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Quantum light sources in the mid‐infrared (MIR) band play an important role in many applications, such as quantum sensing, quantum imaging, and quantum communication. However, there is still a lack of high‐quality quantum light sources in the MIR band, such as the spectrally pure single‐photon source. In this work, the generation of a spectrally‐pure state in an optimized poled lithium niobate crystal using a metaheuristic algorithm is presented. In particular, the particle swarm optimization algorithm is adopted to optimize the duty cycle of the poling period of the lithium niobate crystal. With this approach, the spectral purity can be improved from 0.820 to 0.998 under the third group‐velocity‐matched condition, and the wavelength‐tunable range from 3.0 to 4.0 µm for the degenerate case and 3.0 to 3.7 µm for the nondegenerate case. This work paves the way for developing quantum photonic technologies at the MIR wavelength band. [ABSTRACT FROM AUTHOR]

In article number 2200028, Wu-Hao Cai, Ying Tian, Rui-Bo Jin, and colleagues exploit the particle swarm optimization algorithm to design lithium niobate for generating spectrally-pure single photons at mid-infrared wavelengths. Front Cover: Optimized Design of the Lithium Niobate for Spectrally-Pure-State Generation at MIR Wavelengths Using Metaheuristic Algorithm (Adv. This technique will play as a practical tailoring tool in the crystal structure design and can be utilized for designing special nonlinear crystals to generate exotic quantum states for quantum sensing and metrology. [Extracted from the article]