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Wheeler's Delayed Choice Experiment

Allyson L. O’Brien

Published Jan 1, 2010• 4 citations

Abstract

Wave-particle duality of light has dumbfounded physical scientists for centuries. Sir Isaac Newton contended that light was made up of individual particles while Hooke (who was utterly despised by Newton) wrote on the wave-like properties of light.[6] In 1801, Thomas Young’s double slit experiment demonstrated wavelike properties of light. In 1905, Einstein revisited the theory of light acting as a particle to resolve conflicts between the wave theory of light and certain experimental results such as the photoelectric effect.[11] Experiments confirming particle-like properties of light were not realized until later in the 20th century. In 1981, Grangier, Roger and Aspect published a paper titled Experimental Evidence for a Photon Anticorrelation Effect on a Beam Splitter: A New Light on Single-Photon Interferences. This paper reported on one of the first successful anticoincidence experiments which measured the particle-like behavior of light. The paper also reported on an experiment that used the same source and detection scheme as the anticoincidence experiment, which measured interference. Grangier, Roger and Aspect eloquently depicted the conundrum of the waveparticle duality of light. To what extent does this this complementary nature of light exist? How does light know when to display wave-like or particle-like properties? One popular answer to this was that light can ‘sense’ what the experiment is attempting to measure. Based on the initial ‘feel,’ light would decide whether or not it will display wave-like or particle-like behavior before entering the experiment. This hypothesis is aptly named the conspiracy theory.[2]

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Wave-particle duality of light has dumbfounded physical scientists for centuries. Sir Isaac Newton contended that light was made up of individual particles while Hooke (who was utterly despised by Newton) wrote on the wave-like properties of light.[6] In 1801, Thomas Young’s double slit experiment demonstrated wavelike properties of light. In 1905, Einstein revisited the theory of light acting as a particle to resolve conflicts between the wave theory of light and certain experimental results such as the photoelectric effect.[11] Experiments confirming particle-like properties of light were not realized until later in the 20th century. In 1981, Grangier, Roger and Aspect published a paper titled Experimental Evidence for a Photon Anticorrelation Effect on a Beam Splitter: A New Light on Single-Photon Interferences. This paper reported on one of the first successful anticoincidence experiments which measured the particle-like behavior of light. The paper also reported on an experiment that used the same source and detection scheme as the anticoincidence experiment, which measured interference. Grangier, Roger and Aspect eloquently depicted the conundrum of the waveparticle duality of light. To what extent does this this complementary nature of light exist? How does light know when to display wave-like or particle-like properties? One popular answer to this was that light can ‘sense’ what the experiment is attempting to measure. Based on the initial ‘feel,’ light would decide whether or not it will display wave-like or particle-like behavior before entering the experiment. This hypothesis is aptly named the conspiracy theory.[2]

Delaying Quantum Choice Photons can display wavelike or particle-like behavior, depending on the experimental technique used to measure them. Understanding this duality lies at the heart of quantum mechanics. In two reports, Peruzzo et al. (p. 634 ) and Kaiser et al. (p. 637 ; see the Perspective on both papers by Lloyd ) perform an entangled version of John Wheeler's delayed-choice gedanken experiment, in which the choice of detection can be changed after a photon passes through a double-slit to avoid the measurement process affecting the state of the photon. The original proposal allowed the wave and particle nature of light to be interchanged after the light had entered the interferometer. By contrast in this study, entanglement allowed the wave and particle nature to be interchanged after the light was detected and revealed the quantum nature of the photon, for example, it displays wave- and particle-like behavior simultaneously.

The double-slit experiment in quantum mechanics is an experiment devised by physicist Thomas Young. It shows that light has both a wave nature or characteristic and a particle nature or characteristic, and that these natures are inseparable. So light is said to have wave–particle duality rather than be only a wave or only a particle. The same is true of electrons and other quantum particles. Although Christiaan Huygens thought that light was a wave, Isaac Newton did not. Newton felt that there were other explanations for color, and for the interference and diffraction effects that were observable at the time. Owing to Newton’s tremendous stature, his view generally prevailed. The fact that Huygens’s principle worked was not considered evidence that was direct enough to prove that light is a wave. The acceptance of the wave character of light came many years later when, in 1801, the English physicist and physician Thomas Young (1773–1829) did his now-classic double slit experiment

Wheeler’s gedanken experiment is the most fascinating and subtle illustration of wave-particle duality [1]. If we consider a single-photon light pulse in a Mach-Zehnder interferometer, one can either measure the interference at the output – evidencing the “wave-like” behavior of the photon- or one can detect the path followed by the photon – evidencing its “particle-like” behavior. In order to pinpoint how surprising this is, John Wheeler considered a “delayed-choice” regime where the choice of measuring either property is done only after the photon has entered the interferometer.

Science, as it is generally described at school, starts with experimental data. Scientists then have to find some hypothesis which explains these results. Using this hypothesis, a prediction is made about what might happen in a slightly different experiment. The double-slit experiment is a modified version of the setup first used by Thomas Young in 1801. Young’s version is a classic experiment that, at the time, seemed to have settled the dispute between wave and particle views of light decisively in favour of waves. The idea that particles might display some wavelike properties dates to the mid-1920s when a young PhD student called Louis De Broglie was thinking about the dual wave/particle nature of light. A quantum mechanical description of the double-slit experiment must involve a set of amplitudes governing the passage of an electron through the slits to the far detector.

The wave-particle duality of light has been hanging over the clouds of science as an insuperable mystery. Debates have gone on for centuries as to which of the two aspects of light best represents the natural property of light. On the one hand, the particle theory of light camp made its own submissions a long time ago with many experimental demonstrations to confirm its position. On the other hand, the wave theory of light camp did organize or face some experiments to corroborate its own position. But then, a middle ground interpretation was shoveled-in by Niels Bohr during the second decade of the 20th Century. For him, the two aspects of light are complementary and Louis de Broglie popularized it, despite the halting beginnings of the complementarity Thesis. A double-slit experiment was, however, organized to show that light has these dual aspects. The central thesis of this paper is that though light exhibits these dual aspects, it is fundamentally a wave. The paper adopts the historiographical approach in navigating this lingering issue of the nature of light in the history of science.

The evolution of light theories began with Isaac Newton's corpuscular model, which explained reflection and refraction but could not account for diffraction and interference. In contrast, Christiaan Huygens proposed a wave theory, explaining light's behavior through an ether-based medium, supported by his principle that each point in a wavefront acts as a secondary source. This idea was experimentally supported in the early nineteenth century when Thomas Young's double-slit experiment revealed an interference pattern, affirming light's wave nature. Later, James Clerk Maxwell unified electricity and magnetism, establishing light as an electromagnetic wave and extending the electromagnetic spectrum beyond visible light. In the twentieth century, Einstein's photoelectric effect introduced the concept of wave-particle duality, demonstrating that light behaves as discrete photons. Soon after, Louis de Broglie extended the idea of wave-particle duality to matter, a prediction confirmed in 1927 when Clinton Davisson and Lester Germer observed electron diffraction from a crystal and, independently, G.P. Thomson demonstrated electron diffraction through thin films, both proving that electrons also exhibit wave-like properties. This concept was dramatically visualized by Claus Jonsson's 1961 electron double-slit experiment. Recently, the ALICE collaboration observed quantum interference patterns at the femtometer scale in ultra-relativistic nuclear collisions, pushing quantum interference studies to new frontiers.