Greater Control in the Quantum World: RPTU Researchers Demonstrate an Extraordinary Quantum State

A molecule that should, in principle, fall apart remains stable: researchers at RPTU University Kaiserslautern-Landau have experimentally demonstrated an extraordinary quantum state that was first predicted theoretically more than four decades ago. The findings open up new possibilities for controlling interactions and losses in quantum systems, a key challenge for future quantum technologies.

How can something remain intact when it should naturally break apart? This question has been addressed by researchers at RPTU. Their work has led to the experimental observation of an unusual quantum state in which two atoms remain bound together even though they possess enough energy to separate. The results have been published in the prestigious journal Nature Physics.

“What is remarkable is that the molecular state could decay from an energetic point of view, yet quantum interference suppresses this decay,” explains Professor Artur Widera, head of 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.”

In simple terms, particles such as atoms do not behave solely like tiny objects. They also possess wave-like properties. When such waves overlap, they can either reinforce or cancel one another, a phenomenon physicists refer to as interference. As the new study demonstrates, it is precisely this effect that prevents two atoms from separating.

Bound Despite Being Able to Decay

Under ordinary circumstances, energy is required to separate the atoms that make up a molecule. Below a certain energy threshold, the atoms remain bound together. Above that threshold, they can dissociate.

In the quantum world, however, the rules are not always so straightforward. The RPTU team has experimentally demonstrated a so-called bound state in the continuum. Physicists use this term to describe a state whose energy would ordinarily be sufficient for the constituent atoms to separate. Yet despite this possibility, the atoms remain bound.

The phenomenon has long attracted considerable interest in physics. As early as 1985, theorists predicted that such states could emerge under specific conditions. Related effects have since been observed in optical and acoustic systems. However, the new study now provides the first experimental demonstration that the underlying mechanism can be deliberately realized in collisions of ultracold atoms.

A Principle Also Used in Noise-Cancelling Headphones

The underlying principle is similar to that employed in active noise-cancelling headphones. These devices generate sound waves that selectively cancel unwanted noise.

A comparable process occurs in the RPTU experiment, albeit at the level of individual atoms. A molecule can, in principle, decay into separate atoms through different pathways. Each decay pathway is associated with its own quantum wave. When these quantum waves interfere in just the right way, they can cancel one another out.

Much like two equally strong forces acting in opposite directions, this interference effect blocks the decay process. As a result, the atoms remain bound together even though they possess sufficient energy to separate.

Building on Earlier Research

The experimental breakthrough was made possible by a method the team had previously introduced in Science Advances in 2025. In that work, the researchers demonstrated how rapidly oscillating magnetic fields can be used to precisely control interactions between atoms in an ultracold gas.

For the present study, the team cooled lithium atoms to less than one millionth of a degree above absolute zero. Under such extreme conditions, the quantum behaviour of matter can be observed and manipulated with exceptional precision.

The researchers then brought two molecular resonances into close proximity using carefully tuned magnetic fields. These resonances correspond to states in which atoms interact particularly strongly. This approach enabled the team to create exactly the conditions required to suppress molecular decay.

To verify the existence of the state, the researchers examined, among other observables, how many atoms were lost from the gas during the experiment. Without the protective interference effect, significantly more atoms would have disappeared from the system. In addition, they monitored the dynamics of the atomic cloud and used radio-frequency spectroscopy to probe the molecular states directly. Together, these measurements provided the characteristic experimental signature of the unusual quantum state.

“Our previous work provided us with a tool that allows us to control how atoms interact,” says first author Alexander 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.”

A Foundation for Future Applications

The study not only confirms a decades-old theoretical prediction. It also establishes a new approach for controlling how quantum systems interact with their environment.

The findings are highly relevant in the context of emerging quantum technologies. Quantum systems are regarded as one of the key enabling technologies of the future, with potential applications ranging from highly sensitive sensors for medicine and navigation to secure quantum communication and quantum computers capable of solving certain classes of complex problems far more efficiently than conventional computers.

The research was funded by the German Research Foundation (DFG) through Collaborative Research Centre/Transregio 185 and by the European Research Council (ERC) through an ERC Advanced Grant under the European Union’s Horizon Europe research and innovation programme.

The study:

Alexander Guthmann, Louisa Marie Kienesberger, Felix Lang, Eleonora Lippi and Artur Widera: Bound states in the continuum through quantum interference in ultracold atomic collisions. Nature Physics. https://www.nature.com/articles/s41567-026-03519-6

Scientific Contact:

Prof. Dr. Artur Widera

Individual Quantum Systems

Department of Physics and Research Center OPTIMAS

RPTU University Kaiserslautern-Landau

Email: widera[@]rptu.de

Press contact: 

Christine Pauli

University Communications

RPTU University Kaiserslautern-Landau

Email: presse[@]rptu.de