First observation of antimatter wave interference

Abstract: In 1924 Louis de Broglie introduced the concept of wave-particle duality: the Planck constant $h$ relates the momentum $p$ of a massive particle to its de Broglie wavelength $\lambda=h/p$. 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.

• The paper presents the first empirical demonstration of wave interference using positrons, confirming de Broglie's predictions for antimatter systems.
• Using a specialized Talbot-Lau interferometer and gold-coated silicon nitride gratings, the experiment produced interference patterns with a fringe periodicity of approximately 5.90 µm.
• This work extends quantum mechanics to antimatter, enabling future experiments and providing a method potentially useful for exploring gravitational effects on antimatter.

Observation of Antimatter Wave Interference: A Detailed Examination

In a compelling exploration of quantum mechanics, Ariga et al. present the first empirical demonstration of matter-wave interference using positrons, thus expanding the boundary of tested systems to include antimatter. This empirical work corroborates the theoretical predictions made nearly a century ago by de Broglie, who proposed the wave-particle duality of matter—a concept essential to the foundation of quantum mechanics. The experiment executed by the research team employs a Talbot-Lau interferometer that utilizes material diffraction gratings, specifically designed for use with low-intensity antimatter beams, thus enabling this unique investigation.

Experimental Methodology and Configuration

The experiment was conducted at the L-NESS laboratory, using a monochromatic positron beam derived from the beta decay of a sodium source. The positrons were accelerated to energies in the range of 8-14 keV using an electrostatic system, resulting in a beam with a remarkably low energy spread of less than 0.1%. This controlled setup facilitated the use of nuclear emulsions, offering submicron resolution, to detect positron impacts. Employing gold-coated silicon nitride gratings of varying periodicities, the researchers achieved a Talbot-Lau interference pattern, with fringe periodicity calculated to be about 5.90 ± 0.043 µm.

Key Observations

The contrast of the interference fringes was scrutinized over the energy spectrum from 8 to 14 keV. Observations confirmed that fringe contrast increased as expected from quantum-mechanical interpretations rather than classical projective effects, marking a significant confirmation of antimatter wave interference. Noteworthy results include the peak contrast values at several energies: 0.491 at 14 keV, 0.436 at 11 keV, 0.267 at 9 keV, and 0.144 at 8 keV. These values illustrate a clear decrease in contrast with decreasing energies, a trend consistent with theoretical predictions.

Theoretical and Practical Implications

This achievement extends the scope of wave-particle duality into the antimatter domain, offering a new avenue for quantum mechanical experiments with systems such as positronium and potentially antihydrogen in future studies. The use of Talbot-Lau interferometry, particularly attuned to the challenges posed by antimatter, positions this method as a potential tool for measuring gravitational effects on antimatter. Such applications are of profound interest in exploring gravitational interactions within quantum mechanics, potentially influencing models of fundamental physics.

Conclusion and Future Prospects

The findings exhibit that positrons behave as de Broglie waves, as anticipated by quantum mechanics, validating the interference phenomenon with antimatter. This work sets a precedent for further studies within the QUantum interferometry with Positrons and LASers (QUPLAS) program, particularly in studies involving antimatter and potentially gravitational antigravity experiments. The bridging of quantum mechanics and gravitational physics through experiments on systems like positronium and antihydrogen is within reach and could provide insights with significant implications for theoretical and applied physics. This research thus acts as a foundational step towards a deeper understanding of the behavior of antimatter under various quantum and gravitational conditions, fostering future advancements in the field of quantum interferometry.