Since the first observation of a Bose-Einstein condensate (BEC) made from strongly magnetic atoms, these systems have proven to be a rich source of new and fascinating phenomena arising from the long-range and anisotropic dipole-dipole interaction. Recently, these dipolar quantum gases have proven to be a versatile platform in order to observe the long-sought after supersolid phase—a state that simultaneously manifests a crystalline order and superfluid properties. Here, I will present the latest results from our research on ultracold dipolar quantum gas in Innsbruck, with a particular focus on the combined theoretical and experimental effort to understand the rotational properties of these systems in different phases. The presence of quantized vortices—topological defects of the wavefunction characterized by a 2π phase winding—consists of one of the most distinctive manifestations of their superfluid nature. I will report on the theoretical study and experimental observation of quantized vortices in a strongly magnetic gas of dysprosium atoms in both the unmodulated BEC and supersolid phases [1,2]. Finally, I will show an application of rotating dipolar supersolids, i.e. the possibility to simulate “glitches”, instantaneous jumps of the rotation frequency occurring due to the internal vortex dynamics, akin to observations in neutron stars [3].
[1] L. Klaus, T. Bland et al., Nature Physics 18, 1453–1458 (2022).
[2] E. Casotti, E. Poli et al., arXiv:2403.18510 (2024).
[3] E. Poli et al., Phys. Rev. Lett. 131, 223401 (2023).
]]>Abstract: We present an experimental platform allowing to model arbitrary 2D hamiltonians using alloptical control of fluid of light in a hot Rubidium vapor [1]. Using full-field retrieval of the quantum fluid, we can measure momenta distributions and hydrodynamical observables and use this information to probe the superfluid transition [2, 3] in a time-resolved manner. We also engineer the quantum fluid to study the dynamics of quantized vortices and scale it towards the study of turbulence [4].
References
Glorieux, Q. et al. Hot atomic vapors for nonlinear and quantum optics. en. New Journal of Physics 25, 051201 (2023).
Huynh, J. et al. Two-dimensional superflow past an obstacle of arbitrary penetrability: Exact results for the critical velocity 2023. arXiv: 2305.01293.
Michel, C. et al. Superfluid motion and drag-force cancellation in a fluid of light. Nat. Comm. 9, 2108 (2018).
Abobaker, M. et al. Inverse energy cascade in two-dimensional quantum turbulence in a fluid of light 2022. arXiv: 2211.08441.
When charged particles are placed in a magnetic field, the single-particle energy states form discrete, highly-degenerate Landau levels. Since all states within a Landau level have the same energy, the behaviour of the system is completely determined by the interparticle interactions and strongly-correlated behaviour such as the fractional quantum Hall effect occurs. Here, we present recent experiments from MIT on the microscopy of a rapidly-rotating Bose-Einstein condensate, in which the Coriolis force felt by a massive particle in a rotating frame plays the role of the Lorentz force felt by a charged particle in a magnetic field. In a magnetic field the X and Y coordinates of a particle do not commute, leading to a Heisenberg uncertainty relation between spatial coordinates. We exploit the ability to squeeze non-commuting variables to dynamically create a Bose-Einstein condensate occupying a single Landau gauge wavefunction, and investigate its purely interaction-driven dynamics in the lowest Landau level. We reveal a spontaneous crystallization of the fluid, driven by the interplay of interactions and the magnetic field; increasing the cloud density smoothly connects this quantum behavior to a classical Kelvin-Helmholtz-type hydrodynamic instability, driven by the sheared superfluid flow profile arising from the vector potential. Finally, we project a sharp optical boundary onto our system and demonstrate controllable injection of its associated chiral edge modes, quantifying their speed, excitation energy, and dependence upon wall structure.
]]>A classical system can have multiple equilibrium states at different energies; the absolute ground state is a stable configuration, while the other local energy minima are metastable. In a quantum field theory, quantum fluctuations can trigger the macroscopic tunneling of the field from a metastable state (the false vacuum) to the ground state (true vacuum) through the many-body energy barrier [1]. The decay of a false vacuum, which is thought to manifest via the nucleation of spatially localized bubbles of true vacuum, is a fascinating phenomena for a rich variety of systems, ranging from condensed matter [2] to cosmology [3]. Nevertheless, its experimental verification has been elusive so far, due to the extreme energy scales involved and the lack of tunable parameters. Here, I will present the first experimental evidence of false vacuum decay in a metastable ferromagnetic superfluid [4]. In this novel platform, realized with ultracold sodium atoms [5], the superfluid acts as the quantum field, and macroscopic tunneling events are observed through the spontaneous nucleation of spin bubbles. Our results find good agreement with numerical simulations and instanton theory, opening the way for the simulation of out-of-equilibrium phenomena in a highly controllable and tunable atomic system.
References
[1] The Fate of the False Vacuum. 1. Semiclassical Theory, S. R. Coleman, Phys. Rev. D. 16, 1248 (1977)
[2] False vacuum decay in quantum spin chains, G. Lagnese et al., Phys. Rev. B. 104 L201106 (2021)
[3] Phase transitions in the early universe, M. Hindmarsh et al. SciPost Phys. Lect. Notes 24 (2021)
[4] False vacuum decay via bubble formation in ferromagnetic superfluids, A. Zenesini, A. Berti, R. Cominotti et al, Nat. Phys. (2024).
[5] Ferromagnetism in an Extended Coherently Coupled Atomic Superfluid, R. Cominotti, A. Berti et al.
]]>The contribution of this experimental technique to our understanding of turbulent flows of superfluid helium-4 is reviewed and special emphasis is given to recent results obtained in Prague. Specifically, it is shown that flow visualization data can be employed to estimate (i) the mean distance between quantized vortices, especially in conditions that cannot be accessed by the standard second sound attenuation technique, and (ii) the strength of macroscopic vortical structures, especially in the absence of Eulerian data, which is often the case for flows of superfluid helium-4.
