MAX PLANCK INSTITUTE OF QUANTUM OPTICS Garching, 5 March 2012 Press Release

A Traffic Jam of Quantum Particles LMU/MPQ-scientists discover surprising transport phenomena in

ultracold quantum many body systems. Transport properties such as thermal or electrical conductivity are of great importance for technical applications of materials. In particular the electrical conductivity stems from the behaviour of the electrons in the solid and is very difficult to predict. This is true especially in the case of strongly correlated electrons, when the position and the dynamics of each single electron is strongly influenced by the behaviour of all other electrons. Ultracold atoms in optical lattices can be used as model systems that al-low the study of analogues processes in a clean and well controlled envi-ronment where all relevant parameters can be manipulated by external lasers and magnetic fields. Scientists in the group of Professor Immanuel Bloch (Ludwig-Maximilians-Universität Munich and Max Planck Institute of Quantum Optics, Garching) in collaboration with the theory group of Prof. Achim Rosch (University of Cologne) have now demonstrated that the dynamics of a system of ultracold potassium atoms, trapped in an optical lattice, depend surprisingly strongly on the particle interaction strength (Nature Physics 8, 213-218 (2012), DOI: 10.1038/NPHYS2205). Investigations of this kind give new insights into properties like electrical conductivity, superconductivity or magnetism, and may help to develop materials with ‘tailored’ properties. So-called optical lattices are generated by superimposing several laser beams. The resulting periodic structure of light resembles the geometry of simple solid state crystals. In fact, atoms trapped in such an artificial lattice, at a tempera-ture of a few nano-Kelvin above absolute zero, experience forces similar to the ones that act on electrons in solid state systems. However, concerning their dynamics, only fermionic atoms behave exactly the same way as electrons, which are fermions as well. These particles have to differ in at least one quan-tum property if they happen to be at the same place at the same time. Bosonic particles, on the other hand, prefer to gather in exactly the same quantum state.

In the experiment, atoms of the fermionic isotope potassium-40 are cooled down to an extremely low temperature with the help of laser beams and mag-netic fields. Then they are loaded into an optical lattice as described above. Initially, the edges of the egg carton-like lattice structure are bent upwards (see figure 1, the colours red and green represent different spin states of the atoms) and the particles sit in the centre with a constant density distribution. Subse-quently, the external confining field – responsible for the upwards bending of the lattice – is suddenly eliminated. The egg carton becomes flat and the parti-cle cloud starts to expand. Now the physicists monitor exactly how the density distribution changes during the expansion.

An important feature of this experimental setup is the use of a so-called Feshbach resonance, which makes it possible to change the interaction be-tween the atoms by magnetic fields almost at will. This holds for the sign – at-

Press & Public Relations Dr. Olivia Meyer-Streng Phone: +49 - 89 / 32 905-213 E-mail: olivia.meyer-streng@mpq.mpg.de

Hans-Kopfermann-Str. 1 D-85748 Garching Phone:+49 - 89 / 32 905-0 Fax:+49 - 89 / 32 905-200

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Contact: Prof. Dr. Immanuel Bloch Chair of Quantum Optics LMU Munich, Schellingstr. 4 80799 München, Germany, and Max Planck Institute of Quantum Optics Hans-Kopfermann-Straße 1 85748 Garching b. München Phone: +49 89 / 32905 -138 E-mail: immanuel.bloch@mpq.mpg.de www.quantum-munich.de Dr. Ulrich Schneider Fakultät für Physik LMU Munich, Schellingstr. 4 80799 München, Germany, Phone: +49 89 / 2180 -6129 E-mail: ulrich.schneider@lmu.de www.quantum-munich.de