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Physics Today

Multiparticle Interferometry and the Superposition Principle

Daniel M. Greenberger, Michael Horne, Anton Zeilinger

Published Aug 1, 1993• 377 citations
Publisher

Abstract

Discussing the particle analog of Thomas Young's classic double-slit experiment, Richard Feynman wrote in 1964 that it “has in it the heart of quantum mechanics. In reality, it contains the only mystery.” That mystery is the one-particle superposition principle. But Feynman's discussion and statement have to be generalized. Superposition may be the only true quantum mystery, but in multiparticle systems the principle yields phenomena that are much richer and more interesting than anything that can be seen in one-particle systems.

DOI
10.1063/1.881360

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Discussing the particle analog of Thomas Young's classic double-slit experiment, Richard Feynman wrote in 1964 that it “has in it the heart of quantum mechanics. In reality, it contains the only mystery.” That mystery is the one-particle superposition principle. But Feynman's discussion and statement have to be generalized. Superposition may be the only true quantum mystery, but in multiparticle systems the principle yields phenomena that are much richer and more interesting than anything that can be seen in one-particle systems.
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The double slit interference experiment has been famously described by Richard Feynman as containing the "only mystery of quantum mechanics". The history of quantum mechanics is intimately linked with the discovery of the dual nature of matter and radiation. While the double slit experiment for light is easily undertsood in terms of its wave nature, the very same experiment for particles like the electron is somewhat more difficult to comprehend. By the 1920s it was firmly established that electrons have a wave nature. However, for a very long time, most discussions pertaining to interference experiments for particles were merely gedanken experiments. It took almost six decades after the establishment of its wave nature to carry out a 'double slit interference' experiment for electrons. This set the stage for interference experiments with larger particles. In the last decade there has been spectacular progress in matter-wave interefernce experiments. Today, molecules with over a hundred atoms can be made to interfere. In the following we discuss some of these exciting developments which probe new regimes of Nature, bringing us closer to the heart of quantum mechanics and its hidden mysteries.
Young’s double slit experiment was called as "a phenomenon […] has in it the heart of quantum mechanics. In reality, it contains the only mystery. We cannot make the mystery go away by ‘explaining’ how it works". To study the mystery, we perform the non-crossing-slit-experiments and crossing-slit experiments and show phenomena: (1) in the same double slit or cross double slit experiments, the non-interference patterns gradually evolve to the interference patterns; (2) in the same single slit or cross single slit experiments, the non-diffraction patterns gradually evolve to the diffraction patterns; (3) in the same two-non-parallel-slits experiment, the non-wave patterns gradually evolve to the hybrid pattern (i.e., interference-pattern-embedded-in-diffraction-pattern); (4) in the same curved-double slit experiments, the non-interference patterns evolve to Seashell-shape interference patterns. The above phenomena show new mysteries that are more mysterious than that of the double slit experiments: it is challenge to consistently explain how they work. Those novel phenomena indicate that the standard theories of the double slit are incomplete. Novel experiments bring the search for the new theory of physical/quantum optics.
Complementarity is one of the central mysteries of quantum mechanics, dramatically illustrated by the wave-particle duality in Young's double-slit experiment, and famously regarded by Feynman as "impossible, absolutely impossible to describe classically, [and] which has in it the heart of quantum mechanics" (emphasis original).1 The overarching goal of this thesis is to demonstrate that complementarity is also at the heart of quantum information theory, that it allows us to make (some) sense of just what information "quantum information" refers to, and that it is useful in understanding and constructing quantum information processing protocols.
Young’s double slit experiment was called as "a phenomenon […] has in it the heart of quantum mechanics. In reality, it contains the only mystery. We cannot make the mystery go away by ‘explaining’ how it works". To study the mystery, we perform the non-crossing-slit-experiments and crossing-slit experiments and show phenomena: (1) in the same double slit or cross double slit experiments, the non-interference patterns gradually evolve to the interference patterns; (2) in the same single slit or cross single slit experiments, the non-diffraction patterns gradually evolve to the diffraction patterns; (3) in the same two-non-parallel-slits experiment, the non-wave patterns gradually evolve to the hybrid pattern (i.e., interference-pattern-embedded-in-diffraction-pattern); (4) in the same curved-double slit experiments, the non-interference patterns evolve to Seashell-shape interference patterns. The above phenomena show new mysteries that are more mysterious than that of the double slit experiments: it is challenge to consistently explain how they work. Those novel phenomena indicate that the standard theories of the double slit are incomplete. Novel experiments bring the search for the new theory of physical/quantum optics.