High-Performance Quantum Frequency Conversion from Ultraviolet to Telecom Band

Abstract

Quantum frequency conversion (QFC) is essential for bridging the spectral gap between stationary qubits and low-loss optical communication channels. In this work, we demonstrate a short-wavelength-pumping QFC with the first-order quasi-phase matching period of 3.07 um on thin-film lithium niobate, converting ultraviolet photons to the telecom C-band. By constructing a theoretical model that correlates the normalized conversion efficiency with domain defects in the short-period phase-matched waveguide, we found the critical tolerance of domain defects along the waveguide should be $\le 2$ (excluding the ends). Based on this, we achieved a theoretical limit normalized conversion efficiency of 839%/(W*cm^2) for the fundamental guided mode through fabrication optimization. Furthermore, we propose a robust noise suppression strategy for short-wavelength pumping by utilizing the counter-tuning behaviors of difference-frequency generation and spontaneous parametric down-conversion. By combining these advances with ultra-narrowband filtering, we achieve a record-high external efficiency of 28.8% and an ultra-low noise of 35 counts per second. This high-performance QFC connecting ultraviolet and telecom bands satisfies the stringent requirements for long-lived remote ion-ion entanglement in scalable quantum networks [W.-Z. Liu et al., Nature (2026)].

Figures (4)

• Figure 1: (a) Evolution of the relative efficiency along the waveguide for $N=3$ (orange), 6 (green), and 101 (blue) defects. The black curve represents the ideal growth without defects. Open circles mark the positions where domain defects induce abrupt transitions in efficiency evolution. Inset: highlights the local perturbation at a specific defect site. (b) Comparison of relative efficiency between theory (blue solid line) and experiment (orange dashed line) based on measured defect statistics. Inset: displays a micrograph of the domain structure. (c) Simulated probability of the normalized efficiency exceeding 90% of its ideal value as a function of defect count.
• Figure 2: (a) Images of the fabricated device: (Top) Photograph of the fabricated ridge waveguide chip; (Bottom left) Scanning electron microscopy (SEM) image of the waveguide facet; (Bottom right) SEM image of the periodically poled domain structure revealed by hydrofluoric acid etching. (b) Internal efficiency versus pump power. Filled blue circles: measured points; solid red line: $\sin^2$-type fit to the experimental data; solid green line: the theoretical prediction from the coupled-wave equations including propagation losses.
• Figure 3: (a) Noise counts versus waveguide temperature at a fixed pump power for pump wavelengths of 527.2 nm (blue), 527.3 nm (olive), and 527.37 nm (orange-red). Filled red circles and black diamonds mark the noise counts and corresponding waveguide temperatures at the noise peaks and DFG phase-matching points, respectively. Arrows indicate the opposing temperature shifts versus pump wavelength. (b) Temperatures of the noise peaks and DFG phase-matching points labeled in (a) versus pump wavelength. Filled and open red circles denote theoretical and experimental noise peaks, respectively; filled and open black diamonds denote theoretical and experimental DFG phase-matching points, respectively.
• Figure 4: (a) Experimental setup. The upper section illustrates the DFG module. QWP: quarter-wave plate, HWP: half-wave plate, BS: beam splitter, PM: power meter, NDF: neutral density filter, DM: dichroic mirror, POL: polarizer, LPF: long-pass filter, SPF: short-pass filter, BPF: band-pass filter, PBS: polarizing beam splitter, PPLN WG: periodically poled lithium niobate waveguide. The lower section shows the ultra-narrowband filtering stage. FP: Fabry--Pérot, VBG: volume Bragg grating, FPC: fiber polarization controller, SNSPD: superconducting nanowire single-photon detector. (b) External efficiency and noise counts of the DFG process versus pump power. Filled blue circles show the measured external efficiency, with the solid blue line representing the fitted $\rm sin^{2}$-type model. Filled orange-red squares denote the measured noise counts, and the dashed red lines show the fits based on the noise model [Eq. (\ref{['Eq:modified-noise']})] including propagation losses. Open gray circles indicate the measured external efficiency-to-noise ratio (ENR), with the solid green line showing the fitted trend. The gray vertical line marks the operating point during the experiment.