Bulletin of the American Physical Society
51st Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 65, Number 4
Monday–Friday, June 1–5, 2020; Portland, Oregon
Session C06: Quantum Gases in Low DimensionsLive

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Chair: Dan StamperKurn, University of California, Berkeley Room: E141142 
Tuesday, June 2, 2020 10:30AM  10:42AM Live 
C06.00001: Observation of Dynamical Fermionization Yuan Le, Joshua M. Wilson, Neel Malvania, Yicheng Zhang, Marcos Rigol, David S. Weiss In one dimension, the momentum distribution of a TonksGirardeau (TG) gas of strongly interacting bosons is predicted to evolve from a bosonic to a fermionic shape after turning off the axial confinement [1, 2]. By letting atoms expand in 1D after such a quench, we observe this dynamical fermionization [3]. In this way, we have made the first ever measurement of a distribution of rapidities, which are the momenta of quasiparticles that emerge from interactions in integrable systems. The measurements agree well with numerical simulations of our TG gas. We also observe bosonicfermionic oscillations of the momentum distribution after the trap depth is suddenly changed to a new nonzero value [2, 3]. [1] M. Rigol and A. Muramatsu, PRL 94, 240403 (2005). [2] A. Minguzzi and D. M. Gangardt, PRL 94, 240404 (2005). [3] J. M. Wilson, N. Malvania, Y. Le, Y. Zhang, M. Rigol, and D. S. Weiss, arXiv:1908.05364 (to appear in Science). [Preview Abstract] 
Tuesday, June 2, 2020 10:42AM  10:54AM Live 
C06.00002: Molecular association in a pwave Fermi gas under variable confinement. Kenneth G. Jackson, Denise Braun, Colin Dale, Scott Smale, Servaas Kokkelmans, Joseph H. Thywissen We study molecular formation near a pwave Feshbach resonance in an ultracold Fermi gas. Radiofrequency association line shapes are fit using a thermally weighted FranckCondon overlap function that correctly accounts for asymmetry in the profiles. We can then refine the effective range parameters for pwave scattering in the second lowest magnetic hyperfine level of 40K. Using this model of the threedimensional pwave scattering phase shift, we load the atoms into a deep twodimensional optical lattice and associate onedimensional dimers to test different models of confinement. Specific attention is paid to the resonance position shift and the experimental feasibility of a 1D unitary regime for oddwave scattering. The importance of manybody correlations is determined through measurements of the 1D pwave contacts. [Preview Abstract] 
Tuesday, June 2, 2020 10:54AM  11:06AM Live 
C06.00003: Suppression of threebody loss of $p$wave fermions in quasi1D YaTing Chang, Ruwan Senaratne, Danyel CavazosCavazos, Randall G. Hulet Recent interest in quantum computing has brought attention to the study of $p$wave interactions, which are known to result in intriguing quantum phenomena such as $p + ip$ topological superfluids and Majorana fermions. However, the exploration of these phenomena in ultracold atomic gases has been impeded due to the severe atom losses from threebody recombination collisions near the $p$wave Feshbach resonance in a 3D atomic gas. Previous work predicted\footnote{Lihong Zhou and Xiaoling Cui, Phys. Rev. A 96, 030701 (2017).} that such severe losses could be suppressed in quasi1D. If proven true, this could open a possible avenue for realizing the Kitaev chain model. We characterized the threebody loss in quasi1D using spinpolarized $^6$Li atoms in a twodimensional optical lattice. We measured a reduction in the threebody loss coefficient as a function of lattice depth. The confinement induced shift and the shape of the resonance feature are consistent with coupled channels calculations for $p$wave scattering in quasi1D. [Preview Abstract] 
Tuesday, June 2, 2020 11:06AM  11:18AM Not Participating 
C06.00004: Second Sound and the Superfluid Jump in a Uniform TwoDimensional Bose Gas 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 long range order. In both cases, however, superfluidity manifests itself in the existence of two distinct hydrodynamic sound modes of low lying excitations, first and second sound. In 2D, the detection of second sound, which is directly related to the superfluid density, has remained elusive, besides tremendous efforts both in the context of liquid Helium thin films and ultracold gases. Here we report on the first observation of second sound in a 2D gas confined in a uniform trap of ultracold Bosonic $^{39}$K atoms, accompanied by the simultaneous measurement of the first sound mode. To reach the collisional criteria for the two modes to appear, we tune interactions using a broad magnetic Feshbach resonance available for $^{39}$K. Our results also show the sudden disappearance of second sound at high temperatures, offering a strong link with the BKT theory and its prediction of a universal jump of superfluid density at the critical temperature. [Preview Abstract] 
