High-Performance Quantum Frequency Conversion from Ultraviolet to Telecom Band

Quantum frequency conversion (QFC) is essential for bridging the spectral gap between stationary qubits and low-loss optical communication channels. Practicable QFC interfaces connecting ultraviolet and telecommunication bands are of great significance but have remained elusive, constrained by the dual challenges of a high-performance frequency converter and effective noise suppression in short-wavelength pumping configurations. We address this by demonstrating the phase-matching mechanism with domain defects and differential-tuning behavior of the spontaneous parametric down-conversion noises. Based on a quantitative analysis of performance degradation induced by domain defects, we propose specific fabrication guidelines and experimentally achieve an ultrahigh normalized conversion efficiency of 839%/(W cm^{2}) for the fundamental guided mode in thin-film lithium niobate waveguides. Meanwhile, we reveal a new noise control mechanism for short-wavelength pumping by utilizing the differential-tuning behaviors of difference-frequency generation and spontaneous parametric down-conversion processes. Then a single-stage QFC from 393 nm to the telecom C band with a record-high external efficiency of 28.8% and an ultralow noise level of 35 counts per second is achieved experimentally. The proposed mechanisms would enable other QFCs with higher efficiency and lower noise, advancing the development of scalable quantum networks. Furthermore, it holds significant potential for driving progress in both classical and integrated nonlinear photonics.