# RPTU physicists stabilize a bound state of ultracold 6Li atoms above the dissociation threshold

- Published: 2026-11-10 15:52 UTC
- Section: Science › Physics
- Tags: bound state in the continuum, Friedrich–Wintgen state, Feshbach resonances, ultracold lithium-6, Floquet engineering, quantum interference, non-Hermitian Hamiltonian, dissociation threshold, RPTU Kaiserslautern-Landau, Artur Widera
- URL: https://realtech.media/news/rt-0105b2b8acbeeccc2d8940a5_rptu_physicists_stabilize/

**Floquet-engineered interference between two Feshbach resonances decoupled a molecular state from the continuum in ultracold lithium-6 collisions, the authors report.**

![RPTU physicists stabilize a bound state of ultracold 6Li atoms above the dissociation threshold](https://realtech.media/media/2026/11/14bec5a075c7a414.webp)
*scinexx*

Researchers at the University of Kaiserslautern-Landau (RPTU, Germany), led by Artur Widera, have experimentally demonstrated a bound state in the continuum in collisions of ultracold lithium-6 atoms, according to a paper on nature.com [1]. The molecular state lies above the dissociation threshold, yet it is decoupled from the scattering states and does not decay into free atoms [1]. The atomic gas was cooled to below 600 nanokelvin.

## Interference instead of an energy barrier

In a potential well, states below the dissociation threshold are bound, while states above it are delocalized scattering states. Bound states in the continuum are an exception: they stay localized even though their energy lies within the continuum [1]. In 1985 Friedrich and Wintgen proposed that such a state can arise from interference between two Feshbach resonances coupled to a common continuum [1]. A state of this kind cannot be populated from scattering states, so all resonant scattering signatures vanish [1].

To realize this, the team coherently coupled two tunable Feshbach resonances using Floquet engineering, with a rapidly oscillating magnetic field. The modulation frequency serves as a continuous tuning parameter for the detuning between the resonances [1]. At a critical parameter point, elastic and inelastic coupling to the continuum vanish [1].

## Three ways to identify the state

The team combined three measurements: atom losses in collisions, the motion of the atom cloud after a sudden change of the trap, and radio-wave spectroscopy of molecular states. In a narrow range of magnetic-field parameters, the loss signal typical of a scattering resonance disappeared. The expected collective oscillation of the cloud was also absent, which points to suppression of elastic coupling as well.

As a result, resonant scattering features disappear and the colliding atoms become effectively transparent to one another [1]. Full coupled-channel calculations and a minimal non-Hermitian model quantitatively reproduce the observations and identify the state as a Friedrich–Wintgen bound state in the continuum [1].

Bound states in the continuum have so far been realized mainly in classical systems: photonic crystals, photonic waveguides and acoustic systems [1]. The authors state that the result shows how quantum interference can control the openness of quantum matter and enable the engineering of non-Hermitian Hamiltonians [1]. "The wave nature of matter allows a state to remain bound although we would normally expect it to decay," Widera said.

## Sources

1. [nature.com](https://www.nature.com/articles/s41567-026-03519-6)
2. [scinexx](https://www.scinexx.de/news/physik/quanteninterferenz-haelt-zwei-atome-trotz-ausreichender-zerfallsenergie-gebunden)

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Source: RealTech, realtech.media — https://realtech.media/news/rt-0105b2b8acbeeccc2d8940a5_rptu_physicists_stabilize/