Tuesday, June 2, 2020 11:18AM  11:30AM Live 
C06.00005: Dimensional Crossover of Photon BoseEinstein Condensates Enrico Stein, Axel Pelster In recent years the phenomenon of equilibrium BoseEinstein condensation (BEC) of photons has been studied extensively also within the realm of nonequilibrium condensation. At its core this system consists of a dye solution filling the microcavity in which the photons are trapped. Due to cyclic absorption and reemission processes of photons the dye leads to a thermalisation of the photon gas at room temperature and finally to its BoseEinstein condensation. Because of a nonideal quantum efficiency, those cycles yield in addition a heating of the dye solution, which results in an effective photonphoton interaction [1]. This talk focuses on the theoretical description of a dimensional crossover from a twodimensional photon BEC to a onedimensional photon gas. To this end we extend the semiclassical meanfield equations for a photon BEC [2] by including the matter degrees of freedom. Our special focus lies on the effect of the retarded photonphoton interaction on the dimensional crossover, which we study for a anisotropic box potential. Finally, we characterise the steady state of the resulting onedimensional photon gas.\\ [1] Kl\"ars et al., Appl. Phys. B \textbf{105}, 17 (2011)\\ [2] Stein et al., New J. Phys. \textbf{21}, 103044 (2019) [Preview Abstract] 
Tuesday, June 2, 2020 11:30AM  11:42AM Live 
C06.00006: A subradiant optical mirror formed by a single structured atomic layer Jun Rui, David Wei, Antonio RubioAbadal, Simon Hollerith, Christian Gross, Immanuel Bloch, Johannes Zeiher, Dan StamperKurn Efficient and versatile interfaces for the interaction of light with matter are an essential cornerstone for quantum science. A fundamentally new avenue of controlling lightmatter interactions has been recently proposed based on the rich interplay of photonmediated dipoledipole interactions in structured subwavelength arrays of quantum emitters. Here we report on the direct observation of the cooperative subradiant response of a twodimensional (2d) square array of atoms in an optical lattice. We observe a spectral narrowing of the collective atomic response well below the quantumlimited decay of individual atoms into free space. Through spatially resolved spectroscopic measurements, we show that the array acts as an efficient mirror formed by only a single monolayer of a few hundred atoms. By tuning the atom density in the array and by changing the ordering of the particles, we are able to control the cooperative response of the array and elucidate the interplay of spatial order and dipolar interactions for the collective properties of the ensemble. Bloch oscillations of the atoms out of the array enable us to dynamically control the reflectivity of the atomic mirror. Our work demonstrates efficient optical metamaterial engineering based on structured ensembles of atoms. [Preview Abstract] 
Tuesday, June 2, 2020 11:42AM  11:54AM Live 
C06.00007: Shellshaped BEC with mixtures Patrick Boegel, Alexander Wolf, Matthias Meister, Naceur Gaaloul, Antun Balaz, Maxim A. Efremov, Wolfgang P. Schleich Currently there is a huge interest in the properties of hollow BoseEinstein condensates (BECs) [Phys. Rev. A 98, 013609 (2018)], which are commonly realized with radiofrequency (rf) dressing potentials. As an alternative method we propose to use a dualspecies mixture for the preparation of such hollow BECs. By tuning the interspecies interaction strength and the number of atoms we can realize a ground state where one atomic species is centered in the middle, while the other species forms a shell around it. In order to obtain the width and the form of the shell as well as the spectrum of the collective excitations, from the filled to a hollow BEC, we solve the GrossPitaevskii equation both numerically and within the ThomasFermi approach. For the hollow case the interspecies boundary leads to a change to the collective excitation spectrum. This new method could be more robust for the creation of bubble BECs than the method of rf dressing potentials. [Preview Abstract] 
