On the bound states in the continuum

Xiaojun Zhang, Mingjie 明杰 Zhu 朱, Zhihai Wang

Abstract We investigate phase-controlled bound states in a one-dimensional photonic waveguide coupled to an artificial giant atom at two distant sites. Specifically, we identify the bound state out of the continuum (BOC) and the bound state in the continuum (BIC) and derive the exact existence condition for the BOC. Furthermore, we analytically determine the BIC’s frequency and photonic distribution profile. Remarkably, our analysis reveals quantum beats in both atomic and photonic dynamics, arising from coherent oscillations between the BIC and BOC. These results establish a novel approach for manipulating waveguide quantum electrodynamics via engineered bound states, with potential applications in quantum information processing.

Charles G. Wohl

Kai Chen, Junyan Guan, Zhongming Gu +1 more

Repulsively bound doublons are two-particle composites formed by strong interactions and are usually separated from the scattering continuum. Introducing spin-orbit coupling fundamentally alters the underlying band structure, providing a powerful tuning knob to shift these isolated pairs toward this continuum. However, because entering such a regime typically dictates immediate dissociation, whether this coupling can drive these pairs inside while preserving their bound nature constitutes a fundamental unresolved challenge. Here we show that spin-orbit coupling in the one-dimensional Fermi-Hubbard model can drive doublons into the two-particle scattering continuum. Most of these states hybridize with extended channels and decay, whereas a subset remains decoupled and spatially bound, forming many-body bound states in the continuum (BICs). We map the interacting two-particle problem onto a two-dimensional lattice of coupled acoustic cavities, and experimentally observe both the radiating doublon continuum and the confined BIC states. These results demonstrate that spin-orbit coupling can turn selected doublons into interaction-induced BICs, deepening the understanding of continuum physics for interaction-bound pairs.

В. В. Колосов

Boning Huang, Yongguan Ke, Honghua Zhong +2 more

Bound states in the continuum (BICs) are localized modes residing in the radiation continuum. They were first predicted for single-particle states, and became a general feature of many wave systems. In many-body quantum physics, it is still unclear what would be a close analog of BICs, and whether interparticle interaction may induce BICs. Here, we predict a novel type of multiparticle states in the interaction-modulated Bose-Hubbard model that can be associated with the BIC concept. Under periodic boundary conditions, a so-called quasi-BIC appears as a bound pair residing in a standing wave formed by the third particle. Under open boundary conditions, such a hybrid state becomes an eigenstate of the system. We demonstrate that the Thouless pumping of the quasi-BICs can be realized by modulating the onsite interactions in space and time. Surprisingly, while the center of mass of the quasi-BIC is shifted by a unit cell in one cycle, the bound pair moves in the opposite direction with the standing wave.