Engineering a Bound State in the Continuum via Quantum Interference

TL;DR

This paper demonstrates the creation of a bound state in the continuum (BIC) in ultracold lithium atom collisions by using Floquet engineering to induce quantum interference, effectively decoupling the state from the continuum.

Contribution

It provides the first experimental realization of a Feshbach resonance-based BIC in a genuine quantum system through coherent control techniques.

Findings

Observation of a BIC in ultracold ${}^6$Li collisions.

Decoupling of the molecular state from scattering states at a critical point.

Quantitative agreement with coupled-channel calculations and a non-Hermitian model.

Abstract

Quantum mechanical interaction potentials typically support either localized bound states below the dissociation threshold or delocalized scattering states above it. While bound states are energetically isolated, scattering states embed a quantum system in a continuum of environmental modes, making dissipation and loss intrisic features of open quantum systems. A striking exception are bound states in the continuum (BICs), which remain localized despite lying within the scattering continuum due to destructive interference. It was predicted that such states can arise from the interference of two Feshbach resonances coupled to a common continuum, yet this mechanism has remained experimentally inaccessible in genuine quantum systems. Here we demonstrate the formation of such an interference-stabilized state in ultracold collisions of ${}^6$Li atoms by coherently coupling two tunable Feshbach resonances using Floquet engineering. At a critical parameter point, both elastic and inelastic coupling to the continuum vanish, yielding a molecular state above the dissociation threshold. Loss spectroscopy, quench dynamics, and rf-photoassociation directly reveal the resulting decoupling from scattering states. Our observations are quantitatively captured by full coupled-channel calculations and a minimal non-Hermitian model, identifying a Friedrich-Wintgen BIC. Our results establish quantum interference as a powerful mechanism for controlling openness in quantum matter and for engineering non-Hermitian Hamiltonians.