Counter-propagating parametric conversion processes in non-linear bulk crystals have been shown to feature unique properties for efficient narrowband frequency conversion. In quantum optics, the generation of photon pairs with a counter-propagating parametric down-conversion process (PDC) in a waveguide, where signal and idler photons propagate in opposite directions, offers unique material-independent engineering capabilities. However, realizing counter-propagating PDC necessitates quasi-phase-matching (QPM) with extremely short poling periods. Here, we report on the generation of counter-propagating single-photon pairs in a self-made periodically poled lithium niobate waveguide with a poling period on the same order of magnitude as the generated wavelength. The single photons of the biphoton state bridge GHz and THz bandwidths with a separable joint temporal-spectral behavior. Furthermore, they allow the direct observation of the temporal envelope of heralded single photons with state-of-the art photon counters.

中文翻译：

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非线性波导中反向传播的光子对的产生。

非线性块状晶体中的反向传播参数转换过程已显示出具有有效窄带频率转换的独特特性。在量子光学中，通过在波导中进行反向传播的参数下转换过程（PDC）来生成光子对，信号和闲置光子在相反的方向传播，从而提供了与材料无关的独特工程能力。但是，要实现反向传播的PDC，必须具有极短的极化周期的准相位匹配（QPM）。在此，我们报告了在自制周期极化极化铌酸锂波导中反向传播的单光子对的生成，极化周期与生成的波长在数量级上相同。双光子状态桥的GHz和THz带宽的单光子具有可分离的联合时谱行为。此外，它们允许使用最新的光子计数器直接观察已预告的单个光子的时间包络。