Observation of quantum interference in the plasmonic Hong-Ou-Mandel effect

Surface plasmon polaritons (SPPs) are electromagnetic excitations coupled to electron-charge density waves at metal-dielectric interfaces. They have recently been found to enable a range of nanophotonic devices for controlling systems at the quantum level, including single-photon sources, transistors, and ultracompact quantum circuitry. An important quantum feature of SPPs yet to be fully explored is their bosonic nature. In this work, we report direct evidence of the bosonic nature of SPPs in a scattering-based beam splitter. A parametric down-conversion source is used to produce two indistinguishable photons, each of which is converted into a SPP on a metal-stripe waveguide and then made to interact through a semitransparent Bragg mirror. In this plasmonic analog of the Hong-Ou-Mandel experiment, we measure a coincidence dip with a visibility of 72%, a key signature that SPPs are bosons and that quantum interference is clearly involved. Our work opens up possibilities for the study of fundamental quantum effects in plasmonic systems and their related applications.