Photonic state engineering in waveguide QED is typically based on local light-matter interactions. This limits its control over the spatial structure of bound photonic states. Here, we demonstrate a distinct mechanism arising from the interplay between nonlocal giant-atom coupling and topological band structure. Specifically, we consider giant atoms coupled to a Su-Schrieffer-Heeger waveguide and show that this configuration enables a controllable energy-level crossing protected by the topological gap. Adiabatically sweeping the atomic detuning across the crossing leads to a controlled exchange between distinct photonic bound states. In a two-giant-atom configuration, this mechanism achieves high-fidelity conversion of a spatially splitting state into a combining state. Extending this scheme to three-giant atoms, we further realize robust, shape-preserving photon transfer mediated by sequential in-gap crossings. Our results demonstrate how topology and nonlocal light-matter coupling can be combined to achieve programmable control of bound photonic states in waveguide QED platforms.
Photonic state engineering in waveguide QED is typically based on local light-matter interactions. This limits its control over the spatial structure of bound photonic states. Here, we demonstrate a distinct mechanism arising from the interplay between nonlocal giant-atom coupling and topological band structure. Specifically, we consider giant atoms coupled to a Su-Schrieffer-Heeger waveguide and show that this configuration enables a controllable energy-level crossing protected by the topological gap. Adiabatically sweeping the atomic detuning across the crossing leads to a controlled exchange between distinct photonic bound states. In a two-giant-atom configuration, this mechanism achieves high-fidelity conversion of a spatially splitting state into a combining state. Extending this scheme to three-giant atoms, we further realize robust, shape-preserving photon transfer mediated by sequential in-gap crossings. Our results demonstrate how topology and nonlocal light-matter coupling can be combined to achieve programmable control of bound photonic states in waveguide QED platforms.
The giant-atom topological-waveguide-QED systems have recently emerged as a promising platform for manipulating light-matter interactions. The combination of the multiple-point couplings and topological phase effect could lead to rich physical phenomena and effects. Here, we study single-photon scattering in a Su-Schrieffer-Heeger (SSH) waveguide coupled to either one or two two-level giant atoms. We assume that each giant atom is coupled to the waveguide via two coupling points and hence there exist four and sixteen coupling configurations for the single-giant-atom case and two-giant-atom separate coupling case, respectively. By solving the single-photon scattering problem in the real space, we obtain the exact expressions of the single-photon scattering amplitudes. It is found that a single photon in the SSH waveguide can be completely reflected or transmitted by choosing proper coupling configurations, coupling-point distances, atomic resonance frequency, and dimerization parameter. In addition, under proper parameter conditions, the scattering spectra are periodically modulated by the coupling-point distances. We also find that the collective behavior of the two giant atoms can be adjusted by quantum interference effect and topological effect and that the single-photon scattering spectra can exhibit the Lorentzian, super-Gaussian, electromagnetically induced transparencylike, and asymmetric Fano line shapes for some coupling configurations. This work will inspire the development of controllable single-photon devices based on the giant-atom topological-waveguide-QED systems.
We study the properties of bound states in waveguide-QED systems consisting of multiple giant atoms coupled to a coupled-resonator waveguide. Based on the general analytical expressions for these states and the corresponding energy spectra, we analyze in detail the threshold conditions for the appearance of bound states and the photon-mediated interactions between dressed atoms for different configurations. In addition, when multiple giant atoms are coupled to the waveguide, different types of interacting atomic chain can be obtained by manipulating the coupling configurations. Accordingly, the energy spectra of the bound states form metaband structures in the photonic band gaps. This makes the system a useful platform for quantum simulation and quantum information processing.
Giant atoms, which couple to a waveguide through multiple spatially separated connection points beyond the dipole approximation, provide a versatile route for quantum information processing based on interference-induced bound states in the continuum (BICs). While multi-giant-atom architectures are being developed toward giant-atom quantum networks, the role of direct coupling between the giant atoms, in particular the associated coupling phase, in atomic dynamics remains insufficiently understood. Here we take a first step toward addressing this issue by studying a two-giant-atom waveguide-QED model. We show that the coupling phase can be used to control both the number of BICs and their profiles for both of photon and atoms. More interestingly, the presence of BICs gives rise to a variety of dynamical behaviors, providing an effective mechanism for tailoring quantum-state evolution in giant-atom waveguide-QED systems. Our results highlight coupling-phase engineering as a useful tool for controlling interference, bound states, and quantum dynamics in nonlocal light--matter interfaces.
We investigate the influence of long-range hoppings in a structured photonic bath on atom-photon interactions. Considering a system comprising a two-leg giant atom coupled to an extended Su-Schrieffer-Heeger (SSH) model incorporating beyond-nearest-neighbor hoppings while preserving chiral symmetry, we demonstrate the formation of an unconventional bound state in the continuum (BIC) when the atomic transition frequency resonates with a mode inside the propagation band. In particular, the spatial distribution of the photoinc wavefunction of the BIC can be modified by the long-range hoppings. Consequently, when multiple identical giant atoms are included in the system, the resulting decoherence-free interactions between them are rendered less sensitive to the specific topological configuration (e.g., braided, separated, or nested) of the emitters. Furthermore, we propose two efficient and controllable quantum state transfer protocols enabled by the extended SSH model coupled to normal atoms. We show that by appropriately engineering the long-range hopping strengths, perfect excitation transfer can be achieved between two atoms coupled to the edge sites of an infinite SSH chain. In a second scheme, a single atom coupled to the center of a finite SSH chain can transfer its excitation to one of the topological edge modes. Our paper provides a method for controlling atom-photon interactions with significant implications for quantum information transfer and processing.
To go beyond quantum optics in free-space setups, atom-light interfaces with structured photonic environments are often employed to realize unconventional quantum electrodynamics (QED) phenomena. However, when employed as quantum buses, those long-distance nanostructures are limited by fabrication disorders. In this work, we alternatively propose to realize structured light-matter interactions by engineering multiple coupling points of hybrid giant atom--conventional environments without any periodic structure. We present a general optimization method to obtain the real-space coupling sequence for multiple coupling points. We report a broadband chiral emission for frequency-tunable giant emitters, with no analog in other quantum setups. Moreover, we show that the QED phenomena in the band-gap environment, such as fractional atomic decay and dipole-dipole interactions mediated by a bound state, can be observed in our setup. Numerical results indicate that our proposal is robust against fabrication disorders of the coupling sequence. Our work opens up a route for realizing unconventional light-matter interactions.
Corner states in higher-order topological insulators are typically confined to geometric corners, limiting their flexibility for scalable quantum information processing. We propose a scheme to synthesize topological corner states at arbitrary positions within the bulk of a two-dimensional Su-Schrieffer-Heeger (SSH) lattice by coupling it to giant atoms. By engineering an L-shaped multi-point coupling that satisfies the vacancy-like dressed state (VDS) condition, where the photonic wavefunction vanishes at the coupling sites to form an artificial bulk boundary, we derive the conditions for synthesizing a zero-energy corner state at any target position. We demonstrate that the engineered corner state exhibits high fidelity and spatial localization, remaining robust against realistic disorder. Extending to multi-atom networks, we realize a versatile quantum switch via a giant superatom, enabling multi-channel control over 0D corner states and 1D edge states through the dual-resonance condition. Furthermore, we demonstrate the coherent interactions between two giant atoms mediated by VDS-engineered corner states. Governed by a sublattice selection rule, the coupling activates exclusively in intersecting configurations and decays exponentially with distance. Our work establishes a highly reconfigurable platform for embedding topological boundary modes within the bulk, offering a robust pathway for scalable topological quantum networks.