  Quantum plasmonics experiments have on multiple occasions reported the
observation of quantum coherence of discrete plasmons, which exhibit remarkable
preservation of quantum interference visibility, a seemingly surprising feature
for systems mixing light and matter with high ohmic losses during propagation.
However, most experiments to date used essentially weakly-confined plasmons,
which experience limited light-matter hybridization, thus limiting the
potential for decoherence. Here, we report quantum coherence of plasmons near
the surface plasmon polariton (SPP) resonance frequency, where plasmonic
dispersion and confinement is much stronger than in previous experiments. We
generated polarization-entangled pairs of photons using spontaneous parametric
down conversion and transmitted one of the photons through a plasmonic hole
array designed to convert incident single photons into highly-dispersive single
SPPs. We find the quality of photon entanglement after the plasmonic channel to
be unperturbed by the introduction of a highly dispersive plasmonic element.
Our findings provide a lower bound of 100 femtoseconds for the pure dephasing
time for dispersive plasmons in gold, and show that even in a highly dispersive
regime surface plasmons preserve quantum mechanical correlations, making
possible harnessing the power of extreme light confinement for integrated
quantum photonics.
