Researchers from the Rhineland-Palatinate Technical University (RPTU) in Germany have for the first time experimentally observed an unusual quantum state: two atoms remain bound into a molecule although they have enough energy to fly apart. The results were published in the journal Nature Physics.
"It is surprising that the molecular state could decay from an energy point of view, but quantum interference suppresses this decay," explains the head of the group for individual quantum systems, Professor Artur Widera. "The wave nature of matter allows the state to remain bound, although we would usually expect it to decay."
Interference is a phenomenon in which waves, overlapping, can reinforce or cancel each other. Atoms and other particles behave not only as tiny balls but also as waves. It is precisely the mutual cancellation of waves, as the new work showed, that prevents the two atoms from separating.
Usually, breaking a molecule requires energy. Below a certain threshold the atoms stay together; above it they can fly apart. However, in the quantum world the rules are not always so simple. The researchers obtained a so-called bound state in the continuum - a state whose energy is in principle sufficient to separate the atoms, but they nevertheless remain bound.
Theorists predicted the possibility of such a state back in 1985, and similar effects were later observed in optical and acoustic systems. The new work shows for the first time that the mechanism can be deliberately realized in collisions of ultracold atoms.
The operating principle resembles active noise-cancelling headphones: they create sound waves that cancel unwanted noise. In the RPTU experiment something similar happens at the level of individual atoms. The molecule could decay along different paths, and each path corresponds to its own quantum wave. With the right superposition these waves cancel each other, and the decay is blocked - like two equal forces pulling in opposite directions.
The breakthrough became possible thanks to a method that the team presented earlier, in 2025 in the journal Science Advances: rapidly oscillating magnetic fields allow precise control of the interaction of atoms in an ultracold gas. In the new work, lithium atoms were cooled to a temperature of less than one millionth of a degree above absolute zero. Then, using carefully tuned magnetic fields, the researchers brought two molecular resonances closer together - states in which the atoms interact especially strongly - and created conditions for suppressing the decay.
To confirm the existence of the state, the scientists, among other things, monitored how many atoms are lost from the gas. Without the protective effect of interference, the losses would have been significantly greater. In addition, they observed the dynamics of the atomic cloud and directly studied the molecular states by radio-frequency spectroscopy. Together these measurements gave a characteristic experimental signature of the unusual quantum state.
"Our previous work gave us a tool that allows us to control the interaction of atoms," says first author Alexander Gutman. "Here we used this control to make the two possible decay paths of the molecule cancel each other. This is a vivid example of how quantum physics turns an expected loss mechanism into a protective one."
The work not only confirms a theoretical prediction made forty years ago but also opens a new way to control the interaction of quantum systems with their environment. This could be useful for future quantum technologies - from ultrasensitive sensors for medicine and navigation to secure quantum communication and quantum computers. However, for now this is a fundamental experiment, not ready-made devices.