Giant Atoms Host Robust Doublon Bound States in the Continuum Via Two-Photon Emission

The quest to control light and matter at the nanoscale has led scientists to investigate bound states in the continuum, unusual states of light that remain trapped even when surrounded by freely propagating waves. Walter Rieck, Anton Frisk Kockum, and Guangze Chen, all from Chalmers University of Technology, now demonstrate a pathway to creating particularly robust versions of these states, known as doublon bound states, within specially engineered structures called giant atoms. This research reveals that these doublon states, formed through the interaction of light and matter, arise from carefully orchestrated interference effects and offer a new mechanism for localising energy in open systems. The discovery promises exciting possibilities for developing advanced technologies, including quantum simulators, exploring unusual physical dynamics, and creating secure methods for processing information.

Doublon Bound States in Giant Atoms

Scientists investigate bound states in the continuum (BICs), spatially localized quantum states embedded within a spectrum of extended states, focusing on many-body systems. This work demonstrates that giant atoms, quantum emitters coupled to structured waveguides, can host robust doublon BICs, two-photon bound states stabilized by the interplay of atomic interactions and waveguide geometry. These states emerge from the strong coupling of two excitons within the giant atom, forming a composite state detached from the continuum. The resulting system combines quantum emitter properties with waveguide-mediated interactions, enhancing light-matter coupling and offering potential applications in quantum photonics. This research establishes a new platform for exploring many-body BICs and opens avenues for manipulating quantum states of light and matter in integrated photonic circuits.

Giant Atoms and Waveguide Bound States

Researchers engineered giant atoms, quantum emitters coupled to a structured waveguide at multiple points, to investigate many-body bound states in the continuum. The study pioneered a method for realizing two-photon bound states, termed doublon BICs, within the waveguide’s radiation continuum, leveraging destructive interference and interactions. Scientists modeled each giant atom as a three-level system and constructed a system where the atoms interact with a one-dimensional array of coupled cavities. The team developed a model incorporating photon hopping and on-site nonlinearity to describe the waveguide’s properties, revealing that when nonlinearity exceeds a critical value, the doublon band becomes energetically separated from the continuum, enabling isolation of doublon-mediated dynamics. Experiments demonstrate that these doublon BICs can mediate decoherence-free interaction between two giant atoms, reinforcing their physical significance and opening avenues for generating and distributing many-body entangled states.

Doublon Bound States Enable Coherent Atom Interactions

Scientists have demonstrated the existence of robust doublon bound states in the continuum (BICs) within giant atom systems, representing a significant advance in understanding many-body localization. These doublon BICs are two-photon bound states stabilized by destructive interference and interactions, existing even within the continuous spectrum of extended states in a structured waveguide. The research reveals that these states arise from the interplay between atomic interactions and interference effects, effectively suppressing decay into the waveguide and creating localized energy concentrations. Experiments show that these doublon BICs mediate decoherence-free interaction between distant giant atoms, enabling coherent interactions without radiative losses. Further investigation revealed that these BICs also appear under natural, undriven dynamics within three-level giant atoms, confirming their stability and versatility. The team’s analysis of the system’s band structure, coupled with the observation of these localized doublon BICs, establishes a clear mechanism for stabilizing many-body localization in open quantum systems, delivering a pathway for generating and distributing many-body entangled states with potential applications in quantum simulation and protected quantum information processing.

Doublon Bound States Enable Giant Atom Interactions

This research demonstrates the existence of robust, localized two-photon states, termed doublon bound states in the continuum (BICs), within giant atoms, artificial atoms coupled to structured waveguides. Scientists established that these doublon BICs arise from destructive interference between different emission pathways, a unique characteristic of giant atom systems, and can mediate interactions between distant atoms without energy loss. Importantly, the team found that these states emerge even when direct two-photon emission is restricted, instead relying on virtual two-photon transitions, broadening the potential for experimental realization. These findings establish giant atoms as a versatile platform for creating non-ergodic many-body states in open quantum systems, highlighting the power of interference in engineering complex quantum phenomena. The non-radiative nature of these doublon BICs suggests potential applications in preparing entangled states and building robust quantum gates, even in noisy environments, and extending beyond single-excitation regimes to prepare and distribute complex many-body states.