A defining insight that sets quantum physics apart from classical physics is that matter behaves in unexpected ways at very small scales. One of the most important ideas to emerge was wave-particle duality, which shows that particles can also act like waves.
This behavior was first clearly demonstrated in the double-slit experiment. When electrons passed through two narrow openings, they formed a pattern of alternating light and dark bands on a detector. This pattern revealed that each electron acted like a wave, with its quantum wave function passing through both slits and interfering with itself.
Similar results were later observed with neutrons, helium atoms, and even large molecules, establishing matter-wave diffraction as a fundamental principle of quantum mechanics. However, this effect had not been directly observed in positronium, a short-lived system made of an electron and a positron bound together and orbiting a shared center.
Because both particles have equal mass, scientists have long aimed to understand how such a system would behave when diffracted.
Building on this challenge, researchers from Tokyo University of Science in Japan, led by Professor Yasuyuki Nagashima, along with Associate Professor Yugo Nagata and Dr. Riki Mikami, have now demonstrated matter-wave diffraction in positronium. The beam used in their experiment had the necessary coherence and energy variation to reveal interference effects. The results, published in Nature Communications, provide a new confirmation of wave-particle duality.
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