Two Atoms Stay Bound Despite Enough Energy to Break Apart, Quantum Study Shows

Physicists at the University of Kaiserslautern-Landau (RPTU) in Germany have observed two atoms staying bound together even though they have enough energy to fly apart. The result, published in Nature Physics (DOI 10.1038/s41567-026-03519-6), is the first experimental evidence that the underlying mechanism can be deliberately created in collisions of ultracold atoms.

"What is remarkable is that the molecular state could decay from an energetic point of view, yet quantum interference suppresses this decay," said Artur Widera, a professor who heads the Individual Quantum Systems group at RPTU. "The wave nature of matter allows a state to remain bound even though we would normally expect it to decay."

Under ordinary conditions, a molecule's atoms stay together below a certain energy threshold and can separate above it. Physicists call a state that sits above that threshold yet stays bound a bound state in the continuum. Theorists predicted such states could appear under specific conditions as early as 1985, and related effects have been seen in optical and acoustic systems. Until now, nobody had demonstrated the mechanism in ultracold atoms.

The key is interference. Particles such as atoms behave not only like tiny objects but also like waves, and when waves overlap they can reinforce or cancel each other. A molecule can in principle fall apart through several different pathways, and each pathway has its own quantum wave. When those waves line up so they cancel, the decay is blocked. The team compares the principle to active noise-canceling headphones, which generate sound waves that cancel unwanted noise. In the experiment the cancellation works at the level of individual atoms.

The experiment built on a method the group introduced in Science Advances in 2025, in which rapidly oscillating magnetic fields are used to precisely control the interactions between atoms in an ultracold gas. For the new study, the team, led by Widera and Alexander Guthmann, cooled lithium atoms to less than one millionth of a degree above absolute zero. At those temperatures the quantum behavior of matter can be observed and manipulated directly.

By varying the frequency of the oscillating field, the researchers tuned the interactions until the interference conditions were met and the molecular state became stable against decay. The paper tracks how the state evolves as that modulation frequency changes.

The finding matters mainly as a demonstration of control. It shows that physicists can use time-varying magnetic fields to engineer unusual quantum states in atomic gases, making states that were once only theoretical experimentally accessible. Atom-based setups like this are widely used as clean, adjustable platforms for studying quantum matter, and the researchers say the work extends that toolkit.