  Bose-Einstein condensation is a remarkable manifestation of quantum
statistics and macroscopic quantum coherence. Superconductivity and
superfluidity have their origin in Bose-Einstein condensation. Ultracold
quantum gases have provided condensates close to the original ideas of Bose and
Einstein, while condensation of polaritons and magnons have introduced novel
concepts of non-equilibrium condensation. Here, we demonstrate a Bose-Einstein
condensate (BEC) of surface plasmon polaritons in lattice modes of a metal
nanoparticle array. Interaction of the nanoscale-confined surface plasmons with
a room-temperature bath of dye molecules enables thermalization and
condensation in picoseconds. The ultrafast thermalization and condensation
dynamics are revealed by an experiment that exploits thermalization under
propagation and the open cavity character of the system. A crossover from BEC
to usual lasing is realized by tailoring the band structure. This new
condensate of surface plasmon lattice excitations has promise for future
technologies due to its ultrafast, room-temperature and on-chip nature.
