Ultracold 6Li collisions realize Friedrich-Wintgen bound state in continuum

A team has realized a Friedrich-Wintgen bound state in the continuum in an ultracold gas of 6Li atoms using Floquet engineering of Feshbach resonances. The experiment formed this interference-stabilized state in ultracold collisions by coherently coupling two tunable Feshbach resonances. The bound state was created above the dissociation threshold, counterintuitively stabilizing a state that would normally decay. Alexander Guthmann, Louisa Marie Kienesberger, and colleagues conducted the research, which used loss spectroscopy at cloud temperatures below 600 nanokelvin. The observed state exists at specific Floquet modulation frequencies of 55.95 MHz and 56.74 MHz.

Theoretical foundation and model validation

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. Theory predicts such states can arise through interference between two Feshbach resonances coupled to a common continuum. This situation is effectively described by a non-Hermitian Hamiltonian where a real term describes two bare levels with energies E1 and E2 coupled coherently. The eigenvalues of this model are generally complex, with their real and imaginary parts corresponding to the resonance positions and widths, respectively. The full calculations validated the two-level model, with extracted resonance positions and widths aligning with experimental data.

Control via Floquet modulation and resonance tuning

Strong magnetic-field modulation enables additional Feshbach resonances with tunable positions and widths. In the approach for the Friedrich-Wintgen BIC, the modulation frequency provides an additional tunable control parameter, enabling the realization of a BIC at experimentally accessible collision energies. Coefficients γ1,2 quantify the coupling to the entrance channel continuum, whereas γ'1,2 represent additional coupling to the first Floquet-dressed continuum. An avoided crossing emerges as the detuning between energy levels changes, a critical feature in establishing the BIC.

Interference mechanism and BIC conditions

Destructive interference between two Feshbach resonances localizes the state and prevents decay. Bound states in the continuum are an exception, as this destructive interference keeps them localized within the scattering continuum. At a critical parameter point, elastic and inelastic coupling to the continuum vanish, producing a molecular state above the dissociation threshold. Consequently, resonant scattering features disappear and the colliding atoms become effectively transparent to one another. The state with the minimal width corresponds to a coherent superposition of the two resonances, interfering such that the coupling to the continuum vanishes. This is closely analogous to the dark state in electromagnetically induced transparency. As the detuning δ = E1-E2 changes sign, an avoided crossing forms. Along this avoided crossing, the total resonance width remains constant. A special point is reached when the BIC condition is fulfilled, at which one eigenstate acquires maximal width and the other, minimal width.

Experimental verification and implications

The bound state was created above the dissociation threshold using loss spectroscopy. The numerically calculated inelastic scattering cross section was significantly reduced above the threshold, confirming its stability. Experimental verification used rf photoassociation spectroscopy to probe kinetic energy above the dissociation threshold. The technique enables control over 'openness' in quantum matter, allowing isolation from environmental interaction. The bound state cannot be populated from scattering states due to its decoupling from the continuum. This establishes a novel class of states bound due to quantum interference in addition to those bound by energy gaps or topological invariance. The research demonstrates a novel method for controlling quantum system-environment coupling, relevant to quantum technologies. The work was published in Quantum Zeitgeist, with scientific data available via Zenodo for transparency. The breakthrough hinged on Floquet modulation frequencies of 55.95 MHz and 56.74 MHz.