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

Universality of weakly bound dimers and Efimov trimers close to Li–Cs Feshbach resonances published in New J. Phys. 14.05.2015

We study the interspecies scattering properties of ultracold Li-Cs mixtures in their two energetically lowest spin channels in the magnetic field range between 800 G and 1000 G. Close to two broad Feshbach resonances we create weakly bound LiCs dimers by radio-frequency association and measure the dependence of the binding energy on the external magnetic field strength. Based on the binding energies and complementary atom loss spectroscopy of three other Li-Cs s-wave Feshbach resonances we construct precise molecular singlet and triplet electronic ground state potentials using a coupled-channels calculation. We extract the Li-Cs interspecies scattering length as a function of the external field and obtain almost a ten-fold improvement in the precision of the values for the pole positions and widths of the s-wave Li-Cs Feshbach resonances as compared to our previous work [Pires et al., Phys. Rev. Lett. 112, 250404 (2014)]. We discuss implications on the Efimov scenario and the universal geometric scaling for LiCsCs trimers.

J. Ulmanis et al., Universality of weakly bound dimers and Efimov trimers close to Li–Cs Feshbach resonances, New J. Phys. 17, 055009 (2015), or see our full list of publications
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 analysis 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
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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.

Rydberg physics with ultracold two-electron systems

We are setting up an experiment to study the physics of two-electron Rydberg atoms using a quantum gas of ultracold strontium. The experiment is located at the University of Science and Technology of China (USTC Shanghai Institute for Advanced Studies). First studies will be aiming to explore many-body effects induced by the long-range interactions between highly excited strontium Rydberg atoms, using the inner electron to control the atom's motion and to detect single Rydberg atoms.