  The strong interaction of individual quantum emitters with resonant cavities
is of fundamental interest for understanding light matter interactions.
Plasmonic cavities hold the promise of attaining the strong coupling regime
even under ambient conditions and within subdiffraction volumes. Recent
experiments revealed strong coupling between individual plasmonic structures
and multiple organic molecules, but so far strong coupling at the limit of a
single quantum emitter has not been reported. Here we demonstrate vacuum Rabi
splitting, a manifestation of strong coupling, using silver bowtie plasmonic
cavities loaded with semiconductor quantum dots (QDs). A transparency dip is
observed in the scattering spectra of individual bowties with one to a few QDs,
directly observed in their gaps. A coupling rate as high as 120 meV is
registered even with a single QD, placing the bowtie-QD constructs close to the
strong coupling regime. These observations are verified by
polarization-dependent experiments and validated by electromagnetic
calculations.
