Single photon isolation and nonreciprocal frequency conversion in atom-waveguide systems

In this work, we utilize a two-level atom and a ${\Lambda}$-type atom to link two identical waveguides, subsequently extending the model to a giant-atom configuration. Our analytical solutions and numerical simulations demonstrate that this setup can achieve single-photon isolation and nonreciprocal frequency conversion by tuning the atom-waveguide coupling strengths $g_i$, respectively. We also examine single-photon scattering in the giant-atom model within both the Markovian and non-Markovian regimes. The results reveal that ultranarrow scattering windows are induced by the phases ${\phi}_1$ and ${\phi}_2$ under specific conditions, making them well-suited for precise frequency conversion and sensing. Additionally, in the non-Markovian regime, the spectra exhibit irregular polygonal shapes, offering enhanced opportunities for exploring nonreciprocal frequency conversion in the off-resonant regime. Our work provides a new perspective on achieving optical nonreciprocity at the single-photon level in atom-waveguide systems.