Optical isolation and circulation using quantum frequency conversion on a chip.

Breaking electromagnetic reciprocity enables directional control of light and is essential for both classical and quantum photonic systems. However, realizing integrated optical nonreciprocity that simultaneously combines low loss, broad bandwidth, and compatibility with quantum states of light remains a major challenge. Here, we demonstrate optical nonreciprocity based on quantum frequency conversion in an integrated [Formula: see text] nonlinear waveguide. Operating from classical power levels down to the single-photon level, the device preserves quantum coherence and entanglement of the input photons. We achieve an isolation of 34 decibels with a low on-chip insertion loss of 0.8 decibels and a broad isolation bandwidth, together with a four-port circulator exhibiting an operational fidelity of up to 0.97. Our results establish parametric frequency conversion as a scalable and quantum-compatible route toward integrated nonreciprocal photonic devices for applications in quantum communication and noise-resilient photonic networks.