One of the defining breakthroughs that set quantum physics apart from classical physics was the realization that matter behaves very differently at extremely small scales. Among the most important discoveries was wave-particle duality, the idea that particles can also act like waves.
This concept became widely known through the double-slit experiment. When electrons were fired through two narrow openings, they produced a pattern of alternating light and dark bands on a detector. This pattern revealed that each electron behaved like a wave, with its quantum wave-function passing through both slits at once and interfering with itself. Scientists later confirmed this effect with neutrons, helium atoms, and even larger molecules, establishing matter-wave diffraction as a key principle of quantum mechanics. However, despite these advances, this phenomenon had not been directly observed in positronium. Positronium is a short-lived, two-body system made up of an electron and a positron bound together and orbiting a shared center of mass. Because both components have equal mass, researchers have long sought to understand how such a system would behave when forming a beam and undergoing diffraction.
First Observation of Positronium Wave Behavior
A research team from Tokyo University of Science, Japan, led by Professor Yasuyuki Nagashima and joined by Associate Professor Yugo Nagata and Dr. Riki Mikami, has now achieved that goal. They successfully demonstrated matter-wave diffraction in a beam of positronium. The beam used in their experiment had the necessary energy range and coherence to produce clear interference effects. Their results, published in Nature Communications, provide strong new evidence of wave-particle duality in an unusual system.
“Positronium is the simplest atom composed of equal-mass constituents, and until it self-annihilates, it behaves as a neutral atom in a vacuum. Now, for the first time, we have observed quantum interference of a positronium beam, which can pave the way for new research in fundamental physics using positronium,” says Prof. Nagashima.
Creating a High-Quality Positronium Beam
The breakthrough relied on producing a highly controlled positronium beam. To do this, the researchers first generated negatively charged positronium ions. They then used a precisely timed laser pulse to remove an extra electron, resulting in a fast-moving, neutral, and coherent stream of positronium atoms.
This beam was directed toward a sheet of graphene. The spacing between atoms in the graphene closely matched the de Broglie wavelength of the positronium at the energies used in the experiment. As the positronium atoms passed through the two-to-three-layer graphene sheet, some of them made it through and were detected. The resulting measurements revealed a distinct diffraction pattern, confirming wave-like behavior.
Clear Diffraction Patterns and Quantum Behavior
Compared with earlier techniques, this method produces positronium beams with higher energies, reaching up to 3.3 keV. It also provides a narrower spread of energies and a more tightly directed beam. Conducting the experiment in an ultra-high vacuum kept the graphene surface clean, allowing the diffraction pattern to be observed more clearly.
The results showed that even though positronium consists of two particles, it behaves as a single quantum object. The electron and positron do not diffract separately but instead act together as one wave.
“This groundbreaking experimental milestone marks a major advance in fundamental physics. It not only demonstrates positronium’s wave nature as a bound lepton-antilepton system (a system that behaves like a tiny atom) but also opens pathways for precision measurements involving positronium,” says Dr. Nagata.
The team also investigated whether positronium would produce interference in the same way as a single particle like an electron. Their findings confirmed that it does, reinforcing the idea that it functions as a unified quantum entity.
Future Applications in Materials Science and Antimatter Research
In addition to confirming its quantum properties, positronium diffraction could lead to practical applications. Because positronium carries no electric charge, it may be useful for analyzing material surfaces without causing damage. This makes it especially valuable for studying insulators or magnetic materials that can interfere with charged particle beams.
Looking ahead, experiments involving positronium interference could also make it possible to test how antimatter responds to gravity. This remains an open question, as direct measurements have not yet been achieved, even for electrons.
About Professor Yasuyuki Nagashima from Tokyo University of Science
Dr. Yasuyuki Nagashima is a Professor in the Department of Physics at Tokyo University of Science, Japan, specializing in positron and positronium physics. His research focuses on the properties of negative ions of positronium and the positronium beam. He also studies positron annihilation-induced ion desorption from solid surfaces. In 2020, he received the Hiroshi Takuma Memorial Prize from the Matsuo Foundation. His laboratory conducts fundamental research on exotic particle-matter interactions while developing new positron-based experimental techniques for applied physics.
About Associate Professor Yugo Nagata from Tokyo University of Science
Dr. Yugo Nagata is an Associate Professor in the Department of Physics at Tokyo University of Science, Japan, specializing in positronium and atomic physics. In 2023, he received the Young Scientist Award of the Japanese Positron Science Society.
This work was supported by JSPS KAKENHI (Grants Nos. JP25H00620, JP21H04457, and JP17H01074).