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

Density matrix reconstruction of three-level atoms via Rydberg electromagnetically induced transparency published in J. Phys. B: At., Mol. Opt. Phys.!22.07.2016
Vladislav Gavryusev

The full one-body density matrix of an ultracold gas of three-level atoms under electromagnetically induced transparency conditions can be reconstructed through a combined measurement and analysis of the spatially resolved optical spectrum and of the total excited-atom number.

Our method gives a simple explanation to the counter-intuitive features observed in the spectra and provides the optical susceptibility and the Rydberg density as a function of spatial position, as well as the spatial profile of Rabi frequencies of the coupling laser. These results help elucidate the interplay of matter and light degrees of freedom in three-level media and will facilitate new studies of many-body effects in optically driven Rydberg gases.

V. Gavryusev et al., Density matrix reconstruction of three-level atoms via Rydberg electromagnetically induced transparency, J. Phys. B: At., Mol. Opt. Phys. 49, 164002 (2016), or see our full list of publications
Matthias Weidemüller mit Daniel Kehlmann und Jürgen Neffe auf dem Literaturfestival LIT:potsdam07.07.2016

Was geschah im Moment des Urknalls? Woraus bestehen Raum und Zeit? Welche Rolle spielten Zufall und Notwendigkeit bei der Entstehung der Welt? Auf diese Fragen findet die Physik unterschiedliche Antworten: Albert Einsteins Relativitätstheorie beschreibt die Struktur von Raum und Zeit und damit die Makrowelt, die Welt im Großen. Mit der Quantenphysik lässt sich die Mikrowelt der Atome und Elementarteilchen erklären. Beide Theorien bringen uns den Antworten auf diese großen Fragen ein Stück näher, und doch lassen sie sich nicht miteinander in Einklang bringen. Seit über einem Jahrhundert fasziniert dieser Widerspruch die Wissenschaft und bis heute sucht sie nach einer “Weltformel”, um die unvollendete Geschichte über den Ursprung der Welt zu Ende zu erzählen. (LIT:potsdam)

Antworten auf diesen Fragen suchen auch Matthias Weidemüller mit Daniel Kehlmann und Jürgen Neffe auf dem Literaturfestival LIT:potsdam am 10 Juli 2016.

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Weitere Information und Programm: LIT:potsdam

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
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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.