Quantum physics overturned classical ideas by showing that matter behaves very differently at the smallest scales.<\/p>\n
One of its most surprising discoveries was wave-particle duality, the idea that particles can act like waves under certain conditions.<\/p>\n
This behavior was famously demonstrated in the double-slit experiment, where electrons passing through two slits produced interference patterns instead of simple particle impacts.<\/p>\n
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.<\/p>\n
Despite these advances, one system had remained missing from the experimental record: positronium.<\/p>\n
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.<\/p>\n
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.<\/p>\n
Antimatter shows wave nature<\/p>\n
\u201cPositronium 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,\u201d said Prof. Nagashima.<\/p>\n
The breakthrough was enabled by the creation of a high-quality positronium beam with both sufficient coherence and tunable energy.<\/p>\n
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.<\/p>\n
The researchers directed the beam toward a thin graphene target consisting of two to three atomic layers.<\/p>\n
The spacing between graphene atoms closely matches the de Broglie wavelength of positronium at the energies used, making it an ideal diffraction medium.<\/p>\n
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<\/a> pattern, confirming wave-like behavior.<\/p>\n Single quantum object confirmed<\/p>\n The team also demonstrated that positronium diffracts as a single quantum entity rather than as independent electron and positron components.<\/p>\n This result confirms that the bound system behaves as one coherent particle, despite its composite nature.<\/p>\n \u201cThis groundbreaking experimental milestone marks a major advance in fundamental physics. It not only demonstrates positronium\u2019s 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<\/a>,\u201d said Dr. Yugo Nagata.<\/p>\n Compared with earlier approaches, the new beam achieves higher energies of up to 3.3 keV, a narrower energy spread, and improved directional focus.<\/p>\n The experiment was also carried out in ultra-high vacuum, helping preserve the cleanliness of the graphene surface and ensuring reliable results.<\/p>\n 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.<\/p>\n