Interference between two completely independent photons lies at the heart of many photonic quantum information applications such as quantum repeaters, teleportation, and quantum key distribution. Here, we report the observation of Hong–Ou–Mandel (HOM) interference with two independent continuous-wave coherent light sources that are neither synchronized nor share any common reference. To prepare indistinguishable photons from two independent laser sources, we employ high-precision frequency-stabilization techniques using the $5{\mathrm{S}}_{1/2}(\mathit{F}=3)-5{\mathrm{P}}_{1/2}({\mathit{F}}^{\prime }=3)$ transition line of ${}^{85}\mathrm{Rb}$ atoms. We successfully observe an HOM interference fringe with two independent continuous-wave coherent photons originating from either the frequency-locked or the frequency-modulated lasers. An interference fringe involving two-photon beating is also observed when the frequency difference between the two interfering photons is beyond the spectral bandwidth of the individual coherent photons. We carry out further experiments to verify the robustness of the source preparation regardless of the separation distance between the two independent photon sources.

中文翻译：

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两个独立的连续波相干光子的Hong-Ou-Mandel干涉

两个完全独立的光子之间的干扰是许多光子量子信息应用程序的核心，例如量子中继器，隐形传送和量子密钥分配。在这里，我们报告使用两个独立的连续波相干光源进行的Hong-Ou-Mandel（HOM）干扰观察，这些光源既不同步也不共享任何公共参考。为了从两个独立的激光源制备难以区分的光子，我们采用了高精度的频率稳定技术，该技术使用$5{\mathrm{小号}}_{\mathrm{1个}/2}（\mathit{F}=3）-5{\mathrm{P}}_{\mathrm{1个}/2}（{\mathit{F}}^{\prime }=3）$ 的过渡线 ${}^{85}b$原子。我们成功地观察到带有两个独立的连续波相干光子的HOM干涉条纹，这些光子来自锁频或调频激光器。当两个干扰光子之间的频率差超过单个相干光子的频谱带宽时，也会观察到涉及两个光子跳动的干扰条纹。我们进行了进一步的实验，以验证源制备的稳健性，而与两个独立的光子源之间的分离距离无关。