Quantum dynamics of atomic and molecular systems

Our group studies atomic and molecular quantum systems with respect to their interactions on different levels of complexity. Of special importance is the application and extension of modern methods for the manipulation and quantum control to many-body quantum systems, in particular using coherent light. The systems under investigation range from highly excited Rydberg atoms over atomic and molecular quantum gases to molecular aggregates. The group develops technologies for trapping and cooling of neutral atoms as well as quantum-state sensitive diagnostics.

Latest news from the lab

Analyzing Feshbach resonances: A Li-Cs case study published in Physical Review A01.08.2014

We comprehensively compare three different models for the description of Feshbach resonances: The coupled-channel calculation, the asymptotic bound state model (ABM), and the multichannel quantum defect theory (MQDT). All models describe our previously measured Li-Cs Feshbach resonances accurately. This work demonstrates on the example of Li-Cs, how measured Feshbach resonances can be interpreted by use of simple models.

Since an exact analytical solution of the Schrödinger equation for the collision of two ultracold alkali atoms is not possible, assumptions have to be implemented in order to facilitate the calculation. One model, namely the ABM, which applies such assumptions, did not agree with our experimental findings (see [Repp et al., Phys. Rev. A 87, 010701(R) (2013)]). Spurred by this discrepancy, together with our collaborators we applied and compared three different methods for the calculation of Feshbach resonances. In the course of this investigation, the ABM was extended so that it can also correctly describe the scattering behavior of a system where both a virtual and a bound state play a role, as is the case for Li-Cs. With this analysis we now have a very accurate characterization of the field dependent scattering length, which is required for our study of few- and many-body physics.

R. Pires et al., Analyzing Feshbach resonances: A Li-Cs case study, Phys. Rev. A 90, 012710 (2014), or see our full list of publications
Rico Pires and Hanna Schempp obtain their PhDs24.07.2014

This week both Hanna Schempp from the Rydberg team, and Rico Pires from the mixtures team successfully defended their PhD theses: Congratulations!

R. Pires, Efimov Resonances in an Ultracold Mixture with Extreme Mass Imbalance , PhD thesis
H. Schempp, Formation of Aggregates and Energy Transport in Ultracold Rydberg Interacting Gases, PhD thesis, or see our full list of publications
Observation of Efimov Resonances in a Mixture with Extreme Mass Imbalance published in Phys. Rev. Lett.!25.06.2014

We observed universal Efimov trimer states in a mixed quantum gas, where they appear with a universal scaling factor that is different from homogeneous systems and - in our case - is small enough to reveal a series of trimer states.

Vitaly Efimov predicted already 40 years ago that there is a universal law for any three resonantly interacting particles and that this law has a discrete scaling behavior, i.e. that the resonances appear on a regular basis. We started with an ultracold gas of Li and Cs at temperatures as low as 400 nK and held the strongly interacting mixture in an optical dipole trap. By precisely changing the external magnetic field near a so-called Feshbach resonance, where the interaction energy between Li and Cs is tunable, we let the atoms interfere with several three-body bound states of the underlying scattering resonance. When the Efimov resonance is hit, the atoms experience an enhanced three-body loss and we record the three-body loss rate by a time-resolved measurement of the atom number. The scaling factor between resonances depends on the mass ratio, for which we measure a value of 5 from the series of resonances in good agreement with the theoretical prediction. While the ground Efimov state is about 50 nm in size, the 2nd excited Efimov state is very large with a length scale of almost one µm.

R. Pires et al., Observation of Efimov Resonances in a Mixture with Extreme Mass Imbalance, Phys. Rev. Lett. 112, 250404 (2014), or see our full list of publications

For more information:
University press release: www.uni-heidelberg.de/presse/news2014
wired.com: Physicists Prove Surprising Rule of Threes, http://www.wired.com/Rule_of_Threes

For more highlights see our news page

Research topics

Mixtures of ultracold atoms and molecules

In this experiment we use a mixture of two different alkali metals: cesium and lithium. This gives us the possbility to form ultracold LiCs dimers. These molecules have an extremely large electric dipole moment which promises many new experiments. For example, the molecules can be orientated in an external electric field.

Strongly-correlated Rydberg quantum gases

Rydberg atoms are atoms in highly excited electronic states. These atoms are very sensitive to external fields and experience extremely strong interactions with other Rydberg atoms. This gives us a model system for studying strongly-correlated quantum systems that is highly controllable and completely governed by interatomic interactions.

Collisions of highly charged ions and cold atoms

We are currently setting up this new experiment. Our goal is to investigate multiple electron capture using the combined techniques of magneto-optically cooling and trapping of the target atoms and using recoil ion momentum spectroscopy.

Hybrid ion atom trap for cold chemistry experiments

Interactions between ions and neutrals play an important role in all kind of chemical reactions. In order to gain a full understanding of these systems we are trying to observe reactions at ultra-low temperatures. In this regime the reaction dynamics are no longer concealed by the thermal movement of the particles.