Nonreciprocal single-photon transport is an important functionality for quantum information processing, on-chip quantum photonic devices, and integrated quantum networks. Considerable attention has been devoted to realizing directional single-photon transmission in waveguide quantum electrodynamics (waveguide-QED). However, in many existing schemes, nonreciprocal transport relies mainly on static structural asymmetry, chiral coupling, or fixed propagation phases. Once the device parameters are determined, the corresponding transport direction and isolation characteristics are generally difficult to reconfigure dynamically.
In this work, we theoretically investigate dynamically controllable nonreciprocal single-photon scattering in a time-modulated atom-waveguide system consisting of a two-level atom coupled to two semi-infinite one-dimensional coupled-resonator waveguides. The atom-waveguide coupling strengths are independently and periodically modulated in time. In the high-frequency regime, the periodic modulation induces an effective antisymmetric coupling between the two boundary cavities, whose magnitude and sign can be controlled by the modulation phase difference. Within the discrete-coordinate scattering framework, analytical expressions for the scattering amplitudes of photons incident from opposite directions are derived.
The results show that the nonreciprocity originates from direction-dependent quantum interference between the atom-mediated resonant scattering channel and the Floquet-induced coherent coupling channel. The two propagation directions experience opposite interference conditions, and therefore the preferred transmission direction can be reversibly switched by changing the modulation phase difference. Under appropriate parameter matching, one directional transmission can approach unity while the reverse transmission is completely suppressed, realizing an ideal dynamically switchable single-photon diode. Furthermore, the isolation degree exhibits nonmonotonic dependences on the modulation-induced effective coupling, the static atom-waveguide coupling, and the atomic dissipation. The analytical results indicate that perfect nonreciprocal transmission requires the cooperative interplay of Floquet modulation, resonant atom-waveguide interaction, waveguide dispersion, and dissipation, rather than time-reversal-symmetry breaking alone. These results clarify the physical mechanism of dynamically controllable nonreciprocal single-photon scattering in time-modulated waveguide-QED systems and provide a theoretical basis for reconfigurable single-photon isolators and directional quantum routers in integrated quantum networks.