Afshar experiment

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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.^[1] Criticisms and alternate interpretations have appeared online in blogs, at physics colloquia and academic conferences, and in arXiv e-print archives.

Contents 1 Overview 2 History 3 Experimental setup and Afshar's interpretation 4 Ongoing debate 5 Specific critiques 6 References and notes 7 See also

[edit] 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 can not 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 wave like 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 re-enforce each other or cancel each other out. However when this is performed on with 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 is highly problematic but a 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.^[2] 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).^[2][3][4]

[edit] 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 being a research scholar there. The results were presented at a Harvard seminar in March 2004,^[5] 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.^[6][7] 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.^[8] Afshar presented his work also at the American Physical Society meeting in Los Angeles, in late March of 2005.^[9]

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.^[10]

[edit] 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, 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.

[edit] Ongoing debate

The Afshar experiment has been the subject of heated interpretation and discussion.

It should be noted that Bohr's principle did not forbid one entertaining a violation. Indeed both Bohr and Einstein did just that. The principle is based on the assumption that experimental setups, and their results, allow one to escape concluding a violation, or as Bohr said "escape concluding a paradox":

• "This point is of great logical consequence, since it is only the circumstance that we are presented with a choice of either tracing the path of a particle or observing interference effects, which allows us to escape from the paradoxical necessity of concluding that the behaviour of an electron or a photon should depend on the presence of a slit in the diaphragm through which it could be proved not to pass." - Niels Bohr^[11]

-Afshar's experiment does not yield which way information and demonstrate interference effects for any individual particle (the photon), any more than the classic double slit experiment does, since we already know the photon propagates according to a wave-equation between the slits and any screen (i.e. behaves like a wave until it hits the screen, whereupon is observed as a particle.)

-A "photon propagating according to the wave-equation" is not the same thing as an "interference effect". And the observation of a particle is not the same thing as "tracing the path of a particle".

• "To conclude, in spite of Afshar's claim we still need two experiments in order to exploit the totality of the phenomenon. As pointed out originally by Bohr, we can not use information associated with a same photon event to rebuild in a statistical way (i.e. by an accumulation of such events) the two complementary distributions of photons in the image plane and in the interference plane. The hypothesis of Afshar that we only need some partial information concerning the interference pattern in order to reconstruct the complete interference is only based on the idea that the fringes already exist. The whole reasoning is circular and for this reason misleading." - Aurelien Drezet^[12]

-The claim of complementarity violation in Afshar's experiment is a statistical argument that applies only to large numbers photons, not to individuals (cf "the particle" above in Bohr's statement is a reference to a single photon, not to groups of photons).

-Bohr's "interference effect" is, in his own words, a statistical effect (see below). An interference effect is not the same thing as the wave function. One should not (normally) attempt to reconstruct a wave function from an ensemble of photon detections. Such reconstructions will invariably be psuedo-wave functions - especially in non-solid state experiments. But in the Afshar experiment it is actually impossible to reconstruct a pseudo-wave function from Afshar's particle detections.

• "As a more appropriate way of expression, I advocated the application of the word phenomenon exclusively to refer to the observations obtained under specified circumstances, including an account of the whole experimental arrangement. In such terminology, the observational problem is free of any special intricacy since, in actual experiments, all observations are expressed by unambiguous statements referring, for instance, to the registration of the point at which an electron arrives at a photographic plate. Moreover, speaking in such a way is just suited to emphasise that the appropriate physical interpretation of the symbolic quantum-mechanical formalism amounts only to predictions, of determinate or statistical character, pertaining to individual phenomena appearing under conditions defined by classical physical concepts." - Neils Bohr^[11]

• "an adequate tool for a complementary way of description is offered precisely by the quantum-mechanical formalism"- Neils Bohr^[11]

-If the photons in the experiment obey the precise mathematical laws of quantum mechanics (the formalism), how can Bohr's principle of complementarity be violated by the experiment? ^[13][14][15]

-Bohr's principle of complementarity is both an interpretation of the formalism and more specifically an interpretation of the conditions which otherwise determine the presence/absence of interference effects on the one hand and the tracing of particle paths (or not) on the other. If the formalism is found to remain intact, but the principle not, it just means they are not conflatable.

• "Thus, a sentence like "we cannot know both the momentum and the position of an atomic object" raises at once questions as to the physical reality of two such attributes of the object, which can be answered only by referring to the conditions for the unambiguous use of space-time concepts, on the one hand, and dynamical conservation laws, on the other hand. While the combination of these concepts into a single picture of a causal chain of events is the essence of classical mechanics, room for regularities beyond the grasp of such a description is just afforded by the circumstance that the study of the complementary phenomena demands mutually exclusive experimental arrangements." - Neils Bohr.^[11]

[edit] Specific critiques

A number of scientists, while agreeing that there is which-way information without the grid in Afshar's experiment, have argued that putting in the wire grid leads to erasure of this information.^[16]

• Bill Unruh, Professor of Physics at University of British Columbia proposed an equivalent experiment to Afshar's setup and argued that complementarity principle is not violated.^[13]
• Luboš Motl, Assistant Professor of Physics, Harvard University.^[14]
• Ruth Kastner, Committee on the History and Philosophy of Science, University of Maryland, College Park. Provides exposition of Afshar setup within the light of the transactional interpretation of quantum mechanics^[17][18]
• Aurelien Drezet, University of Graz Institut of experimental physics, Austria,^[12]
• Ole Steuernagel, School of Physics, Astronomy and Mathematics, University of Hertfordshire, UK.^[15]

[edit] References and notes

[edit] See also

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