  We investigate the quantum optical properties of a single photon emitter
coupled to a finite-size metal nanoparticle using a photon Green function
technique that rigorously quantizes the electromagnetic fields. We first obtain
pronounced Purcell factors and photonic Lamb shifts for both a 7-nm and 20-nm
radius metal nanoparticle, without adopting a dipole approximation. We then
consider a quantum-dot photon emitter positioned sufficiently near to the metal
nanoparticle so that the strong coupling regime is possible. Accounting for
non-dipole interactions, quenching, and photon transport from the dot to the
detector, we demonstrate that the strong coupling regime should be observable
in the far-field spontaneous emission spectrum, even at room temperature. The
emission spectra show that the usual vacuum Rabi doublet becomes a rich
spectral triplet or quartet with two of the four peaks anticrossing, which
survives in spite of significant non-radiative decays. We discuss the emitted
light spectrum and the effects of quenching for two different dipole
polarizations.
