Two Atoms Stay Bound With Energy to Spare, Physicists Show

Two atoms in a Kaiserslautern laboratory are staying bound together even though they hold enough energy to fly apart. [Quantum interference keeps two atoms bound despite enough energy to break apart — Phys.org, reporting the Nature Physics paper (November 9)]

The RPTU team, led by Artur Widera with first author Alexander Guthmann, reports the result in Nature Physics. It's the first experimental demonstration of a bound state in the continuum deliberately realized in collisions of ultracold atoms. [Quantum interference keeps two atoms bound despite enough energy to break apart — Phys.org, reporting the Nature Physics paper (November 9)]

The mechanism is interference, the same principle noise-canceling headphones use. A molecule can decay into separate atoms through more than one pathway, and each pathway carries its own quantum wave. Tune the system so the waves cancel, and the decay has nowhere to go: the molecule survives with energy that should have ended it. "The wave nature of matter allows a state to remain bound even though we would normally expect it to decay," Widera said. To build it, the team cooled lithium atoms to less than a millionth of a degree above absolute zero and used rapidly oscillating magnetic fields, a control tool they'd published in 2025, to bring two molecular resonances together until the pathways overlapped. [Quantum interference keeps two atoms bound despite enough energy to break apart — Phys.org, reporting the Nature Physics paper (November 9)]

Verification came from counting: without the protective interference, far more atoms vanish from the gas; the team also tracked the cloud's dynamics and probed the molecular states with radio-frequency spectroscopy. Theorists predicted such states in 1985, and versions have been seen in optical and acoustic systems, but not before in ultracold atomic collisions. "We used that control to make two possible decay pathways of a molecule cancel each other out," Guthmann said. The point isn't the curiosity alone. A state that refuses to decay on cue is a handle on how quantum systems interact with their surroundings, the control problem underneath quantum sensors and quantum computing. [Quantum interference keeps two atoms bound despite enough energy to break apart — Phys.org, reporting the Nature Physics paper (November 9)]