Illuminating a gold nanoparticle with light, one can create so-called surface plasmons, coherent electron oscillations at the nanoparticle surface, and detect their presence by monitoring the power and spectrum of the reflected or transmitted light. These plasmons can be used for optical sensing, for example, of minute molecular adsorption, as the adsorption can cause changes in the plasmon spectrum. Such ultrasensitive sensing requires the use of nonintrusive (thus weak) probing light and a way to boost the signal-to-noise ratio. In this experimental paper, we demonstrate a “quantum spectroscopy” approach that uses entangled photon pairs to probe and read out plasmonic sensors, achieving sensing at a single-photon level and in the presence of a noise level 70 times larger than the signal.

The entangled photon pairs we use are generated through so-called spontaneous parametric down-conversion, so the sum of the frequencies of the two photons in each pair is a constant. We then direct one of the photons at our plasmonic sensor—a hexagonal array of gold nanoparticles—and detect it with a single-photon avalanche photodiode while sending the other photon through a monochromator for frequency selection and detecting it with another photodetector. Coincidence recordings on the two photodetectors and the monochromatic determination of the frequency of the second photon then allow us to measure the frequency shift of the first photon with cancellation of uncorrelated noise. The robustness of the method to noise is demonstrated by its detection of a frequency shift under the severe noise presence characterized by a 1/70 signal-to-noise ratio.

The technique we have presented here can be further developed into more advanced quantum spectroscopy setups for reliable ultrasensitive optical sensing.