The mid-infrared (MIR) band entangled photon source is vital for the next generation of quantum communication, quantum imaging, and quantum sensing. However, the current entangled states are mainly prepared in the visible or near-infrared bands. It is still lacking high-quality entangled photon sources in the MIR band. In this work, we optimize the poling sequence of lithium niobate to prepare two kinds of typical entangled states, the Hermit–Gaussian state and the comb-like entangled state at 3.2 µm. We have also calculated the photon pair rates and estimated the effect of fabrication resolution in the schemes. Our approach will provide entangled photon sources with excellent performance for the study of quantum information in the MIR band.

The Orbital angular momentum (OAM) of light is regarded as a valuable resource in quantum technology, especially in quantum communication and quantum sensing and ranging. However, the OAM state of light is susceptible to undesirable experimental conditions such as propagation distance and phase distortions, which hinders the potential for the realistic implementation of relevant technologies. In this article, we exploit an enhanced deep learning neural network to identify different OAM modes of light at multiple propagation distances with phase distortions. Specifically, our trained deep learning neural network can efficiently identify the vortex beam’s topological charge and propagation distance with 97% accuracy. Our technique has important implications for OAM based communication and sensing protocols.

Hong-Ou-Mandel (HOM) interference is a non-classical effect of photons and plays an important role in quantum optics. <i>β</i>-barium borate (BBO) has high nonlinear efficiency, and is commonly used to generate biphoton states, thereby exhibiting HOM interference. However, in previous experiments, researchers often used band-pass filters, so the resulting spectrum was directly determined by the band-pass filter. As a result, the original spectrum of the BBO crystal, especially the spectrum under tight focusing, was lack of systematic research. In this paper, the biphoton spectral distribution and HOM interference generated by the BBO crystal under the condition of tight focusing were systematically studied for the first time. Theoretical calculations found that using a lens with 50 mm focusing length, the spectral width of the down-converted photons was increased by 7.9 times compared with the non-focused case; the width of the HOM interference fringe decreased by 4 times, and the visibility of the interference fringe was increased from 53.0% to 98.7%. We experimentally prepared the energy-time entanglement state using type II BBO crystal and performed HOM interference, obtaining an interference visibility of <inline-formula><tex-math id="M2">\begin{document}$86.6 \pm 1.0 $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20211783_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20211783_M2.png"/></alternatives></inline-formula>%. The increasing of the HOM visibility is due to the improvement of biphoton's spectral symmetry. In addition, the proposed technique that obtaining different spectral distributions at different incident angles was expected to be applied to the preparation of high-dimensional qudits in the future.

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