We unveil the inherent topological nature from a two-dimensional synthetic parameter space in a single-photon transport problem, with the simple configuration of one waveguide coupled with two four-level atoms driven by external fields. We find that, by tuning the system in the dark-state condition, the system holds two independent phase degrees of freedom from both the photon propagation and the atomic transition. As the direct output, the photon scattering characteristics show the total reflection with the frequency preservation or the complete frequency conversion at the specific choices of two phases. We uncover the underlying topological effect by analyzing the photon scattering amplitudes in the parameter space of two phases, which links to the singularity with nonzero winding numbers. Our results are verified by numerical simulations, and hence point to an alternative perspective for the research at the intersection of single-photon transport and topology, without using the topological geometric arrangement of atoms.
We unveil the inherent topological nature from a two-dimensional synthetic parameter space in a single-photon transport problem, with the simple configuration of one waveguide coupled with two four-level atoms driven by external fields. We find that, by tuning the system in the dark-state condition, the system holds two independent phase degrees of freedom from both the photon propagation and the atomic transition. As the direct output, the photon scattering characteristics show the total reflection with the frequency preservation or the complete frequency conversion at the specific choices of two phases. We uncover the underlying topological effect by analyzing the photon scattering amplitudes in the parameter space of two phases, which links to the singularity with nonzero winding numbers. Our results are verified by numerical simulations, and hence point to an alternative perspective for the research at the intersection of single-photon transport and topology, without using the topological geometric arrangement of atoms.
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.
This work is to investigate the single-photon scattering in a waveguide quantum electrodynamics system consisting of two dipole-coupled giant atoms, each interacting with a separate one-dimensional infinite waveguide at two distinct coupling points. Our primary objective is to establish a theoretical framework for manipulating photon propagation paths via quantum interference induced by multiple coupling points and local phase engineering. Unlike traditional chiral coupling schemes, an innovative method, in which the coupling phases are designed locally at each atom-waveguide interface, is used to achieve effective chiral coupling, thereby introducing novel quantum interference mechanisms. Using a real-space approach, we derive analytical expressions for four-port scattering amplitudes. We establish the conditions for achieving perfect directional routing to any output port and demonstrate the coherent control mechanisms implemented by geometric and local coupled phases. Continuous frequency tunability is primarily achieved through dipole-dipole interaction, and finely tuned through the accumulated phase and local coupling phases. Local phase differences precisely regulate port-specific probability distributions within the waveguides while preserving total routing efficiency. Furthermore, we elucidate the mechanisms of nonreciprocal transport and chiral scattering. The analysis reveals different governing principles: perfect nonreciprocity arises from the interplay of the accumulated phase, local coupling phases, photon-atom detuning, and dipole-dipole interaction. In contrast, perfect chiral scattering depends entirely on the accumulated phase and local coupling phases, and is independent of detuning. Notably, under the phase-matching conditions, the system achieves both perfect chiral and directional routing, and realizes frequency-selective path-asymmetric photon control. These findings provide a comprehensive framework for manipulating quantum interference in multi-atom waveguide systems, highlighting applications in quantum information processing, including tunable single-photon routers, isolators, and chiral quantum nodes. By implementing superconducting circuits, the local phase can be dynamically adjusted, thus proving the feasibility of the experiment.
We propose a topological transport platform for microwave-to-optical conversion at the single-photon level in a Rydberg atom-cavity setting. This setting leverages a hybrid dual-mode Jaynes-Cummings (JC) configuration, where a microwave resonator couples an optical cavity mediated by a Rydberg atom ensemble. Our scheme uniquely enables the formation of Fock-state lattices (FSLs), where photon hopping rates depend on photon numbers in individual sites. We identify an inherent zero-energy mode corresponding to the dark state of the dual-mode JC model. This enables to build a high-efficiency single-photon transducer, which realizes topologically protected photon transport between the microwave and optical modes. Crucially, we show analytically that the FSL features continuous variations of the winding number. Our work establishes a robust mechanism for efficient quantum transduction in synthetic dimensions and opens avenues for exploring topological physics with continuous winding numbers in the atom-cavity system.
We investigate coherent single-photon transport in a waveguide-QED structure containing two giant atoms. The unified analytical expressions of the single-photon scattering amplitudes applicable for different topological configurations are derived. The spectroscopic characteristics in different parameter regimes, especially the asymmetric Fano line shapes and the electromagnetically induced transparency (EIT)-like spectra, are analyzed in detail. Specifically, we find that the appearance of Fano line shapes is influenced by not only the phase delays between coupling points but also the topologies of system. We also summarize the general conditions for appearance of EIT-like spectra by analyzing the master equation and verify these conditions by checking the corresponding analytical expressions of the scattering spectra. These phenomena may provide powerful tools for controlling and manipulating photon transport in future quantum networks.
We investigate the coherent single-photon scattering process in a topological waveguide coupled to a driven three-level atom in the $\mathrm{\ensuremath{\Lambda}}$ configuration, where two ground states are optically connected to a common excited state. We derive an analytical expression for transmittance by using the scattering formalism for three distinct sublattice sites (A, B, and AB) of the waveguide, which couple to the $\mathrm{\ensuremath{\Lambda}}$ system. We have demonstrated that the system's response is topology independent for A and B sublattice-site coupling and becomes topology dependent for AB sublattice-site coupling. In a weak-control-field regime, the system behaves as a perfect mirror in all of these configurations. Upon control field strength enhancement, the transmission spectrum evolves from Electromagnetically Induced Transparency to Autler-Townes splitting in A and B sublattice-site coupling. The manipulation of transmission from opaque to transparent holds the key mechanism of a single-photon switching. Furthermore, the topology-dependent AB sublattice configuration allows multiple-path-induced quantum interference, resulting in the sharper Fano line shape. This topological Fano resonance is not only tunable via the control field and coupling strength but also robust against structural imperfections due to the topological nature of the waveguide. These combined features such as tunability and topological protection make our system a promising platform for next-generation quantum technologies, including single-photon switches, high-resolution sensors, and communication devices.
Abstract We investigate single-photon quantum routing composed of double two-level atoms coupled to two infinite waveguides. By introducing local coupling phases at atom-waveguide connecting points, we engineer asymmetric atom-waveguide interactions, which enables the directional control of photon propagation for achieving high-efficiency quantum routing. Using the real-space approach, we derived exact expressions of the single-photon scattering amplitudes, allowing for a comprehensive analysis of the routing dynamics. For both identical and non-identical atomic configurations, we systematically explored the conditions for achieving perfect photon transfer. Our results demonstrate that complete photon transfer can be realized by precisely tuning the coupling phases, the accumulated phase and atomic frequencies. This work provides a theoretical foundation for developing high-efficiency quantum routers with potential applications in quantum networks and photonic quantum information processing.