Two Atoms That Refuse to Break Apart — Thanks to Quantum Noise-Cancelling
How can something stay intact when it should naturally break apart? That is the question researchers at RPTU Kaiserslautern-Landau in Germany have answered with an experiment that sounds almost impossible: two atoms that remain bound together even though they have more than enough energy to separate.
The results, published on 9 November 2026 in the journal Nature Physics, are the first experimental demonstration of a strange quantum state called a "bound state in the continuum". The idea was predicted by theorists as early as 1985, and similar effects have been seen in light and sound — but this is the first time the underlying mechanism has been deliberately created in collisions of ultracold atoms.
When enough energy is not enough
Normally, the rules are simple. Atoms in a molecule are held together by a bond, and it takes energy to pull them apart. Below a certain energy, they stay bound. Above that energy, they can break free and fly apart.
In the quantum world, however, the rules are not always so straightforward. The RPTU team, led by Professor Artur Widera and first author Alexander Guthmann, showed that a molecule can stay together even above that energy threshold. "What is remarkable is that the molecular state could decay from an energetic point of view, yet quantum interference suppresses this decay," Widera explains. "The wave nature of matter allows a state to remain bound even though we would normally expect it to decay."
The noise-cancelling trick
The secret is interference — the same principle behind noise-cancelling headphones. Atoms are not only tiny objects; they also behave like waves. When waves overlap, they can strengthen each other or cancel each other out. Noise-cancelling headphones create sound waves that cancel unwanted noise. In the RPTU experiment, something similar happens at the level of individual atoms.
A molecule can, in principle, break apart through different pathways, and each pathway has its own quantum wave. When these waves interfere in just the right way, they cancel each other out — like two equally strong forces pulling in opposite directions. The decay is blocked, and the atoms stay bound even though they have enough energy to separate.
Creating those exact conditions took extraordinary control. The team cooled lithium atoms to less than one millionth of a degree above absolute zero, where the quantum behaviour of matter can be observed with exceptional precision. Using rapidly oscillating magnetic fields — a method they had introduced in a 2025 paper — they brought two molecular resonances, states in which atoms interact particularly strongly, close together. That created precisely the conditions needed to suppress the decay.
To prove the state existed, they checked how many atoms were lost from the gas during the experiment. Without the protective interference, far more atoms would have disappeared. They also watched the dynamics of the atomic cloud and used radio-frequency spectroscopy to probe the molecular states directly. Together, these measurements gave the characteristic signature of the unusual state.
"Our previous work provided us with a tool that allows us to control how atoms interact," says Guthmann. "Here, we used that control to make two possible decay pathways of a molecule cancel each other out. It is a striking example of how quantum physics can transform an expected loss mechanism into a protective one."
Why it matters
Beyond confirming a decades-old prediction, the work gives physicists a new way to control how quantum systems interact with their surroundings — something that matters for emerging quantum technologies, from highly sensitive sensors for medicine and navigation to secure quantum communication and quantum computers.
For students, this is quantum physics at its most delightful: the same wave-cancelling idea that silences the hum on a plane can, at nearly absolute zero, hold two atoms together against all expectations. It is also a reminder that in science, an expected loss can sometimes become a protection.
If your child is curious about atoms, waves or how the coldest places on Earth reveal the strangest physics, a first lesson in physics can start with these two stubborn atoms. Book your first lesson
Sources:
- phys.org, "Quantum interference keeps two atoms bound despite enough energy to break apart", 9 November 2026: https://phys.org/news/2026-11-quantum-atoms-bound-energy.html
- Alexander Guthmann et al., "Bound states in the continuum through quantum interference in ultracold atomic collisions", Nature Physics (2026). DOI: 10.1038/s41567-026-03519-6
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