Recent results:

Observation of many-body dynamical localization
Y. Guo, S. Dhar, A. Yang, Z.Chen, H. Yao, M. Horvath, L. Ying, M. Landini, H.-C. Nägerl

The quantum kicked rotor is a paradigmatic model system in quantum physics. As a driven quantum system, it is used to study the transition from the classical to the quantum world and to elucidate the emergence of chaos and diffusion. In contrast to its classical counterpart, it features dynamical localization, specifically Anderson localization in momentum space. The interacting many-body kicked rotor is believed to break localization, as recent experiments suggest. Here, we present evidence for many-body dynamical localization for the Lieb-Liniger version of the many-body quantum kicked rotor. After some initial evolution, the momentum distribution of interacting quantum-degenerate bosonic atoms in one-dimensional geometry, kicked hundreds of times by means of a pulsed sinusoidal potential, stops spreading. We quantify the arrested evolution by analysing the energy and the information entropy of the system as the interaction strength is tuned. In the limiting cases of vanishing and strong interactions, the first-order correlation function exhibits a very different decay behavior. Our results shed light on the boundary between the classical, chaotic world and the realm of quantum physics.

arxiv.org/abs/2312.13880

Bose-Einstein condensation of non-ground-state caesium atoms-
M. Horvath, S. Dhar, A. Das, M. D. Frye, Y. Guo, J. M. Hutson, M. Landini, H.-C. Nägerl

Bose-Einstein condensates of ultracold atoms serve as low-entropy sources for a multitude of quantum-science applications, ranging from quantum simulation and quantum many-body physics to proof-of-principle experiments in quantum metrology and quantum computing. For stability reasons, in the majority of cases the energetically lowest-lying atomic spin state is used. Here we report the Bose-Einstein condensation of caesium atoms in the Zeeman-excited mf = 2 state, realizing a non-ground-state Bose-Einstein condensate with tunable interactions and tunable loss. We identify two regions of magnetic field in which the two-body relaxation rate is low enough that condensation is possible. We characterize the phase transition and quantify the loss processes, finding unusually high three-body losses in one of the two regions. Our results open up new possibilities for the mixing of quantum-degenerate gases, for polaron and impurity physics, and in particular for the study of impurity transport in strongly correlated one-dimensional quantum wires.

Nat. Commun 15, 3739 (2024), preprint at arxiv.org/abs/2310.12025

Anomalous cooling of bosons by dimensional reduction-
Y. Guo*, H. Yao*, S. Dhar, L. Pizzino, M. Horvath, T. Giamarchi, M. Landini, H.-C. Nägerl

*These authors contributed equally to this work

Cold atomic gases provide a remarkable testbed to study the physics of interacting many-body
quantum systems. They have started to play a major role as quantum simulators, given the high
degree of control that is possible. A crucial element is given by the necessarily non-zero temperature.
However cooling to the required ultralow temperatures or even simply measuring the temperature
directly on the system can prove to be very challenging tasks. Here, we implement thermometry on
strongly interacting two- and one-dimensional Bose gases with high sensitivity in the nano-Kelvin
temperature range. Our method is aided by the fact that the decay of the first-order correlation
function is very sensitive to the temperature when interactions are strong. We find that there
may be a significant temperature variation when the three-dimensional quantum gas is cut into two-
dimensional slices or into one-dimensional tubes. Strikingly, the temperature for the one-dimensional
case can be much lower than the initial temperature. Our findings show that this decrease results
from the interplay of dimensional reduction and strong interactions.

Sci. Adv. 10, 6-11 (2024), preprint at: arxiv.org/abs/2308.04144

Observation of the 2D-1D crossover in strongly interacting ultracold bosons-
Y. Guo*, H. Yao*, S. Ramanjanappa, S. Dhar, M. Horvath, L. Pizzino, T. Giamarchi, M. Landini, H.-C. Nägerl

*These authors contributed equally to this work

Dimensionality plays an essential role in determining the nature and properties of a physical system. For quantum systems the impact of interactions and fluctuations is enhanced in lower dimensions, leading to a great diversity of genuine quantum effects for reduced dimensionality. In most cases, the dimension is fixed to some integer value. Here, we experimentally probe the dimensional crossover from two to one dimension using strongly interacting ultracold bosons in variable lattice potentials and compare the data to ab-initio theory that takes into account non-homogeneous trapping and non-zero temperature. From a precise measurement of the momentum distribution we analyze the characteristic decay of the one-body correlation function in the two dimensionalities and then track how the decay is modified in the crossover. A varying two-slope structure is revealed, reflecting the fact that the particles see their dimensionality as being one or two depending on whether they are probed on short or long distances, respectively. Our observations demonstrate how quantum properties in the strongly-correlated regime evolve in the dimensional crossover as a result of the interplay between dimensionality, interactions, and temperature.

