High-efficiency telecom frequency conversion via a diamond-type atomic ensemble*

Telecom frequency conversion (TFC) is essential for fiber-based quantum communications, facilitating the integration of quantum systems with optical fibers while minimizing photon loss. Here, we report high-efficiency TFC from 795 nm to 1367 nm in a cold ⁸⁷Rb ensemble via diamond-type four-wave mixing (FWM), achieving conversion efficiencies of 66% and 80% at optical depths of 75 and 110, respectively. These results exceed previously reported values, enabled by a systematic investigation of the built-in V-type and cascade-type electromagnetically induced transparency spectra, which guided the optimization of FWM efficiency. Theoretical analysis confirms that quantum states are preserved with high fidelity during the conversion process, enabling robust photon transmission through optical fibers and advancing the potential for long-distance quantum communication.