Observing the Birth of Quasiparticles in Real Time

The formation of quasiparticles, such as polarons, in a condensed-matter system usually proceeds in an extremely fast way and is very difficult to observe. In Innsbruck, Rudolf Grimm's physics research group, in collaboration with an international team of theoretical physicists, has simulated the formation of polarons in an ultracold quantum gas in real time. The concept of quasiparticles is a powerful tool to describe processes in many-body quantum systems, such as solid-state materials. For example, when an electron moves through a solid, it generates polarization in its environment because of its electrical charge. This "polarization cloud" moves together with the electron and the resulting "dressed electron" can be theoretically described as quasiparticle or a polaron. "You could picture it as a skier on a powder day," says Grimm. "The skier is surrounded by a cloud of snow crystals.