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Symmetry

Antimatter Quantum Interferometry

Marco G. Giammarchi

Published Oct 5, 2019• 1 citation
Publisher

Abstract

The wave–particle duality hypothesis for massive particles has been confirmed by an overwhelming variety of indirect experimental evidence. In addition, direct interferometric tests have been made on particles like electrons, neutrons and even a few molecules, explicitly showing wave-like diffraction and interference phenomena. Of particular interest in this direction, single particle interference has also been demonstrated, but only for the electron case. No such kind of direct information was available for antiparticles or antimatter in general. After briefly discussing the subjects of antimatter research and interferometry, I present here the first evidence of single particle antimatter interference, made with positrons.

DOI
10.3390/sym11101247

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The wave–particle duality hypothesis for massive particles has been confirmed by an overwhelming variety of indirect experimental evidence. In addition, direct interferometric tests have been made on particles like electrons, neutrons and even a few molecules, explicitly showing wave-like diffraction and interference phenomena. Of particular interest in this direction, single particle interference has also been demonstrated, but only for the electron case. No such kind of direct information was available for antiparticles or antimatter in general. After briefly discussing the subjects of antimatter research and interferometry, I present here the first evidence of single particle antimatter interference, made with positrons.
In 1924 Louis de Broglie introduced the concept of wave-particle duality: the Planck constant hhh relates the momentum ppp of a massive particle to its de Broglie wavelength λ=h/pλ=h/pλ=h/p. The superposition principle is one of the main postulates of quantum mechanics; diffraction and interference phenomena are therefore predicted and have been observed on objects of increasing complexity, from electrons to neutrons and molecules. Beyond the early electron diffraction experiments, the demonstration of single-electron double-slit-like interference was a highly sought-after result. Initially proposed by Richard Feynman as a thought experiment it was finally carried out in 1976. A few years later, positron diffraction was first observed. However, an analog of the double-slit experiment has not been performed to date on any system containing antimatter. Here we present the first observation of matter wave interference of single positrons, by using a period-magnifying Talbot-Lau interferometer based on material diffraction gratings. Individual positrons in the 8-14 keV energy range from a monochromatic beam were detected by high-resolution nuclear emulsions. The observed energy dependence of fringe contrast proves the quantum-mechanical origin of the detected periodic pattern and excludes classical projective effects. Talbot-Lau interferometers are well-suited to the experimental challenges posed by low intensity antimatter beams and represent a promising option for measuring the gravitational acceleration of neutral antimatter.
Abstract Interference of more and more massive objects provides a spectacular confirmation of quantum theory. It is usually regarded as support for “wave–particle duality” and in an extension of this duality even as support for “complementarity”. We first give an outline of the historical development of these notions. Already here it becomes evident that they are hard to define rigorously, i.e. have mainly a heuristic function. Then we discuss recent interference experiments of large and complex molecules which seem to support this heuristic function of “duality”. However, we show that in these experiments the diffraction of a delocalized center-of-mass wave function depends on the interaction of the localized structure of the molecule with the diffraction element. Thus, the molecules display “dual features” at the same time, which contradicts the usual understanding of wave–particle duality. We conclude that the notion of “wave–particle duality” deserves no place in modern quantum physics.
Diffraction of matter waves is a fundamental consequence of quantum mechanics, directly illustrating the core principles of wave-particle duality, quantum superposition, and quantum interference. De Broglie's proposal that particles exhibit wave-like properties has been experimentally confirmed for electrons, neutrons, and composite systems such as helium atoms, molecules and clusters. Here, we report the observation of positronium diffraction using a high-quality, energy-tunable positronium beam transmitted through graphene. Time-of-flight selection and spatially resolved detection reveal a distinct 1st-order diffraction peak at a position consistent with the prediction from matter-wave considerations for positronium. This work provides the direct and definitive evidence of quantum interference in positronium beams and confirms that it behaves as a single quantum entity rather than two independent interfering particles. This groundbreaking experimental milestone marks a major advance in fundamental physics, not only demonstrating positronium's wave nature as a bound lepton-antilepton system but also opening pathways for precision measurements involving positronium.
Matter-wave interference has been observed for a wide range of objects: from electrons to neutrons and complex molecules. On the other hand, a double-slit-like experiment has never been carried out with positrons (the anti-particle of the electron) or with any other system containing antimatter. This thesis provides the theoretical foundations and experimental results of a Talbot-Lau interferometer specifically designed for this goal. Future applications include the measurement of the gravitational acceleration of positronium, to provide a test of the weak equivalence principle on a matter-antimatter symmetric, purely leptonic system.
Neutron interferometry provides a powerful tool to investigate particle and wave features in quantum physics. Single particle interference phenomena can be observed with neutrons and the entanglement of degrees of freedom, i.e., contextuality can be verified and used in further experiments. Entanglement of two photons, or atoms, is analogous to a double slit diffraction of a single photon, neutron or atom. Neutrons are proper tools for testing quantum mechanics because they are massive, they couple to electromagnetic fields due to their magnetic moment, they are subject to all basic interactions, and they are sensitive to topological effects, as well. The 4π-symmetry of spinor wave functions, the spin-superposition law and many topological phenomena can be made visible, thus showing interesting intrinsic features of quantum physics. Related experiments will be discussed. Deterministic and stochastic partial absorption experiments can be described by Bell-type inequalities. Neutron interferometry experiments based on post-selection methods renewed the discussion about quantum non-locality and the quantum measuring process. It has been shown that interference phenomena can be revived even when the overall interference pattern has lost its contrast. This indicates a persisting coupling in phase space even in cases of spatially separated Schrödinger cat-like situations. These states are extremely fragile and sensitive against any kind of fluctuations and other decoherence processes. More complete quantum experiments also show that a complete retrieval of quantum states behind an interaction volume becomes impossible in principle, but where and when a collapse of the wave-field occurs depends on the level of experiment.