This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Quantum networks, systems that connect devices using quantum mechanical effects, could potentially allow users to share information more securely and efficiently. Building these networks typically entails linking stationary qubits, units of quantum information encoded in trapped ions or other physical systems, using photons (particles of light) transmitted through optical fibers.
One challenge when trying to realize quantum networks is that some physical systems that serve as qubits emit ultraviolet light, electromagnetic radiation with wavelengths ranging from approximately 100 to 400 nanometers. This is problematic because optical fibers, the thin light-guiding strands used to link quantum devices, are known to transmit light at these wavelengths poorly. Researchers at the University of Science and Technology of China, Jinan Institute of Quantum Technology and other institutions recently developed a new converter that could help to bridge the gap between the ultraviolet light emitted by these systems and the light wavelengths transmitted with fewer losses through optical fibers.
Their device, introduced in a paper published in Physical Review Letters, relies on thin-film lithium niobate, a crystalline material that enables interactions between light waves with different wavelengths, patterned into a structure that confines and directs light. "Trapped-ion systems with dominant emission in the ultraviolet and blue spectral regions possess exceptional properties including long coherence times and high-fidelity control, making them one of the leading platforms for scalable quantum networks," Qiang Zhang, co-author of the paper, told Phys.org. "However, for those wavelength bands, fiber attenuation is severe." One known approach to converting photons to telecommunications wavelengths that travel through optical fibers with low losses is quantum frequency conversion (QFC).
This method entails changing the frequency of photons while preserving the quantum information they carry. "Previous converters generally suffered from low efficiency and high noise, restricting entanglement distribution in this band to meter-scale distances," said Zhang. "To address this, we aimed to realize high-efficiency, low-noise QFC from the ultraviolet to the telecom band, thereby laying the foundation for the remote interconnection of trapped-ion nodes in scalable quantum networks." The converter developed by the authors operates in accordance with the principle of energy conservation.
This principle states that energy cannot be created or destroyed, so the energy of interacting light must balance while it is converted. "In our converter, driven by an intense 527 nm pump laser, an incoming 393 nm ultraviolet photon splits into a 1550 nm telecom photon and a 527 nm photon," explained Zhang. "This transformation must occur within a nonlinear medium, and its viability is governed by two critical metrics: conversion efficiency and pump-induced noise." The researchers wanted to change the wavelength of light with high efficiency while introducing little noise (i.e., unwanted background light that could obscure useful signals).
To do this, they developed a chip made of lithium niobate and patterned it into a waveguide that can confine and direct light. "We successfully realized this frequency conversion within a periodically poled lithium niobate waveguide chip, overcoming the long-standing challenges of low efficiency and high noise," said Zhang. The team fabricated hundreds of samples of their lithium niobate-based chip and then selected one of the best-performing examples.
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