Hong–Ou–Mandel interference of two independent continuous-wave coherent photons

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 S1∕2(F=3)-5P1∕2(F'=3)transition line of 85 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.