  We present a quantum theory for the interaction of a two level emitter with
surface plasmon polaritons confined in single-mode waveguide resonators. Based
on the Green's function approach, we develop the conditions for the weak and
strong coupling regimes by taking into account the sources of dissipation and
decoherence: radiative and non-radiative decays, internal loss processes in the
emitter, as well as propagation and leakage losses of the plasmons in the
resonator. The theory is supported by numerical calculations for several
quantum emitters, GaAs and CdSe quantum dots and NV centers together with
different types of resonators constructed of hybrid, cylindrical or wedge
waveguides. We further study the role of temperature and resonator length.
Assuming realistic leakage rates, we find the existence of an optimal length at
which strong coupling is possible. Our calculations show that the strong
coupling regime in plasmonic resonators is accessible within current technology
when working at very low temperatures (<4K). In the weak coupling regime our
theory accounts for recent experimental results. By further optimization we
find highly enhanced spontaneous emission with Purcell factors over 1000 at
room temperature for NV-centers. We finally discuss more applications for
quantum nonlinear optics and plasmon-plasmon interactions.

Mesoscale and Nanoscale Physics;Optics;Quantum Physics