This thesis investigates properties of quantum states of light while
travelling as surface plasmon polaritons in plasmonic waveguides
and structures. The bosonic nature of SPPs has been shown in previous
work by performing a Hong-Ou-Mandel interference experiment
with a plasmonic scattering-based beam splitter. Here, we
show the same interference with a higher statistical power thanks to
an improved set-up. A visibility of 59 ± 1 % is obtained in the two-photon
interference, clearly breaking the classical limit of 50 %. The
importance of the phase-relations between the different modes in the
beam splitter is experimentally probed. The output state of the interference
is then further analyzed by a quantum state tomography
set-up. This makes it possible to quantify the entanglement generated
in the interference. As the interference happens in the plasmonic
beam splitter, this shows, to our knowledge for the first time, entanglement
generated in a plasmonic structure. Together with the recent
results in terms of entanglement and coherence preservation of SPPs, this clearly shows the potential of quantum plasmonic devices. To
move into the realm of applications, we also investigate theoretically
the enhancements in sensitivity quantum states of light can deliver
for plasmonic sensing. It is shown that despite the losses, quantum
metrology techniques can be useful in an interferometer with plasmonic
waveguides. Considering the strengths and successes of
plasmonic sensing techniques in a wide range of fields, we envision
that entangled and squeezed states of light will become a new
route to push the limits in sensitivity.