Tuesday, June 2, 2020 11:54AM  12:06PM On Demand 
C06.00008: Designing Quantum Phases at the interface of Atomic and TwoDimensional Dirac Quantum Matter Valeri Kotov, Adrian Del Maestro, Jiangyong Yu, Taras Lakoba Novel twodimensional (2D), atomically flat materials, such as graphene and transitionmetal dichalcogenides, exhibit unconventional Dirac electronic spectra, and we propose that their interactions with cold atoms can be effectively quantum engineered, leading to a synergy between complex electronic and atomic collective quantum phases and phenomena. In particular we discuss theoretically how to manipulate the Casimir / van der Waals (vdW) force between atoms and graphene monolayer through the application of strain, electronic doping, etc., which can lead to selective adsorption and also change the interactions between individual atoms. This allows us to influence fundamental phenomena such as Quantum Reflection, and also analyze manifestations of such 2D effects for many atoms forming a confined BoseEinstein condensate (BEC) placed near 2D materials, which in turn makes the BEC frequency sensitive to the material presence. Finally we discuss the exciting possibility of a novel 2D anisotropic superfluid state formed by helium on strained graphene. This is based on preliminary largescale ab initio quantum Monte Carlo simulations combined with a mapping of the problem to an effective BoseHubbard model. [Preview Abstract] 
Tuesday, June 2, 2020 12:06PM  12:18PM Not Participating 
C06.00009: Nonlinear Damping in a StronglyDriven Two Dimensional Bose Gas in a Box. Maciej Galka, Panagiotis Christodoulou, Nishant Dogra, Julian Schmitt, Zoran Hadzibabic Ultracold atoms constitute a powerful platform to study nonequilibrium phenomena by providing a tractable testbed for microscopic models of the emergent complex macroscopic behavior. In this light, we aim at investigating the response of a quantum manybody system that is driven further and further away from its equilibrium state. Our experiment is based on an interactiontunable 2D Bose gas confined in a box trap, in which we excite the lowestlying long wavelength sound mode by shaking with a magnetic field gradient. Within the linear perturbation regime, this zerotemperature excitation is expected to be undamped due to a gapped energy spectrum and the absence of a (thermal) bath. On increasing the shaking amplitude, we however observe a finite damping rate of this mode, which is solely attributed to its nonlinear coupling to higher excited phonon states arising from interparticle interactions. We find a powerlaw decay of the quality factor $Q$ of the sound mode as a function of the driving strength. We further discover a universal behavior of $Q$ as we vary the interaction strength. Our observations establish a connection between the weakly driven regime of linear excitations and the highly nonequilibrium turbulent state obtained by strongly driving the system. [Preview Abstract] 
Tuesday, June 2, 2020 12:18PM  12:30PM 
C06.00010: Selfinterfering nature of dispersive quantum shock waves in a onedimensional Bose gas Karen Kheruntsyan, S.A. Simmons, S.N. Saadatmand, D. Colas, F. A. Bayocboc, Jr., I.P. McCulloch Shock waves represent examples of farfromequilibrium phenomena for which fundamental understanding of the laws of emergence from the underlying manybody interactions is generally lacking. Here we study dispersive quantum shock waves in a 1D Bose gas, described by the LiebLiniger model and forming from a local density bump expanding into a uniform background. We show that the microscopic mechanism behind the formation of the oscillatory shock wave train is quantum mechanical interference. Our results incorporate the effects of quantum and thermal fluctuations and span the entire range of interaction strengths, from the noninteracting (ideal) Bose gas regime, through the weaklyinteracting or GrossPitaevskii regime, to the regime of infinitely strong interactions corresponding to the TonksGirardeau gas of hardcore bosons. The amplitude of oscillations in the shock wave train, i.e., the visibility of interference fringes, decreases with the increase of both the temperature of the gas and the interaction strength. In both cases this is a consequence of: (a) the reduced phase coherence length in the gas, and (b) the intrinsic fluctuations in the position of the interference pattern from shottoshot due to quantum and thermal fluctuations. [Preview Abstract] 
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