Ruprecht Karls Universität Heidelberg

Ultracold Quantum Gases Group

Our research group at the Physikalisches Institut of Heidelberg University performs basic 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.


Constructing Quantum Matter Atom by Atom

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 is the topic of a PRL Viewpoint written by Cindy Regal and Adam Kaufman from the University of Boulder, CO.

PhD thesis finished

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

Imaging with matter-wave Fourier optics

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.

Master's thesis finished

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!

From Few to Many

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.

More information can be found in the press release (in English or German) from the University of Heidelberg.

Furthermore, our publication is also discussed and explained for a broader audience, for instance in physicsworld and in a science blog.

Few-Body Systems: Cooler Than You Might Think

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.

Move to the new PI

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.

Retreat in Maria Waldrast with the Grimm group

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.

Link to the pictures.

Diploma thesis finished

The new BEC
The new BEC

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.

Fermionization of Two Distinguishable Fermions

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:
2 distinguishable vs. 2 identical atoms
2 distinguishable vs. 2 identical atoms

Paper on deterministic preparation of few fermion systems published

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.

Schematic preparation scheme of the tunable few-fermion system
Schematic preparation scheme of the tunable few-fermion system

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.

New Bubble Chamber at the MPI-K

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.

Another diploma thesis finished

Philipp Simon recently handed in his diploma thesis, which further characterizes the new MOT and Zeeman slower and can be found here:

Paper about Radio-Frequency Association of Efimov Trimers published

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.

Signal of the trimer association (red points) for different interaction strengths.
Signal of the trimer association (red points) for different interaction strengths.

New MOT / diploma thesis finished

The new MOT
The new MOT

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.

New Preprint Online

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.
Two-body loss coefficient of the I1>-I23> mixture vs. magnetic field: the red line is a fit according to universal theory.

First PhD thesis published

Congratulations Timo!
The thesis is available here:
Few-body physics in ultracold Fermi gases

Next diploma thesis finished

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.

now it’s micro: The microtrap is working!

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.

Shown is the optical density of the large volume dipole trap (a), the transfer between dipole trap and microtrap (b) and only the microtrap (c).
Shown is the optical density of the large volume dipole trap (a), the transfer between dipole trap and microtrap (b) and only the microtrap (c).

Another diploma thesis finished

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.

First paper published

Our work on three-component Fermi gases has been published in Physical Review Letters.
It is now available online at PRL 101, 203202.

Quark-Gluon Plasma meets Cold Atoms

The "Quark-Gluon Plasma meets Cold Atoms" introductory workshop at GSI in
Darmstadt took place on september 25th to 27th. Pictures are available online.
For more information visit the EMMI homepage.
Emmi workshop at GSI
Emmi workshop at GSI

next diploma thesis finished

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:

Workshopping trimers in Copenhagen!

Some of us are currently spending an enjoyable time at the Niels-Bohr-Institute in Copenhagen discussing ultracold atoms and Quark-Gluon Plasmas. As you can see on the picture, we share quite some excitement on trimers with the Innsbruck Cesium team!
workshopping trimers: Selim, Francesca and Andre
workshopping trimers: Selim, Francesca and Andre

Experiments on three-component Fermi gases

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.
a.) Magnetic field dependence of the remaining fraction of a three-state mixture
		b.) two-component mixture as a reference
		c.) scattering lengths in all possible channels
a.) Magnetic field dependence of the remaining fraction of a three-state mixture
b.) two-component mixture as a reference
c.) scattering lengths in all possible channels

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.

now it’s ultracold: Our first molecular BEC!

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°.

Our first molecular BEC
Our first molecular BEC

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.

second diploma thesis finished

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:

diploma thesis finished

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.

first atoms trapped

a further step towards a molecular BEC could be reached: The Magneto Optical Trap (MOT)

fluorescence of the trapped atoms
fluorescence of the trapped atoms

mot after some feeding
mot after some feeding