Recent results:

Observation of confinement-induced resonances in a 3D lattice-
D. Capecchi, C. Cantillano, M. J. Mark, A. Schindewolf, M. Landini, A. Saenz, F. Revuelta, H.-C. Nägerl

We report on the observation of confinement-induced resonances for strong three-dimensional (3D) confinement in a lattice potential. Starting from a Mott-insulator state with predominantly single-site occupancy, we detect loss and heating features at specific values for the confinement length and the 3D scattering length. Two independent models, based on the coupling between the center-of-mass and the relative motion of the particles as mediated by the lattice, predict the resonance positions to a good approximation, suggesting a universal behavior. Our results extend confinement-induced resonances to any dimensionality and open up an alternative method for interaction tuning and controlled molecule formation under strong 3D confinement.

Phys. Rev. Lett. 131, 213002 (2023), arxiv: 2209.12504

Quantum engineering of a low-entropy gas of heteronuclear bosonic molecules in an optical lattice-
L. Reichsöllner*, A. Schindewolf*, T. Takekoshi, R. Grimm, and H.-C. Nägerl

*These authors contributed equally to this work.

We demonstrate a generally applicable technique for mixing two-species quantum degenerate bosonic samples in the presence of an optical lattice, and we employ it to produce low-entropy samples of ultracold ⁸⁷Rb¹³³Cs Feshbach molecules with a lattice filling fraction exceeding 30%. Starting from two spatially separated Bose-Einstein condensates of Rb and Cs atoms, Rb-Cs atom pairs are efficiently produced by using the superfluid-to-Mott insulator quantum phase transition twice, first for the Cs sample, then for the Rb sample, after nulling the Rb-Cs interaction at a Feshbach resonance’s zero crossing. We form molecules out of atom pairs and characterize the mixing process in terms of sample overlap and mixing speed. The dense and ultracold sample of more than 5000 RbCs molecules is an ideal starting point for experiments in the context of quantum many-body physics with long-range dipolar interactions.

Phys. Rev. Lett. 118, 073201 (2017), arXiv:1607.06536

Ultracold dense samples of dipolar RbCs molecules in the rovibrational and hyperfine ground state-
T. Takekoshi, L. Reichsöllner, A. Schindewolf, J. M. Hutson, C. R. Le Sueur, O. Dulieu, F. Ferlaino, R. Grimm, H.-C. Nägerl

We produce ultracold dense trapped samples of ⁸⁷Rb¹³³Cs molecules in their rovibrational ground state, with full nuclear hyperfine state control, by stimulated Raman adiabatic passage (STIRAP) with efficiencies of 90%. We observe the onset of hyperfine-changing collisions when the magnetic field is ramped so that the molecules are no longer in the hyperfine ground state. A strong quadratic shift of the transition frequencies as a function of applied electric field shows the strongly dipolar character of the RbCs ground-state molecule. Our results open up the prospect of realizing stable bosonic dipolar quantum gases with ultracold molecules.

Phys. Rev. Lett. 113, 205301 (2014), arXiv:1405.6037

Towards the production of ultracold ground-state RbCs molecules: Feshbach resonances, weakly bound states, and coupled-channel model-
T. Takekoshi, M. Debatin, R. Rameshan, F. Ferlaino, R. Grimm, H.-C. Nägerl, C.R. Le Sueur, J.M. Hutson, P.S. Julienne, S. Kotochigova, E. Tiemann

We have studied interspecies scattering in an ultracold mixture of ⁸⁷Rb and ¹³³Cs atoms, both in their lowest-energy spin states. The three-body loss signatures of 30 incoming s- and p-wave magnetic Feshbach resonances over the range 0 to 667 G have been catalogued. Magnetic field modulation spectroscopy was used to observe molecular states bound by up to 2.5 MHz x h. Magnetic moment spectroscopy along the magneto-association pathway from 197 to 182 G gives results consistent with the observed and calculated dependence of the binding energy on magnetic field strength. We have created RbCs Feshbach molecules using two of the resonances. We have set up a coupled-channel model of the interaction and have used direct least-squares fitting to refine its parameters to fit the experimental results from the Feshbach molecules, in addition to the Feshbach resonance positions and the spectroscopic results for deeply bound levels. The final model gives a good description of all the experimental results and predicts a large resonance near 790 G, which may be useful for tuning the interspecies scattering properties. Quantum numbers and vibrational wavefunctions from the model can also be used to choose optimal initial states of Feshbach molecules for their transfer to the rovibronic ground state using stimulated Raman adiabatic passage (STIRAP).

Phys. Rev. A 85, 032506 (2012), arxiv:1201.1438v2

Molecular spectroscopy for ground-state transfer of ultracold RbCs molecules
-M. Debatin, T. Takekoshi, R. Rameshan, L. Reichsöllner, F. Ferlaino, R. Grimm, R. Vexiau, N. Bouloufa, O. Dulieu, H.-C. Nägerl

We perform one- and two-photon high resolution spectroscopy on ultracold samples of RbCs Feshbach molecules with the aim to identify a suitable route for efficient ground-state transfer in the quantum-gas regime to produce quantum gases of dipolar RbCs ground-state molecules. One-photon loss spectroscopy allows us to probe deeply bound rovibrational levels of the mixed excited (A¹Σ⁺ – b³Π⁰)0⁺ molecular states. Two-photon dark state spectroscopy connects the initial Feshbach state to the rovibronic ground state. We determine the binding energy of the lowest rovibrational level |v“=0,J“=0> of the X¹Σ⁺ ground state to be D^X₀ = 3811.5755(16) 1/cm, a 300-fold improvement in accuracy with respect to previous data. We are now in the position to perform stimulated two-photon Raman transfer to the rovibronic ground state.

Phys. Chem. Chem. Phys. 13, 18926 (2011), arXiv:1106.0129

Former members:

• Deborah Capecchi, 2016-2024, PhD thesis
• Arpita Das, 2021-2023, postdoc
• Andreas Schindewolf, 2013-2018 , PhD, Postdoc
• Lukas Reichsöllner, 2010-2017, PhD thesis
• Silva Mezinska, 2014-2017
• Beatrix Mayr, 2015-2016
• Tetsu Takekoshi, 2010-2015, (now in the group of Prof. Rainer Blatt)
• Francesca Ferlaino, 2009-2014, (now professor at the University of Innsbruck and research director at the IQOQI)
• Michael Kugler, 2012-2013
• Verena Pramhaas, 2012-2013
• Carl Hippler, 2012-2013, diploma thesis
• Markus Debatin, 2008-2013, PhD thesis
• Raffael Rameshan, 2009-2012, diploma thesis
• Bastian Schuster, 2008-2010
• Almar Lercher, 2005-2010, PhD thesis
• Karl Pilch, 2005-2009, PhD thesis
• Andrea Prantner, 2006-2009, diploma thesis