  The concept of quantum plasmonic excitations in plasmonic lattices, which
similarly to Frenkel excitons in molecular crystals propagate by hopping from
one nanoparticle to another, is introduced. A consistent quantum theory of such
plasmonic excitons, beginning with the quantization of localized surface
plasmons in a metal nanoparticle and including the radiative relaxation in both
1D and 2D lattices near a reflective substrate surface, is developed. A
possible room-temperature Bose-Einstein condensation in the quantum gas of
plasmonic excitons is also discussed.
