Research activities

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.

The combination of different quantum objects into a new quantum hybrid system offers intruiging ways for realizing novel building blocks for quantum technology and allows the investigation and realization of completely new coherent interaction mechanisms as a resource for quantum communication and the creation of entanglement. In our experiment we couple ultracold atoms to cryogenically precooled nanomechanical resonators via lightfields. We cool the mechanical oscillator close to its motional ground state employing active feedback cooling and explore various coupling mechanisms involving external as well as internal degrees of freedom of the atoms. Our ultimate goal 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.

Ultrashort laser pulses enable us to observe and manipulate matter on very short time scales, whereas ultracold atoms permit experiments with high precision and controllability. This project aims at manipulating macroscopic quantum systems on ultrafast time-scales by, for example, instantaneously introducing long-range interactions by ionizing atoms in a quantum gas.

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.

Light & Schools begeistert junge Leute durch spannende Experimente für die Physik. Die Angebote richten sich sowohl an Mittel- als auch Oberstufenklassen der Hamburger Stadtteilschulen und Gymnasien. Das Schullabor wurde bereits 2009 im Rahmen der Landesexzellenzinitiative, die durch die Joachim Herz Stiftung gefördert wurde, ins Leben gerufen und wird derzeit im Rahmen des SFB 925 und des Exzellenzclusters „The Hamburg Centre for Ultrafast Imaging“ (CUI) fortgeführt.