  We investigate the use of twin-mode quantum states of light with symmetric
statistical features in their photon number for improving intensity-sensitive
surface plasmon resonance (SPR) sensors. For this purpose, one of the modes is
sent into a prism setup where the Kretschmann configuration is employed as a
sensing platform and the analyte to be measured influences the SPR excitation
conditions. This influence modifies the output state of light that is
subsequently analyzed by an intensity-difference measurement scheme. We show
that quantum noise reduction is achieved not only as a result of the
sub-Poissonian statistical nature of a single mode, but also as a result of the
non-classical correlation of the photon number between the two modes. When
combined with the high sensitivity of the SPR sensor, we show that the use of
twin-mode quantum states of light notably enhances the estimation precision of
the refractive index of an analyte. With this we are able to identify a clear
strategy to further boost the performance of SPR sensors, which are already a
mature technology in biochemical and medical sensing applications.
