Bulletin of the American Physical Society
52nd Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Monday–Friday, May 31–June 4 2021; Virtual; Time Zone: Central Daylight Time, USA
Session X07: Quantum Gases in Low DimensionsLive

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Chair: Cheng Chin, U Chicago 
Friday, June 4, 2021 8:00AM  8:12AM Live 
X07.00001: Observation of first and second sound in a BerezinskiiKosterlitzThouless superfluid Panagiotis Christodoulou, Maciej Galka, Nishant Dogra, Raphael Lopes, Julian Schmitt, Zoran Hadzibabic Superfluidity in two dimensions (2D), unlike its three dimensional counterpart, is associated with the pairing of vortices of opposite circulation as described by the BerezinskiiKosterlitzThouless (BKT) theory, rather than the emergence of true longrange order. BKT superfluidity is characterized by a universal jump in the superfluid density at a critical temperature without any discontinuities in the fluid's thermodynamic properties. One of the hallmarks of superfluidity, predicted by the highly successful twofluid model and observed in both liquid helium and ultracold atomic gases, is the existence of two kinds of sound excitations, the first and second sound. In 2D superfluids the observation of these sound modes has remained elusive. Here we observe first and second sound in a homogeneous 2D atomic Bose gas, and from the two temperaturedependent sound speeds deduce its superfluid density. Our results agree with BKT theory, including the prediction for the universal superfluiddensity jump. 
Friday, June 4, 2021 8:12AM  8:24AM Live 
X07.00002: Progress towards observing quantum fluctuations in matterwave soliton breathers Yi Jin, De Luo, Ricardo Espinoza, Randy Hulet, Boris Malomed, Vladimir Yurovsky, Oleksandr Marchukov, Vanja Dunjko, Maxim Olshanii Solitons are 1D nonlinear waves that propagate without dispersion. Higherorder solitons, i.e. coherent superpositions of fundamental solitons with specific amplitude ratios, are known as breathers, and can be formed from fundamental solitons using a prescribed interaction quench. Breathers are exactly integrable solutions of the meanfield (MF) nonlinear Schrodinger equation and are immune to dissociation. In quantum manybody theory, however, the relative separation of solitons is no longer conserved, thus endowing the breather with exquisite sensitivity to beyondMF effects^{1,2}. We experimentally produce and characterize breathers starting with a bright matterwave soliton, prepared from a BoseEinstein condensate (BEC) of ^{7}Li confined to a quasi1D harmonic potential formed from a single focused IR laser beam. The interactions are initially tuned to be slightly attractive using a Feshbach resonance. An nth order breather is created by quenching the strength of the attractive interactions by a factor of n^{2}, where n is an integer. We realize both the 2nd and 3rd order breathers, and show how their breathing frequency depends on the number of atoms and the aspect ratio of the quasi1D trap potential. Our observations agree well with a quasi1D MF theory. We report the progress made towards observing breather dissociation. 
Friday, June 4, 2021 8:24AM  8:36AM Live 
X07.00003: Simulating ManyBody Open Quantum Systems Beyond the BornMarkov Approximation Stuart Flannigan, François Damanet, Andrew Daley Ultracold atomic systems offer a controllable way of probing the effects of dissipative processes on the dynamics of manybody systems in regimes that go beyond those typical in quantum optics, such as out with a BornMarkov approximation. It is also desirable to develop numerical tools for theoretically analysing these systems, however this requires additions to conventional quantum trajectory techniques as these are only valid when a Lindblad master equation could be used to describe the dynamics. 
Friday, June 4, 2021 8:36AM  8:48AM Live 
X07.00004: Strongly coupling bipolarons and polaron interactions in 1D Bose gases Martin Will, Gregory Astrakharchik, Michael Fleischhauer Bose polarons, quasiparticles composed of mobile impurities surrounded by cold Bose gas, can experience strong interactions mediated by the manybody environment and form bipolaron bound states. Here we present a detailed study of heavy polarons in a onedimensional Bose gas by formulating a nonperturbative theory and complementing it with exact numerical simulations. We developed a semi analytic approach for weak bosonboson interactions and arbitrarily strong impurityboson couplings. It is based on a meanfield theory that accounts for deformations of the superfluid by the impurities and in this way minimizes quantum fluctuations. The meanfield equations are solved exactly in BornOppenheimer (BO) approximation leading to a semi analytic expression for the interaction potential of heavy polarons which is found to be in excellent agreement with quantum MonteCarlo (QMC) results. In the strongcoupling limit the potential substantially deviates from the exponential form valid for weak coupling and has a linear shape at short distances. Taking into account lowestorder BO corrections we calculate bipolaron binding energies and find excellent agreement with QMC results for impurityboson mass ratios as low as 3. 
