  Photonic sensors have many applications in a range of physical settings, from
measuring mechanical pressure in manufacturing to detecting protein
concentration in biomedical samples. A variety of sensing approaches exist, and
plasmonic systems in particular have received much attention due to their
ability to confine light below the diffraction limit, greatly enhancing
sensitivity. Recently, quantum techniques have been identified that can
outperform classical sensing methods and achieve sensitivity below the
so-called shot-noise limit. Despite this significant potential, the use of
definite photon number states in lossy plasmonic systems for further improving
sensing capabilities is not well studied. Here, we investigate the sensing
performance of a plasmonic interferometer that simultaneously exploits the
quantum nature of light and its electromagnetic field confinement. We show
that, despite the presence of loss, specialised quantum resources can provide
improved sensitivity and resolution beyond the shot-noise limit within a
compact plasmonic device operating below the diffraction limit.
