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Q: Fachverband Quantenoptik und Photonik

Q 16: Ultracold Matter III – Fermions (joint session Q/A)

Q 16.8: Vortrag

Dienstag, 3. März 2026, 12:45–13:00, P 2

Bound state in the continuum realized in ultracold gases — •Eleonora Lippi, Alexander Guthmann, Louisa Marie Kienesberger, Felix Lang, and Artur Widera — Department of Physics and Research Center OPTIMAS, RPTU University Kaiserslautern-Landau, 67663 Kaiserslautern, Germany

Quantum mechanical interaction potentials typically support either bound molecular states or unbound scattering states. An interesting exception are bound states in the continuum (BICs), localized quantum states with energies well above the molecular dissociation threshold that have no wavefunction overlap with free scattering states. In 1985, Friedrich and Wintgen (FW) proposed a mechanism to realise such BICs in quantum systems through the interference of two Feshbach resonances. Although BICs have largely been realised in classical systems, an unambiguous quantum-mechanical realisation based on the FW mechanism has so far remained elusive. In this talk, we present our experimental observations of the FW-BIC in an ultracold Li-6 atomic gas. This is achieved through the interference of two tunable Feshbach resonances induced by Floquet engineering, implemented via strong magnetic-field modulation that generates additional Feshbach scattering resonances with controllable positions and widths [1]. We support our observation by full coupled-channel calculations. Additionally, by an effective non-Hermitian Hamiltonian, we interpret the state as a dark-scattering state, in analogy to electromagnetically-induced transparency.

[1] Guthmann, A. et al., Sci. Adv.11, eadw3856 (2025)

Keywords: Fermi gas; Floquet engineering; Feshbach resonances; Bound states; BIC