  We propose a quantum plasmonic sensor using Hong-Ou-Mandel (HOM)
interferometry that measures the refractive index of an analyte, embedded in a
plasmonic beam splitter composed of a dual-Kretschmann configuration, which
serves as a frustrated total internal reflection beamsplitter. The sensing
performance of the HOM interferometry, combined with single-photon detectors,
is evaluated through Fisher information for estimation of the refractive index
of the analyte. This is subsequently compared with the classical benchmark that
considers the injection of a coherent state of light into the plasmonic
beamsplitter. By varying the wavelength of the single photons and the
refractive index of the analyte, we identify a wide range where a 50 % quantum
enhancement is achieved and discuss the observed behaviors in comparison with
the classical benchmark. We expect this study to provide a useful insight into
the advancement of quantum-enhanced sensing technologies, with direct
implications for a wide range of nanophotonic beamsplitter structures.
