Efficient single-photon frequency conversion is investigated in a two-giant-atom waveguide-QED system, where a two-level giant atom and a
Λ-type three-level giant atom couple to a common one-dimensional waveguide. While the
Λ-type atom provides the inelastic channel, the two-level atom induces secondary coherent coupling, creating multi-path interference for the converted photon. Using the real-space approach and within the Markovian approximation, we derive analytical four-channel scattering amplitudes and reveal that the inelastic transmission spectrum, governed by three complex resonance poles, exhibits a multi-peak interference pattern. By introducing a local single-pole approximation, we reduce this complex spectrum to a local Fano lineshape, decomposing it into a coherent superposition of a local background term and a single-pole resonant term. The interplay between these two terms-controlled by the photon propagation phase between the giant atoms' coupling points-determines the conversion efficiency, with the background suppression condition leading to a Lorentzian reduction. Based on the single-pole resonance weight, we formulate a phase-selection criterion for highly efficient conversion. Compared with both the small-atom and single
Λ-type giant-atom models, the two-giant-atom scheme achieves substantially enhanced inelastic transmission over a broader frequency-conversion range. This work reveals how phase-controlled local Fano resonance enables high-efficiency frequency conversion, establishing a general paradigm for engineering resonant light-matter interactions in structured quantum systems.
Abstract Single-photon frequency conversion in the system composed of a Λ -type three-level giant atom chiral couples to a one-dimensional waveguide is investigated. Two configurations, i.e. one of the atomic transitions couples to the waveguide at two separated points while the other one either directly or indirectly couples to the waveguide at a single point, are considered. The scattering amplitudes for representing the frequency conversion in the two kinds of coupling schematics are obtained analytically with the real-space approach. The results show that perfect nonreciprocal transmission with preserving frequency and ideal directional frequency conversion can be realized due to the time-reversal symmetry broken induced by the chiral coupling as well as the self-interference comes from the giant atom. In particular, in the indirect coupling configuration, the scattering spectra exhibit two directional frequency conversion windows due to the atom-cavity coupling, which enables the present system suitable for the simultaneous manipulation of incident photons with different frequencies. These results may have potential applications in various quantum information processing and quantum network engineering.
-type atom coupled to a T-shaped waveguide. By controlling local coupling phases to engineer quantum interference, perfect directional frequency conversion is achieved without an engineered chiral waveguide structure. The multi-path interference of the giant atom, combined with local phase tuning, enables flexible control over photon transmission direction and frequency, supporting multiple directional conversion channels. In the Markovian limit, the cavity-mediated structure realizes two fully efficient directional channels, while under non-Markovian conditions, retardation effects open several distinct frequency conversion windows. This phase-controlled, multi-frequency directional architecture provides a versatile platform for frequency-multiplexed quantum routing and networking.