Building a Quantum Society: Singles, Pairs, Trimers and even more

Recently, ultracold atomic and molecular gases emerged as an ideal playground to study and emulate a variety of systems from condensed matter to nuclear physics. In many quantum phenomena, the interaction between particles plays a fundamental role, acting as a binding agent and determining the “social” structure of particle ensembles. Remarkably, in ultracold gases the interaction can be externally controlled almost at will. I will discuss some of our recent experiments based on the two types of inter- action available in ultracold gases: the contact (short-range) and dipolar (long- range) interaction. As an example of resonant contact interaction, I will tell the story of a young Russian nuclear physicist, Vitaly Efimov, who in 1970 proposed the existence of a geometric series of “giant’” trimer states as a solution to the quantum three-body problem. I will tell how his predictions, first received with skepticism, have been demonstrated in experiments. After three decades of searching in various systems, Efimov’s ideas have come to life in tunable ultracold gases; first in Innsbruck and then in many other labs around the world. Regar- ding dipolar interaction, I will report on the first achievement of a Bose-Einstein condensate of strongly magnetic erbium atoms and on its spectacular dipolar- mediated collapse.