Project A6

H. P. Zahn, V. P. Singh, M. N. Kosch, L. Asteria, L. Freystatzky, K. Sengstock, L. Mathey, and C. Weitenberg

Driving a many-body system out of equilibrium induces phenomena such as the emergence and decay of transient states, which can manifest itself as pattern and domain formation. The understanding of these phenomena expands the scope of established thermodynamics into the out-of-equilibrium domain. Here, we experimentally and theoretically study the out-of-equilibrium dynamics of a bosonic lattice model subjected to a strong dc field, realized as ultracold atoms in a strongly tilted triangular optical lattice. We observe the emergence of pronounced density-wave patterns—which spontaneously break the underlying lattice symmetry—using a novel single-shot imaging technique with two-dimensional single-site resolution in three-dimensional systems, which also resolves the domain structure. Our study suggests that the short-time dynamics arises from resonant pair tunneling processes within an effective description of the tilted Hubbard model. More broadly, we establish the far out-of-equilibrium regime of lattice models subjected to a strong dc field, as an exemplary and paradigmatic scenario for transient pattern formation.

Phys. Rev. X 12, 021014 (2022)

Christof Weitenberg & Juliette Simonet

Floquet engineering is the concept of tailoring a system by a periodic drive, and it is increasingly employed in many areas of physics. Ultracold atoms in optical lattices offer a particularly large toolbox to design a variety of driving schemes. A strong motivation for developing these methods is the prospect to study the interplay between topology and interactions in a system where both ingredients are fully tunable. We review the recent successes of Floquet engineering in realizing new classes of Hamiltonians in quantum gases, such as Hamiltonians including artificial gauge fields, topological band structures and density-dependent tunnelling. The creation of periodically driven systems also gives rise to phenomena without static counterparts such as anomalous Floquet topological insulators. We discuss the challenges facing the field, particularly the control of heating mechanisms, which currently limit the preparation of many-body phases, as well as the potential future developments as these obstacles are overcome.

Nat. Phys. (2021)

Benno S. Rem, Niklas Käming, Matthias Tarnowski, Luca Asteria, Nick Fläschner, Christoph Becker, Klaus Sengstock, and Christof Weitenberg

Nature Physics 15, 917–920 (2019)

M. Pyzh, S. Krönke, C. Weitenberg, P. Schmelcher

Quantum gas microscopes, which image the atomic occupations in an optical lattice, have opened a new avenue to the exploration of many-body lattice systems. Imaging trapped systems after freezing the density distribution by ramping up a pinning lattice leads, however, to a distortion of the original density distribution, especially when its structures are on the scale of the pinning lattice spacing. We show that this dynamics can be described by a filter, which we call in analogy to classical optics a quantum point spread function. Using a machine learning approach, we demonstrate via several experimentally relevant setups that a suitable deconvolution allows for the reconstruction of the original density distribution. These findings are both of fundamental interest for the theory of imaging and of immediate importance for current quantum gas experiments.

New J. Phys. 21, 053013 (2019)

M. Pyzh, S. Krönke, C. Weitenberg, P. Schmelcher

We investigate a few-body mixture of two bosonic components, each consisting of two particles confined in a quasi one-dimensional harmonic trap. By means of exact diagonalization with a correlated basis approach we obtain the low-energy spectrum and eigenstates for the whole range of repulsive intra- and inter-component interaction strengths. We analyse the eigenvalues as a function of the inter-component coupling, covering hereby all the limiting regimes, and characterize the behaviour in-between these regimes by exploiting the symmetries of the Hamiltonian. Provided with this knowledge we study the breathing dynamics in the linear-response regime by slightly quenching the trap frequency symmetrically for both components. Depending on the choice of interactions strengths, we identify 1 to 3 monopole modes besides the breathing mode of the centre of mass coordinate. For the uncoupled mixture each monopole mode corresponds to the breathing oscillation of a specific relative coordinate. Increasing the inter-component coupling first leads to multi-mode oscillations in each relative coordinate, which turn into single-mode oscillations of the same frequency in the composite-fermionization regime.

New J. Phys. 20, 015006 (2018)

Singh V. P., Weitenberg C., Dalibard J., Mathey L.

We investigate the superfluid behavior of a two-dimensional (2D) Bose gas of ⁸⁷Rb atoms using classical field dynamics. In the experiment by R. Desbuquois et al. [Nat. Phys. 8, 645 (2012)], a 2D quasicondensate in a trap is stirred with a blue-detuned laser beam along a circular path around the trap center. Here, we study this experiment from a theoretical perspective. The heating induced by stirring increases rapidly above a velocity vc, which we define as the critical velocity. We identify the superfluid, the crossover, and the thermal regime by a finite, a sharply decreasing, and a vanishing critical velocity, respectively. We demonstrate that the onset of heating occurs due to the creation of vortex-antivortex pairs. A direct comparison of our numerical results to the experimental ones shows a good agreement, if a systematic shift of the critical phase-space density is included. We relate this shift to the absence of thermal equilibrium between the condensate and the thermal wings, which were used in the experiment to extract the temperature. We expand on this observation by studying the full relaxation dynamics between the condensate and the thermal cloud.

Phys. Rev. A 95, 043631 (2017)

M. Lahrz, C. Weitenberg, L. Mathey

Supersymmetric systems derive their properties from conserved supercharges which form a supersymmetric algebra. These systems naturally factorize into two subsystems, which, when considered as individual systems, have essentially the same eigenenergies, and their eigenstates can be mapped onto each other. We propose a Mach-Zehnder interference experiment to detect supersymmetry in quantum-mechanical systems, which can be realized with current technology. To demonstrate this interferometric technique, we first propose a one-dimensional ultracold-atom setup to realize a pair of supersymmetric systems. Here, a single-atom wave packet evolves in a superposition of the subsystems and gives an interference contrast that is sharply peaked if the subsystems form a supersymmetric pair. Second, we propose a two-dimensional setup that implements supersymmetric dynamics in a synthetic gauge field.

Phys. Rev. A 96, 043624 (2017)

M. Tarnowski, M. Nuske, N. Fläschner, B. Rem, D. Vogel, L. Freystatzky, K. Sengstock, L. Mathey, C. Weitenberg
Topological defects in Bloch bands, such as Dirac points in graphene, and their resulting Berry phases play an important role for the electronic dynamics in solid state crystals. Such defects can arise in systems with a two-atomic basis due to the momentum-dependent coupling of the two sublattice states, which gives rise to a pseudo-spin texture. The topological defects appear as vortices in the azimuthal phase of this pseudo-spin texture. Here, we demonstrate a complete measurement of the azimuthal phase in a hexagonal optical lattice
employing a versatile method based on time-of-flight imaging after off-resonant lattice modulation. Furthermore we map out the merging transition of the two Dirac points induced by beam imbalance. Our work paves the way to accessing geometric properties in general multi-band systems also with spin-orbit coupling and interactions.
Phys. Rev. Lett. 118, 240403 (2017)
https://arxiv.org/abs/1703.02813