Quantum physics overturned classical ideas by showing that matter behaves very differently at the smallest scales.
One of its most surprising discoveries was wave-particle duality, the idea that particles can act like waves under certain conditions.
This behavior was famously demonstrated in the double-slit experiment, where electrons passing through two slits produced interference patterns instead of simple particle impacts.
Over time, similar wave-like behavior was confirmed for neutrons, helium atoms, and even large molecules, making matter-wave diffraction a cornerstone of quantum mechanics.
Despite these advances, one system had remained missing from the experimental record: positronium.
Positronium is a short-lived, neutral atom-like system made of an electron and its antimatter counterpart, a positron. Because it consists of two particles with equal mass, observing its quantum interference has long posed a challenge.
Now, researchers from Tokyo University of Science have achieved what had not been done before. Led by Professor Yasuyuki Nagashima, the team has directly observed matter-wave diffraction in a beam of positronium, providing a new confirmation of wave-particle duality in a unique quantum system.
Antimatter shows wave nature
“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 have the way for new research in fundamental physics using positronium,” said Prof. Nagashima.
The breakthrough was enabled by the creation of a high-quality positronium beam with both sufficient coherence and tunable energy.
The team first generated negatively charged positronium ions, then used a precisely timed laser pulse to strip away an extra electron. This produced a fast, neutral positronium beam suitable for interference experiments.
The researchers directed the beam toward a thin graphene target consisting of two to three atomic layers.
The spacing between graphene atoms closely matches the de Broglie wavelength of positronium at the energies used, making it an ideal diffraction medium.
As the positronium atoms passed through the graphene sheet, a portion of the beam was transmitted and detected using a position-sensitive detector. The resulting measurements revealed a clear diffraction pattern, confirming wave-like behavior.
Single quantum object confirmed
The team also demonstrated that positronium diffracts as a single quantum entity rather than as independent electron and positron components.
This result confirms that the bound system behaves as one coherent particle, despite its composite nature.
“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,” said Dr. Yugo Nagata.
Compared with earlier approaches, the new beam achieves higher energies of up to 3.3 keV, a narrower energy spread, and improved directional focus.
The experiment was also carried out in ultra-high vacuum, helping preserve the cleanliness of the graphene surface and ensuring reliable results.
Beyond validating quantum theory, positronium diffraction could have practical implications. Because positronium is electrically neutral, it may allow non-destructive analysis of sensitive materials such as insulators or magnetic surfaces.
In the longer term, it could enable precision experiments involving antimatter, including tests of gravity where direct measurements are still lacking.