  The control of quantum states of light at the nanoscale has become possible
in recent years with the use of plasmonics. Here, many types of nanophotonic
devices and applications have been suggested that take advantage of quantum
optical effects, despite the inherent presence of loss. A key example is
quantum plasmonic sensing, which provides sensitivity beyond the classical
limit using entangled N00N states and their generalizations in a compact system
operating below the diffraction limit. In this work, we experimentally
demonstrate the excitation and propagation of a two-plasmon entangled N00N
state (N=2) in a silver nanowire, and assess the performance of our system for
carrying out quantum sensing. Polarization entangled photon pairs are converted
into plasmons in the silver nanowire, which propagate over a distance of 5 um
and re-convert back into photons. A full analysis of the plasmonic system finds
that the high-quality entanglement is preserved throughout. We measure the
characteristic super-resolution phase oscillations of the entangled state via
coincidence measurements. We also identify various sources of loss in our setup
and show how they can be mitigated, in principle, in order to reach
super-sensitivity that goes beyond the classical sensing limit. Our results
show that polarization entanglement can be preserved in a plasmonic nanowire
and that sensing with a quantum advantage is possible with moderate loss
present.
