Quantum phenomenon observed: Atoms form organized structure from unorganized one

With a Bose-Einstein condensate of cesium atoms, scientists at the Institute for Experimental Physics of the University of Innsbruck have created one dimensional structures in an optical lattice of laser light. In these quantum lattices or wires the single atoms are aligned next to each other with laser light preventing them from breaking ranks. Delete using an external magnetic field allows the physicists to tune the interaction between the atoms with high precision and this set-up provides an ideal laboratory system for the investigation of basic physical phenomena. "Interaction effects are much more dramatic in low-dimensional systems than in three dimensional space," explains Hanns-Christoph Nägerl. Thus, these structures are of high interest for physicists. It is difficult to study quantum wires in condensed matter, whereas ultracold quantum gases provide a versatile tunable laboratory system. And these favorable experimental conditions open up new avenues to investigate novel fundamental phenomena in solid-state or condensed matter physics such as quantum phase transitions.

Quantum phase transition

The Innsbruck physicists have observed a "pinning transition" from a superfluid ("Luttinger liquid") to an insulated phase ("Mott-insulator"). In their experiment they showed that for strongly interacting atoms an additional weak lattice potential was sufficient to pin the atoms to fixed positions along the wire ("pinning"). The atoms were cooled down to nearly absolute zero and were in their quantum mechanical ground state. "It is not thermal fluctuations that induce the phase transition," stresses PhD student Elmar Haller, who is also first author of the study, which has been published in the journal Nature. "In fact, the atoms are already correlated due to strong repulsive interaction and only need a small push to align regularly along the optical lattice," explains Haller. When the lattice is removed, the atoms return to a superfluid state.

Theoretical prediction

The phenomenon observed by the experimental physicists was proposed by three theorists two years ago, two of whom -- Wilhelm Zwerger and Hans Peter Büchler -- also worked at the University of Innsbruck. This research work is funded by the Austrian Science Fund (FWF), the European Science Foundation (ESF) and by European Union research programs.