  A canonical quantization scheme for localized surface plasmons (LSPs) in a
metal nanosphere is presented based on a microscopic model composed of
electromagnetic fields, oscillators that describe plasmons, and a reservoir
that describes excitations other than plasmons. The eigenmodes of this fully
quantum electrodynamic theory show a spectrum that includes radiative
depolarization and broadening, including redshifting from the quasi-static LSP
modes, with increasing particle size. These spectral profiles correctly match
those obtained with exact classical electrodynamics (Mie theory). The present
scheme provides the electric fields per plasmon in both near- and far-field
regions whereby its utility in the fields of quantum plasmonics and nano-optics
is demonstrated.
