We theoretically investigated spectrally uncorrelated biphotons generated in a counter-propagating spontaneous parametric downconversion (CP-SPDC) from periodically poled $M{\rm TiO}X{{\rm O}_4}$ ($M = {\rm K}$, Rb, Cs; $X = {\rm P}$, As) crystals. By numerical calculation, it was found that the five crystals from the KTP family can be used to generate heralded single photons with high spectral purity and wide tunability. Under the type-0 phase-matching condition, the purity at 1550 nm was between 0.91 and 0.92, and the purity can be maintained over 0.90 from 1500 nm to 2000 nm wavelength. Under the type-II phase-matching condition, the purity at 1550 nm was 0.96, 0.97, 0.97, 0.98, and 0.98 for PPKTP, PPRTP, PPKTA, PPRTA, and PPCTA, respectively; furthermore, the purity can be kept over 0.96 for more than 600 nm wavelength range. We also simulated the Hong–Ou–Mandel interference between independent photon sources for PPRTP crystals at 1550 nm, and interference visibility was 92% (97%) under the type-0 (type-II) phase-matching condition. This study may provide spectrally pure narrowband single-photon sources for quantum memories and quantum networks at telecom wavelengths.

中文翻译：

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由周期性极化的M TiO X O ₄（M  = K，Rb，Cs; X  = P，As）在1550 nm处反向传播的光谱不相关双光子

我们从理论上研究从周期性极化的反向传播的自发参量下变频（CP-SPDC）产生的频谱不相关biphotons $ M {\ RM的TiO}×{{\ RMö} _4} $（$ M = {\ RMķ} $，Rb，Cs；$ X = {\ rm P} $，As）晶体。通过数值计算，发现来自KTP家族的五种晶体可用于产生具有高光谱纯度和宽可调性的预示单光子。在0型相位匹配条件下，在1550nm处的纯度在0.91至0.92之间，并且从1500nm至2000nm的波长可以将纯度维持在0.90以上。在II型相位匹配条件下，PPKTP，PPRTP，PPKTA，PPRTA和PPCTA在1550 nm处的纯度分别为0.96、0.97、0.97、0.98和0.98；此外，在600 nm以上的波长范围内，纯度可保持在0.96以上。我们还模拟了PPRTP晶体在1550 nm处独立光子源之间的Hong-Ou-Mandel干涉，在0型（II型）相位匹配条件下，干涉可见度为92％（97％）。