Quantum Photon Source in Lithium Niobate Waveguide Achieves 54.4% Efficiency and 99THz Bandwidth

Broadband sources of light are essential for numerous applications, and researchers continually seek more efficient methods for generating them, particularly at quantum wavelengths. Xiao-Xu Fang from Shandong University, Guoliang Shentu from the Wuhan Institute of Quantum Technology, and He Lu demonstrate a significant advance in this field by creating a new type of quantum photon source. They designed and fabricated a waveguide using a specially engineered lithium niobate crystal, achieving remarkably broad bandwidth and brightness in the generation of entangled photon pairs. This achievement paves the way for more practical and powerful quantum technologies, promising improvements in areas such as quantum communication, sensing, and computation.

The team fabricated a 6. 82-millimeter-long step-chirped periodically poled lithium niobate waveguide on lithium niobate on insulator, enabling efficient second-harmonic generation and spontaneous parametric down-conversion over a broad bandwidth.

Waveguide Fabrication for Quantum Photon Pair Generation

Scientists engineered a 6. 82-millimeter-long step-chirped periodically poled lithium niobate waveguide on lithium niobate on insulator to efficiently generate photon pairs for quantum optics. Researchers fabricated a waveguide structure with a 600-nanometer-thick lithium niobate film bonded to a silicon substrate, carefully controlling layer thicknesses and material properties to optimize performance. The team designed a waveguide with a top width of 2200 nanometers, an etching depth of 240 nanometers, and a 70-degree sidewall angle, precisely tailoring the geometry to confine light and enhance nonlinear interactions.

This waveguide structure efficiently converts a pump photon into a pair of lower-energy signal and idler photons through spontaneous parametric down-conversion. To achieve broad bandwidth operation, scientists implemented a step-chirped poling period, varying the structure of the lithium niobate to compensate for different wavelengths of light. The design incorporates 101 sections, each containing 15 fixed periods, with the poling period increasing from 4. 45 micrometers to 4. 55 micrometers, resulting in a total waveguide length of 6.

8175 millimeters. This precise control over the poling period enables efficient phase matching across a wide range of wavelengths. Researchers utilized electron beam lithography to create chromium electrodes on the lithium niobate on insulator, followed by pulsed voltage application to induce domain poling and reactive ion etching to define the waveguide pattern. Characterization using piezoresponse force microscopy confirmed the successful fabrication of the step-chirped periodically poled structure, demonstrating the precision of the fabrication process. The resulting device achieves an average second-harmonic generation efficiency of 54. 4 percent per watt per centimeter across a wavelength range of 1510 to 1620 nanometers and generates photon pairs with a maximum full bandwidth of 99 terahertz and a brightness of 20 gigahertz per milliwatt per nanometer when pumped at 775, 780, and 785 nanometers.

Broadband Photon Pairs from Lithium Niobate Waveguide

Scientists have achieved efficient broadband photon pair generation using a step-chirped periodically poled lithium niobate waveguide on lithium niobate on insulator. Experiments demonstrate an average second-harmonic generation efficiency of 54. 4 percent per watt per centimeter, measured across a first-harmonic wavelength range of 1510 to 1620 nanometers. This high efficiency paves the way for realizing spontaneous parametric down-conversion across a wide range of pump wavelengths. For spontaneous parametric down-conversion, the team generated broadband photon pairs by tuning the pump wavelength to 775 nanometers, 780 nanometers, and 785 nanometers.

Measurements reveal a maximum full bandwidth of 99 terahertz, corresponding to a central wavelength of 846 nanometers. The team also recorded a maximum brightness of 20 gigahertz per milliwatt per nanometer. These results demonstrate an efficient and experiment-friendly approach for generating broadband photon pairs, which holds significant promise for advancing applications in quantum technologies and optical communications.

Broadband Frequency Conversion on Lithium Niobate

Researchers have successfully designed and fabricated a step-chirped periodically poled lithium niobate waveguide on a lithium niobate on insulator platform, demonstrating its capability for both efficient second-harmonic generation and spontaneous parametric down-conversion. The fabricated waveguide achieves a quasi-phase matching bandwidth exceeding 110 nanometers, alongside an average normalized efficiency of 54. 4 percent per watt per centimeter squared across a wavelength range of 1510 to 1620 nanometers for second-harmonic generation. This performance establishes a strong foundation for applications requiring efficient frequency doubling of light.

For spontaneous parametric down-conversion, the team generated broadband photon pairs by employing pump wavelengths of 775, 780, and 785 nanometers, achieving full bandwidths up to 99 terahertz and brightness levels reaching 20 gigahertz per milliwatt per nanometer. These photon pairs exhibit characteristics suitable for Hong-Ou-Mandel interference with narrow dips, which is beneficial for quantum metrology applications. The authors acknowledge that further optimisation of the fabrication process could potentially enhance the performance of the waveguide. Future work may focus on exploring the device’s capabilities in more complex quantum photonic circuits and applications, building upon this demonstration of efficient broadband photon pair generation.