]]>The static and dynamic properties of vortices in dipolar Bose-Einstein condensates (dBECs) can be considerably modified relative to their nondipolar counterparts by the anisotropic and long-ranged nature of the dipole-dipole interaction (DDI). In the context of a uniform, three-dimensional dBEC, I will present recent results on the structure of single vortex lines and the dynamics of pairs of vortices across a range of scenarios. For a single vortex, by isolating the effects of the DDI from those of external trapping, I demonstrate intrinsic differences in its density and phase structure owing to the effective interaction between the core and the bulk. These discrepancies can be controlled by tuning the parameters that pertain to the DDI. The ramifications of this are then elucidated for the dynamics of pairs of straight vortex lines, where both same-signed vortex-vortex trajectories and opposite-signed vortex-vortex translational velocities are demonstrated to diverge from the point-vortex paradigm in a dipole-dependent manner. I will also discuss preliminary results on the reconnection dynamics of a pair of vortex lines, where the influence of the DDI on the curvature of the vortices results in dipole-mediated reconnection times and post-reconnection vortex structures. These findings open the door to a deeper understanding of the mechanisms that underlie many-vortex phenomena in dipolar superfluids such as vortex lattice dynamics and quantum turbulence, and could be studied experimentally in optically generated three-dimensional box-trapping potentials.
]]>Recent experimental advancements in ultracold atoms are leading to the controlled creation of complex topological excitations in spinor Bose-Einstein condensates (BECs) [1]. In this webinar, I will address defects and their dynamics in a spin-2 BECs, where the rich variety of order-parameter symmetries allows exotic phenomena such as fractional vortex charges, monopoles, and non-singular spin textures. In particular, I will show how energy relaxation causes a monopole in the uniaxial-nematic phase to deform into a spin-Alice ring, exhibiting a composite core with distinct short- and long-distance topologies [2]. Numerical simulations reveal dynamical oscillations between the spin-Alice ring and a split-core hedgehog configuration. Moreover, I will also address hybrid defect connections and their stability across optically induced topological interfaces in uniaxial-to-biaxial nematic (UN-BN), cyclic-to-ferromagnetic (C-FM), and cyclic-to-biaxial nematic (C-BN) phases, where a sudden topology change occurs in a fashion similar to that of early-universe theories [3].
[1] Xiao, Y. et al., Nat. Commun. 13, 4635 (2022).
[2] Baio, G. & Borgh M. O., arXiv:2401.04103 (2024).
[3] Baio, G. et al., Phys. Rev. Research 6, 013046 (2024).
]]>I will present our investigations of a low-energy effective field theory of a two-dimensional superfluid vortex crystal which reduces to a scalar Lifshitz theory in the linearized approximation. General symmetry considerations allow us to determine non-linear terms that fix a decay rate of a unique collective gapless Tkachenko mode which disperses quadratically at low momenta. I will also introduce a linearized fracton-elasticity duality adapted to the vortex crystal that allows to incorporate naturally topological crystalline defects such as disclinations and dislocations. Based on that duality, I will discuss possible scenarios of melting of the vortex crystal due to quantum fluctuations. Finally, I will present our first steps towards a non-linear extension of the elasticity duality which gives rise to a dynamical theory of gravity.
]]>In typical fluids like air or water viscosity plays a crucial role in flight. It allows for the development of a boundary layer that separates from an airfoil when it is accelerated from rest; this separation forms the starting vortex that allows for the generation of lift. It is then interesting to ask what happens in a fluid, like a superfluid, that does not possess any viscosity whatsoever. To investigate this we study the development of superfluid flow around an airfoil accelerated to a finite velocity from rest. Using both simulations of the Gross-Pitaevskii equation and analytical calculations we find striking similarities to viscous flows: from the production of starting vortices to the convergence of the airfoil circulation onto a quantized version of the classical Kutta-Joukowski circulation. Using a phenomenological argument we predict the number of vortices nucleated by a given foil and find good agreement with numerics. Finally we analyze the lift and drag acting on the airfoil. Our simulations suggest that flight is indeed possible in a superfluid despite its lack of viscosity.
]]>The talk is an overview of selected topics of quantum turbulence (QT) [1] - the stochastic motion of quantum fluids He II and 3He-B, recently described in detail in Ref. [1]. We focus on unified phenomenological description of 3D QT [2] (especially of the Vinen and Kolmogorov forms of QT, emerging as a direct consequence of quantum mechanical constraint on circulation in a superfluid) and description of transition to QT [3] (based on quantum channel flows, high-Stokes-number oscillatory flows and spherical counterflow [4]), both in the zero temperature limit and at finite temperatures where these superfluids display the two-fluid behavior.
[1] C.F. Barenghi, L. Skrbek and K.R. Sreenivasan, Quantum turbulence, Cambridge Univ. Press, 2023 [2] L. Skrbek, D. Schmoranzer, S. Midlik, K.R. Sreenivasan, Phenomenology of quantum turbulence in superfluid helium, Proc. Natl. Acad. Sci., U.S.A. 118, e2018406118 (2021). [3] L. Skrbek, D. Schmoranzer, K.R. Sreenivasan, Phenomenology of transition to quantum turbulence in superfluid helium, Proc. Natl. Acad. Sci., U.S.A, in print. [4] F. Novotný, Y. Huang, J. Kvorka, Š. Midlik, D. Schmoranzer, Z. Xie, L. Skrbek, Spherical thermal counterflow of superfluid 4He, submitted to Phys. Rev. Fluids
]]>