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.
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.
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