshar experiment

The Afshar experiment is an optical experiment which may challenge the principle of complementarity in quantum mechanics, although there is as yet no consensus on this in physics. The result of the experiment, which was first devised and carried out by Shahriar Afshar in 2001, appears to be in accordance with the standard predictions of quantum mechanics; however, it is claimed to violate the Englert-Greenberger duality relation.

The interpretation and significance of the experiment have engendered some controversy in the physics community. Afshar has published descriptions of the experiment in the American Institute of Physics and SPIE conference proceedings, and a peer-reviewed article appeared in Foundations of Physics. Criticisms and alternate interpretations have appeared online in blogs, at physics colloquia and academic conferences, and in arXiv e-print archives.

Overview

The principle of complementarity states that two complementary physical observables cannot both be measured for any given quantum particle without one measurement disturbing the other. An example of two complementary physical observables in quantum mechanics is the observation of the wave and particle nature of light simultaneously. The application of complementarity in this case states that we cannot observe and measure the purely wave and particle-like behavior of a single photon (the particle of light) at the same time. The most basic way of showing the wavelike nature of light is to create interference patterns between two sources of coherent light (coherence meaning that the two sources of light have a fixed phase relationship). When this is done, an interference pattern is created as the peaks and troughs of the two waves reinforce each other or cancel each other out. However, when this is performed on a stream of photons (each photon thus seemingly a particle), the surprising result is that the interference pattern remains. This raises the question of which way the photon went (i.e. which hole it passed through). This highly problematic solution for how to think about light was formulated in the Copenhagen interpretation of quantum mechanics. In essence, this interpretation states that if it is known which way the photon goes, it is impossible to demonstrate interference. The demonstration of this is to block one of the holes, at which point the interference pattern is replaced by an apparently clear path that the photon must have taken. Some interpretations of the Afshar experiment claim that it disproves the Copenhagen interpretation, while others claim that the results are perfectly consistent with it.

Afshar's experiment uses a variant of the classic Thomas Young double-slit experiment. Such interferometer experiments typically have two "arms" or paths a photon may take. 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).

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. The results were presented at a Harvard seminar in March 2004, and published as conference proceeding by the International Society for Optical Engineering (SPIE). The experiment was featured as the cover story in the July 24, 2004 edition of New Scientist. 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 Cramer's response. Afshar presented his work also at the American Physical Society meeting in Los Angeles, in late March 2005.

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 John G. Cramer, Afshar's results support Cramer's own transactional interpretation of quantum mechanics and challenges the many-worlds interpretation of quantum mechanics.

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 photon-detector (Fig. 1). In this setup, Afshar argues that a photon that goes through pinhole number one impinges only on detector number one, and similarly, if it goes through pinhole two. Therefore according to Afshar, 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 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.

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. 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.

Specific critiques

A number of scientists have published criticisms of Afshar's interpretation of his results. While united in their rejection of Afshar, in many cases they explicitly disagree among themselves as to why he is wrong.

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

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

Kastner's criticism, published in a peer-reviewed paper, proceeds by setting up a gedanken experiment and applying Afshar's logic to it to expose its flaw. She proposes that Afshar's experiment is equivalent to preparing an electron in a spin-up state and then measuring its sideways spin. This does not imply that one has found out the up-down spin state and the sideways spin state of any electron simultaneously. Applied to Afshar's experiment: "Nevertheless, even with the grid removed, since the photon is prepared in a superposition S, the measurement at the final screen at t₂ never really is a 'which-way' measurement (the term traditionally attached to the slit-basis observable $mathcal\; O$), because it cannot tell us 'which slit the photon actually went through.' In addition she underscores her conclusion with an analysis of the Afshar setup within the framework of the transactional interpretation of quantum mechanics.A follow-up e-print by Kastner [arXiv:0801.4757v2 [quant-ph]: "On Visibility in the Afshar Experiment"] argues that the commonly referenced inverse relationship between visibility parameter V and which-way parameter K does not apply to the Afshar setup, which post-selects for "which slit" after allowing interference to take place.

