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 single-photon source in mid infrared (MIR) band is very important for the next generation of quantum sensing, quantum communication and quantum imaging. At present, the commonly used method of generating MIR single-photon source is based on the spontaneous parametric down conversion (SPDC) process in the periodically poled lithium niobate (PPLN) crystal. However, the spectral purity of single-photon source based on ordinary PPLN is not high, and the maximum value is only about 0.82, which will affect the fidelity of quantum information processing scheme. In this paper, 4000 polarized domains in a 30-mm-long LN crystal are customized by using the domain design theory. The sidelobe in the phase matching function is eliminated, and the Gaussian distribution is obtained. The calculated spectral purity of the single-photon source can reach 0.99, and its tunable range is 2.7∼3.3 µm. The customized poled lithium niobate (CPLN) is expected to provide a single-photon source with excellent performance for the study of quantum information in the MIR band.