In the 17th century, there existed two theories of light, namely, the wave theory of Huygens and the corpuscular theory of Newton. At the beginning of 19th century Thomas Young demonstrated the wave character of light using diffraction experiments. James Clerk Maxwell developed his electromagnetic wave equation and showed that light is an electromagnetic wave. This chapter considers a Mach–Zehnder interferometer experiment, an equivalent experiment of the double slit experiment. To test whether light priorily get the information about the experimental set up in the double slit experiment John Archibald Wheeler proposed a gedanken delayed-choice experiment with the Mach–Zehnder interferometer. This thought experiment was then realized experimentally using various systems. An experiment was proposed to observe simultaneously both particle and wave character of the quantum through quantum entanglement. In quantum mechanics the relative temporal order of measurement of events is not relevant.
In the 17th century, there existed two theories of light, namely, the wave theory of Huygens and the corpuscular theory of Newton. At the beginning of 19th century Thomas Young demonstrated the wave character of light using diffraction experiments. James Clerk Maxwell developed his electromagnetic wave equation and showed that light is an electromagnetic wave. This chapter considers a Mach–Zehnder interferometer experiment, an equivalent experiment of the double slit experiment. To test whether light priorily get the information about the experimental set up in the double slit experiment John Archibald Wheeler proposed a gedanken delayed-choice experiment with the Mach–Zehnder interferometer. This thought experiment was then realized experimentally using various systems. An experiment was proposed to observe simultaneously both particle and wave character of the quantum through quantum entanglement. In quantum mechanics the relative temporal order of measurement of events is not relevant.
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.
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.
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
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.
Quantum theory is a mathematical formalism that allows physicists to predict the probability of observing different outcomes in experiments on sub-atomic systems. It has been tested more rigorously than any theory in science, and has never been wrong. However, strictly speaking it does not “explain” the behavior of sub-atomic systems, since it does not propose a mechanism to account for it. It tells us that they will behave in certain ways, but not why . Thus, quantum mechanics is not a theory in the sense familiar to social scientists, namely a body of laws that explains some part of reality. The explanatory question is the subject of the interpretive debates that surround the theory, not of the theory per se. That said, the literature routinely refers to quantum mechanics as a theory, and I shall use the terms interchangeably below. A common way to introduce quantum theory is through some of the key experiments that have confirmed its predictions. In this chapter I describe three of the most well-known: the Two-Slit Experiment, the Bell Experiments, and the Delayed-Choice Experiment. While painting the same overall picture, each reveals distinct features of the quantum realm. While their interpretation is contested, here I will do my best to present just the theory's findings, saving interpretive questions for later. The Two-Slit Experiment This experiment actually had its origins long before the quantum revolution. One of the most controversial questions in classical physics was whether light was made of particles or waves. In keeping with his atomistic worldview, Newton favored the particle or “corpuscular” theory of light, which was the majority view among physicists through the eighteenth century. However, in 1801 Thomas Young performed an experiment that seemed to prove conclusively that the wave theory was correct. In his experiment, Young set up a light source behind an opaque screen with one small opening or slit, through which the light passed in a concentrated beam.