• Daniel Reitzner (Research Center for Quantum Information, Institute of Physics, Slovak Academy of Sciences, Bratislava, Slovakia), Comment on Afshar’s expriments, ArXiv:quant-ph/0701152 (2007).

Reitzner performed numerical simulations, published in a preprint, of Afshar's arrangement and obtained the same results that Afshar obtained experimentally. From this he argues that the photons exhibit wave behavior, including high fringe visibility but no which-way information, up to the point they hit the detector: "In other words the two-peaked distribution is an interference pattern and the photon behaves as a wave and exhibits no particle properties until it hits the plate. As a result a which-way information can never be obtained in this way."

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.

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

Unruh, who has published his objections on the web pages of his university, is probably the most prominent critic of Afshar's interpretation. He, like Kastner, proceeds by setting up an arrangement that he feels is equivalent but simpler. The size of the effect is larger so that it is easier to see the flaw in the logic. In Unruh's view that flaw is, in the case that an obstacle exists at the position of the dark fringes, "drawing the inference that IF the particle was detected in detector 1, THEN it must have come from path 1. Similarly, IF it were detected in detector 2, then it came from path 2." In other words, he accepts the existence of an interference pattern but rejects the existence of which-way information when Afshar puts in the wire grid.

• Tabish Qureshi (Physics Dept., Jamia Millia Islamia, New Delhi), Complementarity and the Afshar experiment, ArXiv:quant-ph/0701109 (2007).

Qureshi, in his preprint, considers like Kastner a spin system as a simplified but equivalent experiment. Like Kastner he also argues that there is no which-way information, but for a different reason. While Kastner says there is no determinate which-way information with or without the grid as long as both slits are open, Qureshi argues that there is such information when the photon passes the slits, but it is erased when the two parts of the wave overlap, irrespective of whether or not the interference pattern is actually detected.

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

• Luboš Motl, Former Assistant Professor of Physics, Harvard University.

Motl's criticism, published in his blog, is based on an analysis of Afshar's actual setup, instead of proposing an equivalent experiment like Unruh and Kastner. In contrast to Unruh and Kastner, he believes that which-way information always exists, but argues that the measured contrast of the interference pattern is actually very low: "Because this signal (disruption) from the second, middle picture is small (equivalently, it only affects a very small portion of the photons), the contrast V is also very small, and goes to zero for infinitely thin wires." He also argues that the experiment can be understood with classical electrodynamics and has "nothing to do with quantum mechanics".

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

Drezet's preprint argues that the classical concept of a "path" leads to much confusion in this context, but "The real problem in Afshar’s interpretation comes from the fact that the interference pattern is not actually completely recorded." The argument is similar to that of Motl, that the observed visibility of the fringes is actually very small. Another way he looks at the situation is that the photons used to measure the fringes are not the same photons that are used to measure the path. The experimental setup he analyzes is only a "slightly modified version" of the one used by Afshar.

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

In his preprint, Steuernagel makes a quantitative analysis of the various transmitted, refracted, and reflected modes in a setup that differs only slightly from Afshar's. He concludes that the Englert-Greenberger duality relation is strictly satisfied, and in particular that the fringe visibility for thin wires is small. Like some of the other critics, he emphasizes that inferring an interference pattern is not the same as measuring one: "Finally, the greatest weakness in the analysis given by Afshar is the inference that an interference pattern must be present."

Specific support for Afshar interpretation

There also is support for the Afshar interpretation:

• http://www.analogsf.com/0410/altview2.shtml (by John G. Cramer [Analog, [October 2004])
• http://www.physics.ohio-state.edu/~perry/p632/articles/Boskone_0402.ppt#1 (Powerpoint by J. Cramer [2004])
• http://arxiv.org/abs/quant-ph/0608202 (by Paul O'Hara [2006/09])
• http://arxiv.org/abs/quant-ph/0702210 (by Eduardo Flores and Ernst Knoesel [2007/02])
• http://arxiv.org/abs/0706.3966 (by R. Mir et al. [2007/06])

See also

• Wheeler's delayed choice experiment
• Delayed choice quantum eraser

References and notes