
The latest project in the research group of Prof. H.C. Nägerl is aimed at studying quantum gas mixtures of cesium (Cs) and potassium (K). Mixed quantum gases offer a wealth of research opportunities, ranging from precision measurements to studies of various quantum manybody regimes. The research presently focuses on two main directions that both complement each other: Dipolar quantum matter and topological quantum matter. Within the first line of research, we aim to build a molecular quantum simulator based on ultracold polar KCs groundstate molecules. Our goal is to study novel types of dipolar quantum matter in optical lattice geometries under a highresolution microscope that shall allow us to observe the molecules with singlesite resolution. Within the second line of research, we aim at realizing exotic quantum manybody phases of fermionic and bosonic mixtures in lattice geometries with nontrivial band structures. Presently, we are focusing on ultracold bosons with tunable interactions loaded into twodimensional Lieb lattices. Such a system is particularly interesting because it features two dispersive bands that form a Dirac cone and a flat band that intersects the Dirac point. A topological state of matter that is exclusively driven by interactions is expected to form.





Degenerate Raman sideband cooling of ^{39}KM. Gröbner, P. Weinmann, E. Kirilov, and H.C. Nägerl We report on a realization of subDoppler laser cooling of ^{39}K atoms using degenerate threedimensional Raman sideband cooling. We take advantage of the wellresolved excited hyperfine states on the D_{1} optical transition to produce spinpolarized samples with 1.4 x 10^{8} atoms at temperatures of 1.8 μK. The phasespace densities are ≥10^{4}, which significantly improves the initial conditions for a subsequent evaporative cooling step. The presented cooling technique using the D_{1} line can be adapted to other atomic species and is applicable to highresolution imaging schemes in far offresonant optical lattices. 

Observation of interspecies Feshbach resonances in an ultracold ^{39}K^{133}Cs mixture and refinement of interaction potentialsM. Gröbner, P. Weinmann, E. Kirilov, H.C. Nägerl, P. S. Julienne, C. R. Le Sueur, and J. M. Hutson We observe interspecies Feshbach resonances due to swave bound states in ultracold ^{39}K^{133}Cs scattering for three different spin mixtures. The resonances are observed as joint atom loss and heating of the K sample. We perform leastsquares fits to obtain improved KCs interaction potentials that reproduce the observed resonances, and carry out coupledchannel calculations to characterize the scattering and boundstate properties for ^{39}KCs, ^{40}KCs and ^{41}KCs. Our results open up the possibilities of tuning interactions in KCs atomic mixtures and of producing ultracold KCs molecules. 

A new quantum gas apparatus for ultracold mixtures of K and Cs and KCs groundstate moleculesM. Gröbner, P. Weinmann, F. Meinert, K. Lauber, E. Kirilov, and H.C. Nägerl We present a new quantum gas apparatus for ultracold mixtures of K and Cs atoms and ultracold samples of KCs groundstate molecules. We demonstrate the apparatus' capabilities by producing BoseEinstein condensates (BEC) of ^{39}K and ^{133}Cs in a manner that will eventually allow sequential condensation within one experimental cycle, precise sample overlap, and magnetic association of atoms into KCs molecules. The condensates are created independently without relying on sympathetic cooling. Our approach is universal and applicable to other species combinations when the two species show dramatically different behavior in terms of loss mechanisms and post laser cooling temperatures, i.e. species combinations that make parallel generation of quantum degenerate samples challenging. We give an outlook over the next experiments involving e.g. sample mixing, molecule formation, and transport into a science chamber for highresolution spatial imaging of novel quantummany body phases based on KCs. 

Compact, robust, and spectrally pure diodelaser system with a filtered output and a tunable copy for absolute referencingE. Kirilov, M. J. Mark, M. Segl, H.C. Nägerl We report on a design of a compact laser system composed of an extended cavity diode laser with high passive stability and a prefilter FabriPerot cavity. The laser is frequency stabilized relative to the cavity using a serrodyne technique with a correction bandwidth of ≥6 MHz and a dynamic range of ≥700 MHz. The free running laser system has a power spectral density (PSD) ≥100 Hz^{2}/Hz centered mainly in the acoustic frequency range. A highly tunable, 0.5–1.3 GHz copy of the spectrally pure output beam is provided, which can be used for further stabilization of the laser system to an ultrastable reference. We demonstrate a simple onechannel lock to such a reference that brings down the PSD to the subHz level. The tuning, frequency stabilization and sideband imprinting is achieved by a minimum number of key elements comprising a fibered EOM (electrooptic modulator), AOM (acoustooptic modulator) and a NLTL (nonlinear transmission line). The system is easy to operate, scalable, and highly applicable to atomic/molecular experiments demanding high spectral purity, longterm stability, and robustness. 
2012–2016 ERC Starting Grant MicroQuant: Microscopy of Tunable ManyBody Quantum Systems 

2016–2019 Project P 29602: Experiments with PotassiumCesium Quantum gas Mixtures 

2016– Doktoratskolleg W 1259: Atoms, Light, and Molecules 

2018–2023 Wittgensteinpreis Z 336: exp. quantum physics, quantum gases, lowdimensional quantum systems, ultracold molecules 