Few-Body Physics
The interaction of particles is a key element of Nature for building up nuclei, atoms, molecules, gaseous, liquid or solid states of matter, and even the system of planets with galaxies. Systems of few interacting particles are of particular interest as they often exhibit universal properties.

We perform theoretical research on the few-body physics of atomic, nuclear, and elementary particle systems. Of particular interest are:

Borromean rings The Borromean rings, heraldic symbol of the princes of Borromeo, are carved in stone at their castle on an island in Lago Maggiore in Northern Italy. The three rings are interlocked in such a way that if any of them were removed, the other two would also fall apart. The three-body quantum analogue is one where the three-body system is bound, but none of the binary subsystems. Ground state two-proton radioactivity (true three-body decay) was predicted by V. Goldansky in 1960 for nuclei beyond the proton dripline as an inclusive quantum-mechanical phenomenon. True three-body decay, in his terms, is a situation where the sequential emission of the particles is energetically prohibited and all the final state fragments are emitted simultaneously. Resonances in binary subsystems are located at higher energies than in the three-body system. This situation is in a sense similar to the Borromean property of bound halo nuclei situated in the vicinity of the neutron dripline. This type of radioactivity was first experimentally confirmed in 45Fe only in 2002, which makes it a very active field of experimental and theoretical studies.

Optical lattice The study of ultracold atom gases is one of the most rapidly advancing fields of contemporary physics. Following the experimental realization of a Bose-Einstein condensate in 1995, the field has ever since developed into a truly interdisciplinary area, involving quantum optics, nuclear, atomic and molecular physics, condensed matter physics and more recently, also nanoscience. Part of the vitality of the field is due to the fact that the systems investigated are very clean, both experimentally and theoretically. In addition, the fact that they can be investigated optically has made possible measurements with a precision difficult to match in other systems, and the systems may be manipulated by the use of magnetic and optical traps. Exciting observations, such as the very recent first evidence of an Efimov spectrum [Zaccanti et al., Nature Phys., vol. 5 (Aug 2009) p586], enables a direct test of universal properties of quantum few-body systems. Efimov states are named after the Russian physicist Vitaly Efimov, who predicted that three quantum particles subjected to a resonant pair-wise interaction can join into an infinite number of loosely bound states, even if each pair of particles cannot bind. The properties of these aggregates, such as the peculiar geometric scaling of their energy spectrum, are universal, that is, independent of the microscopic details of their components.