Surface plasma waves are typically quantized by direct analogy to electromagnetic waves in free space. As a result, the quantum theory of these waves predicts that their quanta—surface plasmons—should exhibit the same quantum phenomena that photons do.
Here we report on two experiments that test this analogy between photons and surface plasmons. The first is a plasmonic version of the Hong-Ou-Mandel experiment in which we observe two-photon quantum interference (TPQI) between plasmons with a visibility of 93%, comparable to what we observe using dielectrically-guided photons. To make this measurement, we produce pairs of single photons by spontaneous parametric down-conversion and couple them into low-loss silicon nitride waveguides that deliver them to and collect them from plasmonic directional couplers. This hybrid dielectric-plasmonic platform enables us to couple single photons into and out of plasmonic components with relatively high efficiency, resulting in high count rates and error bars of order 1%.
In the second experiment, we extend this platform to investigate path entanglement in circuits that involve plasmonic elements. We use TPQI at a dielectric 50-50 directional coupler to prepare a path-entangled two-photon state, then send the photons through plasmonic waveguides, and finally let them interfere at a second dielectric coupler to determine whether they remain entangled. Unlike previous experiments that converted polarization- or frequency-entangled photons into plasmons, in our experiment any information about the mere presence or absence of a plasmon could potentially distinguish between the components of the entangled state and cause decoherence. We have observed path entanglement in a dielectric circuit with 90% contrast and will discuss measurements in plasmonic circuits as well.