  We investigate the dynamics of a plasmonic oscillation over a metal
nanoparticle when it is strongly coupled to a quantum emitter (e.g. quantum
dot, molecule). We simulate the density matrix evolution for a simple model;
coupled classical--quantum oscillators system. We show that lifetime of the
plasmonic oscillations can be increased several orders of magnitude, upto the
decay time of the quantum emitter. This effect shows itself as the narrowing of
the plasmon emission band in the spaser (surface plasmon amplification by
stimulated emission of radiation) experiment [{\it Nature}, {\bf 2009}, 460,
1110], where a gold nanoparticle interacts with the surrounding molecules.
Enhancement of the plasmonic excitation lifetime enables stimulated emission to
overcome the spontaneous one. The enhancement occurs due to the emergence of a
phenomenon analogous to electromagnetically induced transparency (EIT). The
effect can find applications in many areas of nanoscale physics, such as in
quantum information with plasmons and in increasing solar cell efficiency.
