Researchers at Aalto University and the University of Eastern Finland have created a Bose-Einstein condensate in femtoseconds (fs) — a record-breaking time for producing this state of matter. Bose-Einstein condensation is a quantum phenomenon in which a large number of particles start to behave as if they were one. In the quantum, just as in the classical world, particles must first lose their energy in order to condense to the lowest possible energy state. This process typically takes from thousandths to trillionths of a second. The researchers combined a lattice of plasmonic nanoparticles with dye molecule solution at the strong coupling regime and pumped the molecules optically. The emitted light revealed three distinct regimes: one-dimensional lasing, incomplete stimulated thermalization, and two-dimensional multimode condensation. The sample (inside a glass side). Courtesy of Aaro Väkeväinen and Konstantinos Daskalakis/Aalto University. The researchers showed that formation of a condensate with a pronounced thermal distribution was possible at a 200-fs timescale. They attributed this fast thermalization to partially coherent dynamics due to highly stimulated processes and strong light-matter coupling. “This means that the effective interaction of photons, which leads into condensation, accelerates when the number of photons increases,” researcher Aaro Väkeväinen said. “Such a phenomenon is the key for the speed-up.” The condensate was achieved by matching the thermalization rate with the lattice size and occurred only for pump pulse durations below a critical value. The results of the experiments provide a method to control and monitor thermalization processes and condensate formation at a subpicosecond timescale. Many different systems have been used to observe Bose-Einstein condensates, but none have demonstrated the speed of the Finnish researchers’ condensate. It was challenging for the team to prove that condensation happened as quickly as it did, since even advanced lab cameras could not measure the time resolution. “When we pumped energy into the molecules in 50 fs, the condensate was observed. But with 300-fs pump pulse we did not see it, which indicated that the condensation must be triggered even faster,” researcher Antti Moilanen said. The intensity of the light emitted by the condensate shows a distribution in energy that matches the predictions by Bose and Einstein. The formation of the condensate occurred in a few hundreds of femtoseconds. Courtesy of Aalto University/Sofia Heikkinen. “The condensate produces a coherent light beam that is 100,000 times brighter than the first surface plasmon polariton condensate we observed in a metal nanorod array two years ago,” professor Päivi Törmä said. The large number of photons in the beam allowed the researchers to clearly observe the distribution of photons at different energies, as predicted by Bose and Einstein almost 100 years ago. “The brightness of the beam makes it easier to explore new areas of fundamental research and applications with these condensates,” Törmä said. An invention that emerged from the group’s condensate research has been granted a patent and will be further developed. The research was published in Nature Communications (www.doi.org/10.1038/s41467-020-16906-1).