Two-photon interference between continuous-wave coherent photons temporally separated by a day

An understanding of the phenomenon of light interference forms the kernel underlying the discovery of the nature of light from the viewpoints of both classical physics and quantum physics.Here we report on two-photon interference with temporally separated continuous-wave coherent photons by using a temporal post-selection method with an arbitrary time delay.Although the temporal separation of a day between the photons is considerably longer than the coherence time of the light source,we observe the Hong–Ou–Mandel(HOM)interference of the pairwise two-photon state.Furthermore,we experimentally demonstrate the HOM interference observed in one of the interferometer-output modes by using only one single-photon detector for a large temporal separation.