Research

Ultracold atoms in optical lattices are nearly perfect realizations of various kinds of Hubbard models and, as such, can be used as quantum simulators. In this project, we investigate some challenging open questions of condensed matter physics.

The periodic driving of the lattice constitutes the core of the recent experiments. The resulting inertial force generates an effective complex tunneling parameter|J|eiѲ. The experimental control over the Peierls phase Ѳ extends the quantum simulation abilities of ultracold atomic systems further, e.g. to quantum Hall physics or magnetism.

In this project we investigate ground state and dynamical properties of Bose-Fermi mixtures and Fermi-spin-mixtures in optical lattices. Recently we have been looking at the intriguing dynamical properties of fermionic high-spin systems emplyoing the F=9/2 hyperfine manyfold of 40K. We observed fascinating phenomena ranging from collective spin oscillation of bulk systems to complex tensorial yet fully controllable spin-waves. In a deep optical lattice we performed benchmark experiments exploring spin changing dynamics on a two-body level. We are further interested in simulating highly relevant model systems to explore unknown and thus far unaccessible terrain towards a better understanding of phenomena like high-Tc superconductivity and elements of SU(N) magnetism as well as geomtrical frustration on a Fermionic triangular lattice and Graphene like systems in hexagonal periodic potentials.

The major goal of this project is to combine the benefits of optical lattice clocks and ultracold degenerate Fermi gases. For this purpose we have set up a machine capable of the production of ultracold ytterbium quantum gas mixtures inside an optical lattice. The specific properties of ytterbium, namely a very favorably distribution of natural abundances of various isotopes, the existence of a very narrow so called optical clock transition and a correspondingly long lived meta-stable state as well as the SU(N) symmetric interaction of the fermionic isotopes allows for the investigation of exotic quantum matter. In particular the meta-stable excited state opens the door for the study of lattice systems with an additional orbital degree of freedom. To this end we strive to investigate the Kondo-lattice model, which is a paradigm for strongly correlated many-body systems. Controlled losses can also be invoked in ytterbium quantum gases and allow to investigate open quantum systems, which we use to study the dissipative Fermi-Hubbard model.

In this project we aim at the coupling of an ensemble of either cold or ultracold atoms to a nano-mechanical oscillator such as e.g. a SiN Membrane at cryogenic temperatures. In first experiments we want to evaluate and compare different methods of cooling the high quality nano-mechanical oscillator to its absolute quantum mechanical ground state by established means of laser cooling on the one hand and by indirect cooling via a coupling to cold and ultracold atoms using different coupling methods. The ultimate goal of our research project is the coherent coupling of two macroscopic quantum objects namely the ground-state-cooled oscillator and a Bose-Einstein condensate.

We are setting up a new lithium quantum gas experiment, in which we want to study small quantum systems of just a few atoms in a microtrap. The new technique of single-atom resolved fluorescence imaging will allow for detection of these small atom numbers and for the study of correlation functions. Lithium is a good choice, because it has fermionic and bosonic species and a broad Feshbach resonance for tuning the interactions.

Atoms in artificial optical lattices offer outstanding prospects for the comparison between theory and experiment. Since Hubbard has introduced the “the Hubbard model” in the 60’s, it has established itself as a dominant description for strongly correlated electrons. In contrast to complex and “dirty“ solids, optical lattices – as extremely pure and controllable systems – represent a perfect environment for looking beyond the Hubbard model. Optical lattices shed new light on orbital physics and interaction-induced beyond-Hubbard processes. Recently, more complex lattice structures are investigated such as the graphene lattice and superlattices. Single-particle manipulation and imaging paves the way towards the quantum simulation of molecules.

In this project we are setting up a versatile quantum gases machine. The experiment aim is to produce degenerate gases of fermionic 40K and bosonic 87Rb or 39K atoms.

This novel experimental setup shall trigger external collaborations within the guest program of Center of Quantum Technologies (ZOQ - Hamburg), as it is intended to serve as a user facility where guest researchers can perform forefront research. This experiment also plays a central role in the education of undergraduate students at the Hamburg University offering an attractive insight into the latest developments in quantum optics.

In earthbound experiments, studying the free evolution of a BEC is limited to less than 100 ms as the atoms fall out of the interrogation volume once released from their trap. This project overcomes this limitation by dropping a complete quantum gas setup down the 110 m high free fall tower at ZARM in Bremen allowing for a free expansion of 1 s. The system facilitates the investigation of the condensate's coherence in new regimes, the operation of highly sensitive atom interferometers and is an important stepping stone for future experiments on ballistic rockets or satellites.

In this project we want to refine a new method based on laser cooling and trapping to determine ratios of different isotopes of Kr with unprecedented sensitivity. Being a characteristic fission product in Plutonium breeding 85Kr is a sensitive tracer to detect non-compliance with the Non-Proliferation Treaty (NPT). We put special emphasis on an highly efficient all optical production of the rare-gas metastable states in Kr which are required for laser cooling in order to increase the overall efficiency. A strobed double-MOT set up with single-atom detection capability is at the heart of our experimental. This project is carried out in the Zentrum für Naturwissenschaft und Friedensforschung.