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

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 nature.comnature.comnature.comOpen source ↗. The molecular state lies above the dissociation threshold, yet it is decoupled from the scattering states and does not decay into free atoms. 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. In 1985 Friedrich and Wintgen proposed that such a state can arise from interference between two Feshbach resonances coupled to a common continuum. A state of this kind cannot be populated from scattering states, so all resonant scattering signatures vanish.

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. At a critical parameter point, elastic and inelastic coupling to the continuum vanish.

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 nature.comnature.comnature.comOpen source ↗. 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.

Bound states in the continuum have so far been realized mainly in classical systems: photonic crystals, photonic waveguides and acoustic systems. 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. "The wave nature of matter allows a state to remain bound although we would normally expect it to decay," Widera said.