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

Optical signals are increasingly used to transfer and process information, because of their high speed, bandwidth, and propagation distances in transparent optical networks. However, the miniaturization of photonic devices faces a hard block: the diffraction limit. One proposed means to go beyond this limit is to use surface plasmons polaritons (SPPs), charge density waves at the interface between a metal and a dielectric, which can be excited with light, as the carrier of the information [1]. It has been demonstrated that SPPs can be confined in sub-wavelength volumes. So far, the study of plasmons has been confined to the classical regime, but recently more attention is given to the quantum regime. Indeed, several interesting devices taking advantage of the large field confinement provided by SPPs have been proposed, such as single photon switches or transistors [2]. However, at this stage, little is known about the behaviour of SPPs in the quantum regime, i.e. when they are excited one at a time for instance. In this presentation, we report direct evidence of the bosonic nature of SPPs in a scattering-based beamsplitter, in one of the most simple experiments of quantum optics - done using SPPs instead of photons propagating in air. 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 [3].