Two-way photonic interface for linking the Sr⁺ transition at 422 nm to the telecommunication C band

We report a single-stage bidirectional interface capable of linking Sr+ trapped-ion qubits in a long-distance quantum network. Our interface converts photons between the Sr+ emission wavelength at 422 nm and the telecom C band to enable low-loss transmission over optical fiber. We have achieved both up- and down-conversion at the single-photon level with efficiencies of 9.4% and 1.1%, respectively. Furthermore, we demonstrate noise levels that are low enough to allow for genuine quantum operation in the future.