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.
In our 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 recently been published in Physical Review Letters (PRL 115, 215301 (2015), also available at arXiv:1507.01117) and was selected for an Editors' Suggestion.
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
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.
Our latest paper was recently published in Physical Review Letters 114, 230401 (also available at arXiv:1409.5373). Here, we investigate a two-dimensional Fermi gas in the BEC-BCS crossover. We measure the pair momentum distribution of the gas by means of our matter wave focusing technique and observe a clear signature for condensation at low temperatures. By tuning the temperature as well as the interaction strength in the gas, we are able to map out the phase diagram of the quasi-2D Fermi gas in the BEC-BCS crossover.
The paper received PRL's "Editors' Suggestion" and was also featured in a nicely written Viewpoint in Physics by Pierbiagio Pieri from the University of Camerino.
More information can be found in the press release from Heidelberg University: English version, German version
The article was also featured on the Facebook page of Physics Today.
In this publication published in Physical Review Letters ( PRL 114, 080402 or also arXiv:1410.8784), we realize a simple system consisting of a pair of atoms in a two-site “minicrystal” and reproduce the physics of a widely used model of electrons in a solid. By combining many such “minicrystal” we aim for the deterministic preparation of larger systems and new insights into complex quantum many-body physics.
The publication was selected for a Viewpoint in Physics, which was written by Cindy Regal and Adam Kaufman from the University of Boulder, CO. It was also featured as a Brennpunkt in the Physik Journal of the DPG, written by Dieter Jaksch.
More information can be found in the press release (in English or German) from the University of Heidelberg.
Martin recently finished his PhD thesis. It describes the preparation of a 2D Fermi gas in the BEC-BCS crossover, the observation of pair condensation in this system and the measurement of its phase diagram.
The thesis is now available online: A Two-Dimensional Fermi Gas in the BEC-BCS Crossover
In our recent paper published in Physical Review A (also available at arXiv:1408.4680), we describe a simple technique to measure the momentum distribution of a strongly interacting quantum gas. We have used well known concepts from Fourier optics to investigate three operations on the gas: focusing, collimation and magnification. When we apply these techniques in our experiment, we see a prominent occupation of low momentum states that was not visible in the in-trap density distribution.
Sebastian finished his Master's thesis on the BKT-phase transition in a strongly interacting 2d Bose gas. We're very sad to see you go and wish you all the best, Sebastian!
In our recent paper which was published in the current issue of Science 342, 457 (2013) (also available at arXiv:1307.3443), we study the formation of a Fermi sea one atom at a time. We investigate the crossover from few to many-body physics in a quasi one-dimensional system using a single impurity atom which interacts with an increasing number of majority atoms that are in a different state. We could show that for the investigated quantum impurity systems as few as four particles are enough to describe the system with the many-body theory.
Our work on finite fermionic systems was recently covered in a science blog by Chad Orzel. The article nicely summarizes and explains why few-body systems are interesting and what we are able to do with these kind of systems.
In December last year we moved our experiments from the MPIK in Heidelberg to the Physikalisches Institut, also in Heidelberg. In this new building, the rooms for our labs had just been finished before. We decided to keep the optics and vacuum chambers mounted on the tables for the move. This allowed us to start up our experiments quickly: As of now, one experiment is running as good as before the move, the other experiment is steadily getting there.
We just went on a short retreat with the Grimm group from Innsbruck. The two days at the monastry Maria Waldrast in the Austrian alps were full of interesting discussions, beautiful landscapes and nice hiking.
Johanna Bohn recently finished her diploma thesis, which describes the setup of our new dipole trap and the molecular BEC we are able to create in this trap.
In our latest paper we make two distinguishable atoms behave as if they were fermions, and verify this by comparing them to two identical fermionic atoms. The paper has been selected as an editor's suggestion by PRL, and was highlighted by a Physics synopsis:
We sucessfully prepared a tunable quantum system consisting of one to ten fermions. We prepare such a system using ultracold Lithium atoms in a micrometer sized optical dipole trap. Due to Pauli's principle one quantum state in the trap is occupied with one atom per spin state (red and blue balls). By tilting the trap we can control the number of remaining quantum states and thus the number of particles.
