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

Sympathetic cooling in multipole rf-traps10.06.2016

It has been common wisdom, starting from early work of Dehmelt and others, that one cannot cool ions in a radiofrequency (RF) trap with a buffer gas if the mass ratio between the buffer gas atom and ion exceeds a critical value. In this paper, we theoretically show that one can overcome this dogma by using a spatially localized buffer gas and/or a higher multipole order for the radial trapping potential. These approaches make use of the fact, that the principle hindrance of sympathetic cooling inside an RF trap arises from a collisionally induced energy transfer between the RF-driven micromotion and the macromotion (see Figure). Thus, spatially restricting collisions to the volume of minimal micromotion and/or reducing the average micromotion altogether, leads to an increased critical mass ratio, enabling the use of heavier buffer gases.

The comprehensive model presented in this paper provides an intuitive picture of collisions in an RF trap based on a favorable frame transformation, where the micromotion is assigned to the neutral buffer gas. Using numerical simulations, we find three distinct dynamical regimes, characterized by analytical expressions for the ion's equilibrium energy distribution. These results not only comprise earlier studies on collisional cooling of ions but also predict a novel regime of stable cooling of ions beyond the critical mass ratio. In this regime one can actively tune the ions temperature by controlling the buffer gas' extension and/or the RF-trap fields (forced sympathetic cooling). Our findings are directly applicable to cooling of ions with laser cooled atoms or He buffer gas in Paul traps (as used in the quantum information and quantum simulation communities) or multipole traps (as used in the chemical reaction and astrochemistry communities). Especially for experiments investigating interactions of ions with an ensemble of ultracold atoms, the prospect of using heavier atom species, makes a whole new range of possible systems available that have not been studied yet.

B. Höltkemeier et al., Buffer-Gas Cooling of a Single Ion in a Multipole Radio Frequency Trap Beyond the Critical Mass Ratio, Phys. Rev. Lett. 116, 233003 , or see our full list of publications
German Research Foundation funds the Collaborative Research Center ISOQUANT2.06.2016

German Research Foundation (DFG) funds the Collaborative Research Center on "Isolated quantum systems and universality in extreme conditions" (ISOQUANT). The center will explore universal properties of different quantum systems that range from particle and nuclear to atomic, molecular, and solid state physics. A special focus will lie on conditions, under which the behaviour of such apparently disparate systems can be identical, despite the vastly different temperature, density, or field strength that characterizes them. ISOQUANT comprises research groups and projects from Institute for Theoretical Physics, Kirchoff-Institute for Physics, Physics Institute, and Max-Planck Institute for Nuclear Physics in Heidelberg and Vienna University of Technology.

Our group is embedded in the framework of ISOQUANT with two projects. We aim to employ strong and tunable interactions in ultracold Bose-Fermi mixtures of lithium and caesium atoms to explore the properties of composite quantum particles that consist of an impurity, which is strongly coupled to a surrounding quantum gas. Potentially, phenomena occurring in solid-state physics can be simulated in this way. The second project addresses quantum systems with long-range interactions, which are ideal for the investigation of non-equilibrium dynamics at strong coupling. Our experiments will employ ultracold Rydberg atoms to identify common characteristics that govern quantum fluctuations and relaxation in strongly-coupled spin systems and quantum fluids.

For more information:
DFG press release (in German): DFG fördert 20 neue Sonderforschungsbereiche
Heidelberg University press release (in German): Universität Heidelberg mit vier Förderanträgen für Sonder­forschungs­bereiche erfolgreich
Collaborative Research Center homepage (in German): SFB 1225 ISOQUANT

Matthias Weidelmüller receives the first Hefei Friendship Award3.05.2016

Matthias Weidelmüller has received the first Hefei Friendship Award. This year ten foreign experts from seven different countries received the newly established and prestigious prize, which recognizes outstanding contributions to Hefei's economic and social progress. The award ceremony was held on 19th April in University of Science and Technology (USTC), China. Congratulations!

For more information:
USTC press release: Weidemüller wins first Hefei Friendship Award
Hefei local news (in Chinese): Towards better world with quantum gases

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.

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.