We design an atom-waveguide system to achieve nonreciprocal frequency conversion. Single-photon frequency conversion is achieved through inelastic scattering mediated by the V-type giant atom with one transition coupled to a one-dimensional coupled-resonator waveguide (CRW) and the other transition coherently driven by a classical field. To quantitatively characterize the frequency-conversion efficiency, we define scattering flows by incorporating the corresponding group velocities. The reciprocity and nonreciprocity of the system can be characterized by the conversion-flow contrast. It is found that the single-photon frequency-conversion flow is closely related to the size of the giant atom and the phase difference between the coupling strengths at the two atom-CRW coupling sites. Under reciprocal conditions, the maximum conversion flow is strictly bounded by $1/2$, regardless of how the giant atom size and the phase difference are tuned. When reciprocity is broken, tuning the coupling phase at a fixed giant atom size significantly enhances the maximum conversion flow, which can even reach unity. Meanwhile, the conversion-flow contrast reaches its extremal values, corresponding to perfect single-photon nonreciprocity.

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• Non-reciprocal transmission
• Quantum communication
• Quantum optics