Manipulating Excitation Dynamics in Structured Waveguide Quantum Electrodynamics

TL;DR

This work introduces a structured wQED framework where each emitter carries a local directionality $D_\mu$ modulated by a global chirality $η$, enabling programmable excitation transport in atom–nanophotonic interfaces. By analyzing four configurations S1–S4, the authors reveal distinct dynamical regimes—centering, wave-like propagation, leap-frog transfer, and dispersion—driven by interference among subradiant non-Hermitian eigenmodes. The study connects transport behavior to spectral properties and mode structure, showing robustness against nonguided losses with coupling efficiencies $β\geq 0.99$, and demonstrates practical feasibility in superconducting-qubit and quantum-dot platforms. The results establish structured wQED as a flexible route to control localization, coherence, and transport for quantum information routing and state transfer in integrated photonic networks.

Abstract

Waveguide quantum electrodynamics (wQED) has become a central platform for studying collective light-matter interactions in low-dimensional photonic environments. While conventional wQED systems rely on uniform chirality or reciprocal emitter-waveguide coupling, we propose a structured wQED framework, where the coupling directionality of each emitter can be engineered locally to control excitation transport in an atom-nanophotonic interface. For different combinations of patterned coupling directionalities of the emitters, we identify four representative configurations that exhibit distinct dynamical behaviors: centering, wave-like, leap-frog, and dispersion excitations. Spectral analysis of the effective non-Hermitian Hamiltonian reveals that these dynamics originate from interferences among subradiant eigenmodes. Variance analysis further quantifies the spreading of excitation as functions of interatomic spacing and global chirality, showing tunable localization-delocalization transitions. Including nonguided losses, we find that the transport characteristics remain robust for realistic coupling efficiencies (beta >= 0.99). These results establish structured wQED as a practical route to manipulate excitation localization, coherence, and transport through programmable directionality patterns, paving the way for controllable subradiant transport and chiral quantum information routing.

Figures (7)

• Figure 1: (a) Equivalence between a parallel double-waveguide system (upper panel) and an engineered single-waveguide configuration (lower panel). In the double-waveguide configuration, waveguides A and B support opposite unidirectional propagations (red arrows), while the atoms are positioned at an angle with respect to each other, leading to nonuniform coupling strengths and unequal decay rates $\gamma_{\mu}^A$ and $\gamma_{\mu}^B$ into the respective waveguides. In the equivalent single-waveguide mapping, the same effective dynamics can be reproduced by tailoring the transverse positions of atoms to control the coupling strength and by setting the local directionality. (b) An example to show the equivalence between double-waveguide and structured wQED systems where the two configurations (left and right panels) yield identical dynamics. In this example, the single-waveguide configuration uses homogeneous coupling strengths for all atoms, while the directionality is varied site-by-site.
• Figure 2: Excitation dynamics for four structured directionality configurations S1–S4. (a) Schematic representation of the four investigated configurations, labeled S1, S2, S3, and S4. (b–e) Corresponding excitation transport dynamics for each structure: (b) S1 – centering excitation, (c) S2 – wave-like excitation, (d) S3 – leap-frog excitation, and (e) S4 – dispersion excitation. The array consists of $N=54$ atoms separated by $\xi = \pi/2$. (f) Total population dynamics on a logarithmic scale. Solid lines indicate S1 (green), S2 (blue), S3 (red), and S4 (cyan). Black dashed line in (e) represent exponential decays $e^{-\gamma t}$, serving as references for comparing decay rates.
• Figure 3: A middle-localized subradiant mode found in structure S1, which the $D_\mu = 0$ placed in sites 23 to 32, with $\eta = 0.999$. (a) The population profile of the most subradiant mode ($\lambda_1 = -0.5250\gamma - i0.00581\gamma$). (b) The time evolution of the system initially quenched at this subradiant mode. Other parameters, including the interatomic spacing $\xi$ and the number of atoms $N$, are the same as in Fig. \ref{['Fig2']}(b).
• Figure 4: Two middle-localized subradiant modes and their beating dynamics in the S2 configuration, with $D_\mu = 0$, placed at sites 11 and 44. Population profile of a subradiant mode with frequency shifts (a) $\omega_5 = -0.126\gamma$ and (b) $\omega_{12} = 0.291\gamma$. (c) Time evolution of a superposition of these two modes, showing a clear beating pattern with a frequency $\omega_{\mathrm{beat}} = \omega_{12} - \omega_{5}$ that reproduces the wave-like excitation observed in Fig. \ref{['Fig2']}(c). Other system parameters are the same as in Fig. \ref{['Fig3']}.
• Figure 5: Two edge modes and distinct beating dynamics in the S4 configuration. (a,b) Population profiles of two nearly degenerate eigenmodes localized at the first and last unit cells, corresponding to edge states arising from the broken translational symmetry of the directionality pattern. (c,d) Time evolution of the superposition of two subradiant modes with a high beat frequency (c) $\omega_{\mathrm{beat}} = 0.467\gamma$, forming a herringbone-like interference pattern and (d) $\omega_{\mathrm{beat}} = 0.048\gamma$, contributing to the global dispersive behavior across the array. Other system parameters are the same as in Fig. \ref{['Fig3']}.