Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:


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.

Figure 1: A system of fermionic atoms in an optical lattice (top) is brought out of equilibrium and exhibits different dynamics for non-interacting (left) and interacting atoms (right). Grafik: MPQ

Ultracold atoms in optical lattices can be used as model systems that allow the study of analogues processes in a clean and well controlled environment 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 temperature 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 quantum 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 magnetic 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. Subsequently, the external confining field – responsible for the upwards bending of the lattice – is suddenly eliminated. The egg carton becomes flat and the particle 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 between the atoms by magnetic fields almost at will. This holds for the sign – attractive or repulsive – as well as for the strength of the interaction. In fact, the interaction can be switched off completely. In this case the atoms don’t ‘see’ each other. They move through the lattice unhindered, and their velocity depends on the lattice depth only. During this free expansion, the symmetry of the cloud changes from the spherical initial density distribution to a square symmetry that is governed by the symmetry of the lattice (figure 1, left).

As soon as there are small interactions present the atoms collide and ‘hinder’ each other, such that the expansion velocity of the cloud decreases. For larger interactions, more and more atoms ‘remain stuck’ in the core of the cloud, which remains spherical. For very strong interactions the dynamics of the high density core change qualitatively: the essentially frozen core dissolves by emitting particles and therefore shrinks in size, similarly to a melting ice cube (figure 1, right).
Surprisingly, only the magnitude, but not the sign of the interaction matters. The observed dynamics of the expansion is identical for repulsive and attractive interactions, as long as they are of the same strength. “This symmetry between attractive and repulsive interaction is an interesting feature of these lattice systems,” Dr. Ulrich Schneider, project leader at this experiment, explains. “In free space, interactions with opposite signs would give rise to opposite effects. Here they can lead to a quantum mechanical entanglement of distant atoms and allow the generation of either ‘normally’ or ‘repulsively’ bound particle pairs.”

Former experiments with fermionic atoms in optical lattices focused on the properties of systems in equilibrium. Here, on the contrary, the scientists observe the dynamics of the atoms in an out-of equilibrium system. These measurements are an important step towards a better understanding of the electronic motion in condensed matter. The physicists hope that this knowledge will lead to an explanation of complex phenomena in solid state physics and material science, and consequently to new tailored materials. [Olivia Meyer-Steng]

Original publication:
Ulrich Schneider, Lucia Hackermüller, Jens Philipp Ronzheimer, Sebastian Will, Simon Braun, Thorsten Best, Immanuel Bloch, Eugene Demler, Stephan Mandt, David Rasch and Achim Rosch

Fermionic transport and out-of-equilibrium dynamics in a homogeneous Hubbard model with ultracold atoms
Nature Physics 8,213-218 (2012), DOI: 10.1038/NPHYS2205 (AOP, 15 January 2012)

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
Dr. Ulrich Schneider
Fakultät für Physik
LMU Munich, Schellingstr. 4
80799 München, Germany,
Phone: +49 89 / 2180 -6129
Dr. Olivia Meyer-Streng
Press & Public Relations
Max Planck Institute of Quantum Optics
Press & Public Relations
Phone: +49 89 / 32905 -213

Dr. Olivia Meyer-Streng | Max-Planck-Institut
Further information:

More articles from Physics and Astronomy:

nachricht New NASA study improves search for habitable worlds
20.10.2017 | NASA/Goddard Space Flight Center

nachricht Physics boosts artificial intelligence methods
19.10.2017 | California Institute of Technology

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Neutron star merger directly observed for the first time

University of Maryland researchers contribute to historic detection of gravitational waves and light created by event

On August 17, 2017, at 12:41:04 UTC, scientists made the first direct observation of a merger between two neutron stars--the dense, collapsed cores that remain...

Im Focus: Breaking: the first light from two neutron stars merging

Seven new papers describe the first-ever detection of light from a gravitational wave source. The event, caused by two neutron stars colliding and merging together, was dubbed GW170817 because it sent ripples through space-time that reached Earth on 2017 August 17. Around the world, hundreds of excited astronomers mobilized quickly and were able to observe the event using numerous telescopes, providing a wealth of new data.

Previous detections of gravitational waves have all involved the merger of two black holes, a feat that won the 2017 Nobel Prize in Physics earlier this month....

Im Focus: Smart sensors for efficient processes

Material defects in end products can quickly result in failures in many areas of industry, and have a massive impact on the safe use of their products. This is why, in the field of quality assurance, intelligent, nondestructive sensor systems play a key role. They allow testing components and parts in a rapid and cost-efficient manner without destroying the actual product or changing its surface. Experts from the Fraunhofer IZFP in Saarbrücken will be presenting two exhibits at the Blechexpo in Stuttgart from 7–10 November 2017 that allow fast, reliable, and automated characterization of materials and detection of defects (Hall 5, Booth 5306).

When quality testing uses time-consuming destructive test methods, it can result in enormous costs due to damaging or destroying the products. And given that...

Im Focus: Cold molecules on collision course

Using a new cooling technique MPQ scientists succeed at observing collisions in a dense beam of cold and slow dipolar molecules.

How do chemical reactions proceed at extremely low temperatures? The answer requires the investigation of molecular samples that are cold, dense, and slow at...

Im Focus: Shrinking the proton again!

Scientists from the Max Planck Institute of Quantum Optics, using high precision laser spectroscopy of atomic hydrogen, confirm the surprisingly small value of the proton radius determined from muonic hydrogen.

It was one of the breakthroughs of the year 2010: Laser spectroscopy of muonic hydrogen resulted in a value for the proton charge radius that was significantly...

All Focus news of the innovation-report >>>



Event News

ASEAN Member States discuss the future role of renewable energy

17.10.2017 | Event News

World Health Summit 2017: International experts set the course for the future of Global Health

10.10.2017 | Event News

Climate Engineering Conference 2017 Opens in Berlin

10.10.2017 | Event News

Latest News

NRL clarifies valley polarization for electronic and optoelectronic technologies

20.10.2017 | Interdisciplinary Research

Metallic nanoparticles will help to determine the percentage of volatile compounds

20.10.2017 | Materials Sciences

Shallow soils promote savannas in South America

20.10.2017 | Earth Sciences

More VideoLinks >>>