Ruprecht Karls Universität Heidelberg

Ultracold Quantum Gases Group

Our research group at the Physikalisches Institut of Heidelberg University performs fundamental research in the fields of quantum and atomic physics. In our experiments, we use ultracold atom clouds to understand how complex quantum systems behave. In particular, we are interested in questions related to how strong interactions, reduced dimensionality (1D and 2D) and finite system size affects the physical properties of a quantum system. More information on current research projects and the experimental setups can be found here.





Equation of State of Ultracold Fermions in the 2D BEC-BCS Crossover Region

last modified on February 03 2016 16:16

Our most recent paper in collaboration with Igor Boettcher and Tilman Enss shows the equation of state of ultracold fermions in the 2D BEC-BCS crossover region. It was published this week in Physical Review Letters (Physical Review Letters 116, 045303 (2016), also available at arXiv:1509.03610), with a Viewpoint in Physics by Meera Parish.


Christmas Party

last modified on December 22 2015 14:47

We want to thank all our collaborators and colleagues for a very successful year and wish a merry Christmas and a happy New Year.

Antiferromagnetic Heisenberg spin chains

last modified on November 20 2015 10:20

In our most recent publication, we prepared Heisenberg spin chains of up to four atoms in their antiferromagnetic ground state. Without the need for an external periodic potential, these spin chains are stabilized by strong repuslive interactions and a one-dimensiional trapping geometry. This work constitutes the first observation of quantum magnetism with ultracold fermions that exceeds nearest-neighbor correlations.
The paper has been recently published in Physical Review Letters (PRL 115, 215301 (2015), also available at arXiv:1507.01117) and was selected for an Editors' Suggestion.

Ground-state systems of up to four atoms (shown in green and blue) are prepared in a strongly-elongated optical dipole trap (red) in which they can move in one dimension only. When the atoms do not interact, they can move freely in the trap (top picture) and they have no discernible order. When repulsive interactions between the atoms become strong (bottom picture), they can not pass each other anymore and independently arrange themselves in an antiferromagnetic spin chain.

Group retreat at Trifels

last modified on August 3 2015

We recently held a group retreat in Annweiler near the famous castle Trifels in the Pfalz. Experimental methods for wood-cutting and campfire-making were established. Pictures can be found in the image gallery



Rock climbing at Trifels

Superfluidity and long-range coherence in a 2D Fermi gas

last modified on July 01 2015 13:09

In our recent experiments, we observed the Berezinskii-Kosterlitz-Thouless (BKT) phase transition in a 2D Fermi gas. We have measured, for the first time, the non-local correlations of a many-body system. We saw the onset of power-law behavior at low temperatures. More interestingly, we measured the scaling exponents and observed that the inhomogeneity of the system leads to a change of the universal critical exponents predicted by the BKT Theory.
The results of this work have been recently published in Physical Review Letters 115, 010401 (2015) (also available at arXiv:1505.02123). The paper was also selected for an Editors' Suggestion!



Image: An artist's impression of the complex phase of a two-dimensional superfluid which is described by the Berezinskii-Kosterlitz-Thouless (BKT) theory. The two most prominent features of BKT theory are spatial phase fluctuations and bound pairs of topological vortices. The phase fluctuations have a peculiar spectrum which leads to algebraically decaying correlations, which are measured in this work for a fermionic superlfuid. Above a critical temperature the vortex pairs unbind and the individual vortices disorder the system such that superfluidity is lost.



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