  The theoretical method to study strong coupling between an ensemble of
quantum emitters (QEs) and surface plasmons excited by the nanoparticle cluster
has been presented by using a rigorous first-principles electromagnetic Green's
tensor technique. We have demonstrated that multi-qubit entanglement for
two-level QEs can be produced at different frequencies simultaneously, when
they locate in hot spots of metallic nanoparticle clusters. The duration of
quantum beats for such an entanglement can reach two orders longer than that
for the entanglement in a photonic cavity. The phenomenon originates from
collective coupling resonance excitation of the cluster. At the frequency of
single scattering resonance, the entanglement cannot be produced although the
single QE spontaneous decay rate is very big
