  Reducing the noise below the shot-noise limit in sensing devices is one of
the key promises of quantum technologies. Here, we study quantum plasmonic
sensing based on an attenuated total reflection configuration with single
photons as input. Our sensor is the Kretschmann configuration with a gold film,
and a blood protein in an aqueous solution with different concentrations serves
as an analyte. The estimation of the refractive index is performed using
heralded single photons. We also determine the estimation error from a
statistical analysis over a number of repetitions of identical and independent
experiments. We show that the errors of our plasmonic sensor with single
photons are below the shot-noise limit even in the presence of various
experimental imperfections. Our results demonstrate a practical application of
quantum plasmonic sensing is possible given certain improvements are made to
the setup investigated, and pave the way for a future generation of quantum
plasmonic applications based on similar techniques.