The paper is published in the current issue of Science. The ArXiv version can be found here and there is also a press release (in German) by the University of Heidelberg.
The ultracold quantum gases group proudly present its new bubble chamber implemented at the Max-Planck Institute. Due to space constraints it has been temporarily placed inside Selim's office.
Philipp Simon recently handed in his diploma thesis, which further characterizes the new MOT and Zeeman slower and can be found here:
We have associated an univeral three-body bound state, so called Efimov state, and measured its binding energy as a function of interaction strength with radio-frequency spectroscopy.
The paper has been published in the current issue of Science. The arxiv version can be found here.
We recently finished the construction of a new magneto-optical trap (MOT). With loading rates of 3·10^8 and higher, it will serve as a fast and efficient precooling stage for future experiments. The background collision limited lifetime of atoms in the trap is approximately 23 minutes.
More details on the apparatus can be found in Martin Ries' diploma thesis, which was handed in recently.
It is now available online: diplomarbeit_martin.pdf.
We have conducted a study of atom-dimer scattering in an ultracold three-component Fermi gas consisting of 6Li atoms in three different hyperfine states |1>,|2> and |3>.
For this we have prepared three different mixtures of atoms and dimers and observed their decay via inelastic atom-dimer collisions. In the |1>-|23> atom-dimer mixture we have observed resonant enhancement of the loss at the crossings of two Efimov-like trimer states with the |1>-|23> atom-dimer threshold. We have also found that these trimer states can cause a suppression of |3>-|12> → |1>-|23> exchange reactions in the |3>-|12> mixture, which allows to control the rate constants for these processes.
For more details on these results see arXiv:1003.0600
Two-body loss coefficient of the I1>-I23> mixture vs. magnetic field: the red line is a fit according to universal theory..
The thesis is available here:
Few-body physics in ultracold Fermi gases
Recently Gerhard Zürn handed in his diploma thesis which is about the
"Realization of an Optical Microtrap for a Highly Degenerate Fermi Gas" .
It is now available online: thesis-gerhard.pdf.
In the last few weeks we accomplished another milestone on our way to a mesoscopic Fermi system the realization of a small volume optical dipole trap with high trapping frequencies ("microtrap"). More information will be provided in the diploma thesis of G. Zürn which will appear in a few weeks.
Andre Wenz just handed in his diploma thesis with the title
"Few-Body Physics in a Three-Component Fermi Gas".
It is now available online at: thesis-andre.pdf.
Our work on three-component Fermi gases has been published in Physical Review Letters.
It is now available online at PRL 101, 203202.
The next diploma thesis done in our group was just handed in by Matthias Kohnen.
The title is: "Ultracold Fermi Mixtures in an Optical Dipole Trap"
and it is now available online at:
Recently we managed to prepare a three-component degenerate Fermi gas of 6Li with 50000 atoms in each state at a temperature of 215 nK, which corresponds to T/TF=0.37.
We studied the collisional stability of the gas in the magnetic field range from 0 to 750 G and found a broad and pronounced loss feature at 130 G. At approximately 580 G the sample remains quite stable, with 1/e lifetimes exceeding 30 s.
For a more quantitative analysis we deduced the three-body loss coefficient K3 from lifetime measurements.
The new results are available at arXiv:0806.0587.
We have reached condensation of 80000 6Li2 molecules in a dipole trap using an all-optical approach.
After being precooled in our magneto-optical trap (MOT) the 6Li atoms are transferred into the optical dipole trap, for which we use a 200 W fibre laser. The trap is established by two focussed beams with a waist of 50 µm crossing under an angle of about 7°.
Picture: Bimodal (condensate and thermal cloud) distributions at different stages of evaporation with final laser power between 120 - 61 mW. Before taking the image (after a time of flight of 10ms) the laser power is adiabatically ramped back to 120mW for all images.
the second diploma thesis (from Thomas Lompe) with the title:
"An apparatus for the production of molecular Bose-Einstein condensates"
was just finished. And is now available online:
The first diploma thesis with the title
"The setup of a Magneto Optical Trap for the preparation of a mesoscopic degenerate Fermi gas"
is now online.
a further step towards a molecular BEC could be reached: The Magneto Optical Trap (MOT)