We use frequency-entangled photons, generated via spontaneous parametric down conversion, to
measure the broadband spectral response of an array of gold nanoparticles exhibiting Fano-type plasmon
resonance. Refractive-index sensing of a liquid is performed by measuring the shift of the array resonance.
This method is robust in excessively noisy conditions compared with conventional broadband transmission
spectroscopy. Detection of a refractive-index change is demonstrated with a noise level 70 times higher
than the signal, which is shown to be inaccessible with the conventional transmission spectroscopy. Use
of low-photon fluxes makes this method suitable for measurements of photosensitive biosamples and