Turning Off System-Continuum Coupling: Engineering a Bound State in the Continuum

Open quantum systems inevitably couple to their environment, leading to dissipation and loss that limit control and coherence. In this talk, I will show how destructive quantum interference can be used to suppress this coupling and realize a long-sought bound state in the continuum in an ultracold atomic gas. Using Floquet engineering, we coherently couple two tunable Feshbach resonances and tune them through an avoided crossing. At a critical parameter point, interference eliminates all coupling to the scattering continuum, yielding a molecular state above the dissociation threshold. We identify this state through atom-loss spectroscopy, quench-induced collective dynamics, and rf photoassociation, which directly probes the scattering wavefunction. Our observations are supported by coupled-channel calculations and a minimal non-Hermitian model, establishing interference as a powerful tool for controlling openness and engineering dissipation in quantum matter.