Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

A Traffic Jam of Quantum Particles

05.03.2012
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)

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
Dr. Ulrich Schneider
Fakultät für Physik
LMU Munich, Schellingstr. 4
80799 München, Germany,
Phone: +49 89 / 2180 -6129
E-mail: ulrich.schneider@lmu.de
Dr. Olivia Meyer-Streng
Press & Public Relations
Max Planck Institute of Quantum Optics
Press & Public Relations
Phone: +49 89 / 32905 -213
E-mail: olivia.meyer-streng@mpq.mpg.de

Dr. Olivia Meyer-Streng | Max-Planck-Institut
Further information:
http://www.quantum-munich.de

More articles from Physics and Astronomy:

nachricht Basque researchers turn light upside down
23.02.2018 | Elhuyar Fundazioa

nachricht Attoseconds break into atomic interior
23.02.2018 | Max-Planck-Institut für Quantenoptik

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: Attoseconds break into atomic interior

A newly developed laser technology has enabled physicists in the Laboratory for Attosecond Physics (jointly run by LMU Munich and the Max Planck Institute of Quantum Optics) to generate attosecond bursts of high-energy photons of unprecedented intensity. This has made it possible to observe the interaction of multiple photons in a single such pulse with electrons in the inner orbital shell of an atom.

In order to observe the ultrafast electron motion in the inner shells of atoms with short light pulses, the pulses must not only be ultrashort, but very...

Im Focus: Good vibrations feel the force

A group of researchers led by Andrea Cavalleri at the Max Planck Institute for Structure and Dynamics of Matter (MPSD) in Hamburg has demonstrated a new method enabling precise measurements of the interatomic forces that hold crystalline solids together. The paper Probing the Interatomic Potential of Solids by Strong-Field Nonlinear Phononics, published online in Nature, explains how a terahertz-frequency laser pulse can drive very large deformations of the crystal.

By measuring the highly unusual atomic trajectories under extreme electromagnetic transients, the MPSD group could reconstruct how rigid the atomic bonds are...

Im Focus: Developing reliable quantum computers

International research team makes important step on the path to solving certification problems

Quantum computers may one day solve algorithmic problems which even the biggest supercomputers today can’t manage. But how do you test a quantum computer to...

Im Focus: In best circles: First integrated circuit from self-assembled polymer

For the first time, a team of researchers at the Max-Planck Institute (MPI) for Polymer Research in Mainz, Germany, has succeeded in making an integrated circuit (IC) from just a monolayer of a semiconducting polymer via a bottom-up, self-assembly approach.

In the self-assembly process, the semiconducting polymer arranges itself into an ordered monolayer in a transistor. The transistors are binary switches used...

Im Focus: Demonstration of a single molecule piezoelectric effect

Breakthrough provides a new concept of the design of molecular motors, sensors and electricity generators at nanoscale

Researchers from the Institute of Organic Chemistry and Biochemistry of the CAS (IOCB Prague), Institute of Physics of the CAS (IP CAS) and Palacký University...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

VideoLinks
Industry & Economy
Event News

2nd International Conference on High Temperature Shape Memory Alloys (HTSMAs)

15.02.2018 | Event News

Aachen DC Grid Summit 2018

13.02.2018 | Event News

How Global Climate Policy Can Learn from the Energy Transition

12.02.2018 | Event News

 
Latest News

Basque researchers turn light upside down

23.02.2018 | Physics and Astronomy

Finnish research group discovers a new immune system regulator

23.02.2018 | Health and Medicine

Attoseconds break into atomic interior

23.02.2018 | Physics and Astronomy

VideoLinks
Science & Research
Overview of more VideoLinks >>>