Two-photon interference of multimode two-photon pairs produced by an optical
parametric oscillator has been observed for the first time with an unbalanced
interferometer. The time correlation between the multimode two photons has a
multi-peaked structure. This property of the multimode two-photon state induces
two-photon interference depending on delay time. The nonclassicality of this
interference is also discussed.Comment: 5 pages, 4 figure