Nature Physics 20, 934 (2024), preprint at arxiv.org/abs/2308.00411

Bloch oscillations in the absence of a lattice-
F. Meinert, M. Knap, E. Kirilov, K. Jag-Lauber, M. B. Zvonarev, E. Demler, H.-C. Nägerl

The interplay of strong quantum correlations and far-from-equilibrium conditions can give rise to striking dynamical phenomena. We experimentally investigated the quantum motion of an impurity atom immersed in a strongly interacting one-dimensional Bose liquid and subject to an external force. We found that the momentum distribution of the impurity exhibits characteristic Bragg reflections at the edge of an emergent Brillouin zone. Although Bragg reflections are typically associated with lattice structures, in our strongly correlated quantum liquid they result from the interplay of short-range crystalline order and kinematic constraints on the many-body scattering processes in the one-dimensional system. As a consequence, the impurity exhibits periodic dynamics, reminiscent of Bloch oscillations, although the quantum liquid is translationally invariant. Our observations are supported by large-scale numerical simulations.

Science 356, 945 (2017), arXiv:1608.08200

Floquet engineering of correlated tunneling in the Bose-Hubbard model with ultracold atoms

– F. Meinert, M. J. Mark, K. Lauber, A. J. Daley, and H.-C. Nägerl

We report on the experimental implementation of tunable occupation-dependent tunneling in a Bose-Hubbard system of ultracold atoms via time-periodic modulation of the on-site interaction energy. The tunneling rate is inferred from a time-resolved measurement of the lattice site occupation after a quantum quench. We demonstrate coherent control of the tunneling dynamics in the correlated many-body system, including full suppression of tunneling as predicted within the framework of Floquet theory. We find that the tunneling rate explicitly depends on the atom number difference in neighboring lattice sites. Our results may open up ways to realize artificial gauge fields that feature density dependence with ultracold atoms.

Phys. Rev. Lett. 116, 205301 (2016)  , arXiv:1602.02657

Probing the Excitations of a Lieb-Liniger Gas from Weak to Strong Coupling-
F. Meinert, M. Panfil, M.J. Mark, K. Lauber, J.-S. Caux, H.-C. Nägerl

We probe the excitation spectrum of an ultracold one-dimensional Bose gas of Cesium atoms with repulsive contact interaction that we tune from the weakly to the strongly interacting regime via a magnetic Feshbach resonance. The dynamical structure factor, experimentally obtained using Bragg spectroscopy, is compared to integrability-based calculations valid at arbitrary interactions and finite temperatures. Our results unequivocally underly the fact that hole-like excitations, which have no counterpart in higher dimensions, actively shape the dynamical response of the gas.

Phys. Rev. Lett. 115, 085301 (2015), arXiv:1505.08152

Observation of Density-Induced Tunneling
-O. Jürgensen, F. Meinert, M. J. Mark, H.-C. Nägerl, D.-S. Lühmann

We study the dynamics of bosonic atoms in a tilted one-dimensional optical lattice and report on the first direct observation of density-induced tunneling. We show that the interaction affects the time evolution of the doublon oscillation via density-induced tunneling and pinpoint its density- and interaction-dependence. The experimental data for different lattice depths are in good agreement with our theoretical model. Furthermore, resonances caused by second-order tunneling processes are studied, where the density-induced tunneling breaks the symmetric behavior for attractive and repulsive interactions predicted by the Hubbard model.

Phys. Rev. Lett. 113, 193003 (2014), arXiv:1407.0835

Former members:

• Katharina Lauber, 2018, PhD thesis
• Florian Meinert, 2012-2016, PhD thesis, (SAMOP Prize, IQOQI Dissertation Prize and Liechtenstein Prize)
• Emil Kirilov, 2012-2016, (now K-Cs experiment and senior scientist in the group of Prof. Rudolf Grimm)
• Manfred Mark, 2006-2014, PhD thesis, diploma thesis (now senior scientist in the group of Prof. Francesca Ferlaino)
• Michael Gröbner, 2011-2013, master thesis (afterwards K-Cs team)
• Benjamin Rutschmann, 2011-2012, master thesis
• Johann Danzl, 2005-2012, PhD thesis (Liechtenstein Prize, now professor at the IST Austria)
• Mohamed Rabie, 2010-2011, diploma thesis
• Oliver Krieglsteiner, 2010-2011, diploma thesis
• Andreas Klinger, 2009-2011, master thesis
• Elmar Haller, 2005-2011, PhD thesis (DAMOP Prize, now lecturer at the Unicersity of Strathclyde, Glasgow)
• Russell Hart, 2008-2010, Marie Curie postdoc
• Lukas Reichsöllner, 2009-2010, diploma thesis (afterwards Rb-Cs team)
• Mattias Gustavsson, 2003-2009, PhD thesis
• Gabriel Rojas-Kopeinig, 2006-2007, diploma thesis
• Anton Flir, 2005-2006, diploma thesis
• Peter Unterwaditzer, 2004, diploma thesis