Friday, June 4, 2021 8:48AM  9:00AM Live 
X07.00005: Density fluctuations in a twodimensional SU(N) Fermi gas Chengdong HE, Entong ZHAO, Zejian Ren, Elnur Hajiyev, Ka Kwan Pak, Gyuboong Jo In an interacting SU(N) Fermi gas, interaction effects are enhanced by SU(N) symmetry and therefore thermodynamics is expected to depend on spin multiplicity N. Here, we demonstrate a direct measurement of spindependent density fluctuations of twodimensional degenerate Fermi gasses with SU(N) symmetry. The scaling of compressibility with spin multiplicity is extracted from the fluctuationdissipation theorem. This work extends the quantitative thermodynamic study to lower dimensions. 
Friday, June 4, 2021 9:00AM  9:12AM 
X07.00006: Generation of spin currents by a temperature gradient in strongly interacting onedimensional systems Rafael E Barfknecht, Angela Foerster, Nikolaj T Zinner, Artem Volosniev We argue that the finitetemperature dynamics of a strongly interacting onedimensional system can be modeled using a Heisenberg spin chain whose couplings are defined by the local temperature [see arXiv:2101.02020]. This allows us to introduce a microscopic theory in which spin currents are generated by a temperature gradient. We demonstrate the generation of a spin current due to the temperature gradient by studying the dynamics that follows a spinflip of an atom in the chain. A temperature gradient accelerates the atom in one direction more than in the other, leading to an overall spin current, which is similar to the spin Seebeck effect. 
Friday, June 4, 2021 9:12AM  9:24AM Live 
X07.00007: Topological pumping of a 1D dipolar gas into strongly correlated quantum manybody scar states KuanYu Lin, Wil Kao, KuanYu Li, Sarang Gopalakrishnan, Benjamin L Lev Longlived excited states of interacting quantum systems that retain quantum correlations and evade thermalization are of great fundamental interest. We create nonthermal states in a bosonic onedimensional (1D) quantum gas of dysprosium by stabilizing a superTonksGirardeau gas against collapse and thermalization with repulsive longrange dipolar interactions. Stiffness and energyperparticle measurements show that the system is dynamically stable regardless of contact interaction strength. This enables us to cycle contact interactions from weakly to strongly repulsive, then strongly attractive, and finally weakly attractive. We show that this cycle is an energyspace topological pump (caused by a quantum holonomy). Iterating this cycle offers an unexplored topological pumping method to create a hierarchy of increasingly excited quantum manybody scar states. 
Friday, June 4, 2021 9:24AM  9:36AM Live 
X07.00008: Superfluid BKT transition of 2D bubbletrapped condensates Andrea Tononi, Axel Pelster, Luca Salasnich A relevant role in the superfluid transition of ultracold bosonic atoms is played by the complex interplay between the system topology and the external trapping potential. We discuss here the qualitative and quantitative aspects of the superfluid BKT transition of twodimensional bubbletrapped superfluids. Due to technical limitations in groundbased experiments, these curved quantum systems are currently produced in the Cold Atom Lab (CAL), a microgravity facility for studying BoseEinstein condensates on board of the International Space Station. In our work, relying on a new derivation of the equation of state of these topologicallynontrivial systems, we analyze the nonuniversal character of BKT physics, and we discuss how the vanishing of the superfluid density affects the hydrodynamic modes. Our theoretical predictions are of immediate experimental interest, being tailored on the typical parameters and regimes of CAL experiments. 
Friday, June 4, 2021 9:36AM  9:48AM Live 
X07.00009: Improving the grasp of harmonically trapped fermions in low dimensions Lukas Rammelmüller, David Huber, Artem Volosniev Experiments with only a few atoms shed light on the crossover from few to manybody physics. Theoretical treatment of these experiments is challenging because the underlying Hilbert space grows rapidly with particle number. The standard effective treatment for shortranged interactions relies on the δpotential, for which the exact diagonalization in a truncated Hilbert space converges slowly. Moreover, in spatial dimensions larger than one, the δpotential requires careful renormalization to converge at all. Here we exploit a particularly effective renormalization procedure adapted from nuclear physics: We fix the lowest part of the twobody spectrum using the exact solution. This provides us with an efficient tool to obtain physical observables free from a cutoff dependence at significantly reduced computational cost. 
Friday, June 4, 2021 9:48AM  10:00AM Live 
X07.00010: Impurityimpurity interaction mediated by a onedimensional Bose gas Fabian Brauneis, Hans W Hammer, Mikhail Lemeshko, Artem Volosniev The first step for understanding a Bose gas with impurity atoms is to study a single impurity, i.e. the Bose polaron problem. However, the physics of systems with many impurities can be drastically different from the Bose polaron, because the Bose gas mediates interactions between impurities. We use the flow equation approach to calculate repulsive and attractive impurityimpurity correlations mediated by a weakly interacting onedimensional Bose gas in the BornOppenheimer approximation. We find that leading order perturbation theory, which leads to Yukawatype potentials, fails when the bosonimpurity interaction is stronger than the bosonboson interaction. For attractive impurityimpurity potentials, we also calculate the potential within a meanfield approximation (MFA). Our results show that the MFA is useful to study mesoscopic and large systems with two impurities. In particular, it allows one to calculate the induced impurityimpurity interaction potential beyond leading order perturbation theory in a simple manner, i.e., without including any information about beyondmeanfield bosonboson correlations. 
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