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 5S1/2(F=3)−5P1/2(F′=3) transition line of Rb85 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：

---

两个独立连续波相干光子的红欧曼德尔干涉

两个完全独立的光子之间的干涉是许多光子量子信息应用（例如量子中继器、隐形传态和量子密钥分发）的核心。在这里，我们报告了对 Hong-Ou-Mandel (HOM) 干涉与两个独立的连续波相干光源的观察，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，我们使用 Rb85 原子的 5S1/2(F=3)−5P1/2(F'=3) 过渡线采用高精度频率稳定技术。我们成功地观察到来自锁频或调频激光器的两个独立连续波相干光子的 HOM 干涉条纹。当两个干涉光子之间的频率差超出单个相干光子的光谱带宽时，也会观察到涉及双光子拍频的干涉条纹。我们进行了进一步的实验以验证源制备的稳健性，而不管两个独立光子源之间的间隔距离如何。