Two-plasmon quantum interference

Surface plasma waves on metals arise from the collective oscillation of many free electrons in unison. These waves are usually quantized by direct analogy to electromagnetic fields in free space(1-3), with the surface plasmon, the quantum of the surface plasma wave, playing the same role as the photon. It follows that surface plasmons should exhibit all the same quantum phenomena that photons do. Here, we report a plasmonic version of the Hong-Ou-Mandel experiment(4), in which we observe unambiguous two-photon quantum interference between plasmons, confirming that surface plasmons faithfully reproduce this effect with the same visibility and mutual coherence time, to within measurement error, as in the photonic case. These properties are important if plasmonic devices are to be employed in quantum information applications(5), which typically require indistinguishable particles.