  The possibility of using weak optical signals to perform sensing of delicate
samples constitutes one of the main goals of quantum photonic sensing.
Furthermore, the nanoscale confinement of electromagnetic near fields in
photonic platforms through surface plasmon polaritons has motivated the
development of highly sensitive quantum plasmonic sensors. Despite the enormous
potential of plasmonic platforms for sensing, this class of sensors is
ultimately limited by the quantum statistical fluctuations of surface plasmons.
Indeed, the fluctuations of the electromagnetic field severely limit the
performance of quantum plasmonic sensing platforms in which delicate samples
are characterized using weak near-field signals. Furthermore, the inherent
losses associated with plasmonic fields levy additional constraints that
challenge the realization of sensitivities beyond the shot-noise limit. Here,
we introduce a protocol for quantum plasmonic sensing based on the conditional
detection of plasmons. We demonstrate that the conditional detection of
plasmonic fields, via plasmon subtraction, provides a new degree of freedom to
control quantum fluctuations of plasmonic fields. This mechanism enables
improvement of the signal-to-noise ratio of photonic sensors relying on
plasmonic signals that are comparable to their associated field fluctuations.
Consequently, the possibility of using weak plasmonic signals to sense delicate
samples, while preserving the sample properties, has important implications for
molecule sensing, and chemical detection .
