The use of magnetic fields to manipulate the interactions of ultracold atoms at temperatures below one microkelvin has allowed the study of exotic phenomena in quantum physics. But until now, this research has mainly used simple atoms that have relatively simple interactions. Ferlaino and her team at the Institute for Experimental Physics in Innsbruck worked instead with a complex atomic species, erbium, and combined theory and experiment to demonstrate the signature of quantum chaos in the collisions between two erbium atoms. The researchers used a tightly focused laser to trap samples of around 100,000 erbium atoms at a temperature of about 400 nanokelvins.
They tuned an applied magnetic field to a fixed value between 0 and 7 millitesla and counted the number of atoms remaining after holding them in this "trap" for 400 milliseconds. The authors found that the number of atoms left in the trap depended strongly on the particular value of the magnetic field selected. Such atom losses are well known in cold-atom physics, and are used to locate features known as "Feshbach resonances". After varying the magnetic field in each experimental cycle and repeating the experiment 14,000 times, the physicists identified 200 resonances. "We were fascinated by how many resonances of this type we found. This is unprecedented in the physics of ultracold quantum gases", says team member Albert Frisch.
To explain the high density of resonances, the researchers used statistical methods, mainly random matrix theory. Nobel laureate Eugene Wigner formulated random matrix theory to describe complex systems in the 1950s. Today random matrix theory is applied not only in physics but also in number theory, wireless information technology and risk management models in finance. Now, the erbium team has experimentally shown chaotic behaviour of particles in a quantum gas. "For the first time, we have been able to observe quantum chaos in the scattering behaviour of ultracold atoms", says an excited Ferlaino. This work also provides new inroads to the investigation of ultracold gases and, thus, ultracold chemistry. Ferlaino is convinced: "Our work represents a turning point in the world of ultracold gases."
The experiment and statistical analysis were carried out at the Institute for Experimental Physics at the University of Innsbruck. Theoretical support was provided by John L. Bohn from the Joint Institute for Laboratory Astrophysics in Boulder, Colorado, USA and the team of Svetlana Kotochigova at Temple University in Philadelphia, Pennsylvania, USA. The Austrian researchers are supported by the Austrian Science Fund and the European Research Council ERC. (© University of Innsbruck, AcademiaNet)