Session N03: Quantum Gases in Low Dimensions

Berezinskii-Kosterlitz-Thouless Transition in a Two-dimensional Dipolar Bose Gas

Instead of statistic-driven Bose-Einstein-Condensate transition, interaction-driven Berezinskii-Kosterlitz-Thouless(BKT) transition connects the normal phase and superfluid phase in two dimension(2D) where a quasi long-range order reveals superfluidity. The BKT transition has been extensively studied with ultracold quantum gases in quasi-2D confinement, but only with contact interactions so far. In this talk, we will present the latest study on 2D quantum gases with long-range and anisotropic dipole-dipole interaction (DDI) utilizing magnetic erbium atoms in a quasi-2D harmonic trap. We characterize the dipole-angle-dependent BKT transition point by measuring the onset of extended coherence and equation of states (EoS). We also investigate the scale invariance and universal behavior around the BKT critical point in the presence of DDI. Additionally, we observe a unique anisotropic number fluctuation in the dipolar 2D superfluid with tilted dipoles, which reflects the anisotropic density-density correlation in the system.   

High Resolution Imaging of Quantum Gases: Tracking Impurities in 2D Bose Gases

We report on our recent experiments on impurities in a degenerate two-dimensional K39 Bose gas.

For both 3D and 2D, we measure the polaron spectrum over a wide range over interactions by employing a interstate Feshbach resonance, and can model the line shapes in good agreement with observation when including excited polaronic states.

Furthermore, we experimentally investigated spatially and temporally resolved impurity dynamics in the 2D BKT gas.

For repulsive interactions we observe an outward motion of the impurities and ultimately the expulsion of the impurities from the bath. We can accurately describe this motion when including friction of the impurities with the superfluid for velocities above the speed of sound.   

Superfluidity in inhomogeneous 2D Bose-Einstein condensates

Superfluidity phenomena in the context of thermodynamics, nonlinear, and out-of-equilibrium physics can all be studied with the spectroscopic tool offered by the excitation of sound waves in a Bose-Einstein condensate.

In our experiment, we prepare a quasi-2D ²³Na Bose-Einstein condensate confined in a 2D box potential. We spatially modulate the condensate to create an inhomogeneous density profile, on which the dynamics of low-lying sound excitation is observed. We measure the speed of sound along and orthogonal to the 1D inhomogeneities; the case of a 1D lattice, studied in [1, 2], serves as a reference case. The anisotropic speed of sound reveals a reduction of the component of the superfluid density tensor along the modulation direction.  Different inhomogeneities potentially change the superfluid density differently, and we compare each case to the result of Leggett [3].

[1] G. Chauveau, C. Maury, F. Rabec, C. Heintze, G. Brochier, S. Nascimbene, J. Dalibard, J. Beugnon, S. M. Roccuzzo, and S. Stringari, Superfluid Fraction in an Interacting Spatially Modulated Bose-Einstein Condensate, Phys. Rev. Lett. 130, (2023).

[2] J. Tao, M. Zhao, and I. B. Spielman, Observation of Anisotropic Superfluid Density in an Artificial Crystal, Phys. Rev. Lett. 131, (2023).

[3] A. J. Leggett, Can a Solid Be “Superfluid”?, Phys. Rev. Lett. 25, 1543 (1970).   

Phase coherence and dynamical crystallization in attractive 1D Bose gases

Unveiling non-equilibrium instability dynamics of a many-body system is a current frontier across diverse fields. Low-dimensional quantum gases offer a versatile platform for studying microscopic details of instability-induced dynamics. An intriguing example is modulation instability (MI), which emerges when the interaction is quenched from repulsive to attractive. It is commonly known that MI amplifies initial density fluctuations, resulting in the formation of solitonic excitations observed in one-dimensional (1D) and two-dimensional (2D) quantum gases. In this talk, we present the first observation of multi-mode breather excitations resulting from the nonlinear stage of MI seeded by quantum and thermal fluctuations. We prepare two parallel quasi-1D gases in an integrability-preserving box potential. By performing both in-situ and interferometric imaging of two quenched samples, we directly measure density and phase modulations along the samples. Unlike phase-incoherent solitonic excitations, we observe a form of dynamical crystallization in which periodic density modulations recur dynamically in a time-evolution intertwined with reduction and recovery of global phase coherence. At longer times, we show that a dephased 1D gas can regain phase coherence after the interaction is ramped back to repulsion without significant atom losses. Those series of observations open a pathway to investigate very rich MI-induced dynamics, ranging from breathers to phase transition across zero scattering length.   

Matter-wave interference with molecular superfluids

In this work, we investigate the phase transformation from atomic to molecular Bose-Einstein condensates (BEC). Coupling between atoms and diatomic molecules can be expressed in the interaction Hamiltonian as a combination of annihilation and creation operators where we annihilate a molecule to create two atoms and vice versa.  If we include the phase of the atomic and molecular wavefunctions into the interaction, the system will remain invariant as long as the phase of the molecular wavefunction is twice as that of the atomic wavefunction. To probe this phase doubling, we implement an interferometric technique utilizing vortices. Quantum vortices are characterized by the phase winding of the macroscopic wavefunction around the annulus.  The phase of the vortex is doubled when we convert an atomic BEC containing a single vortex into a molecular one. To detect this phase transformation, we perform interferometry using an optical lattice to create interference fringes. The vorticity will affect the formation of interference fringes. We will report on our current progress.   

Phase Fluctuations in Concentric Rings of Fermionic Superfluid

Fluctuations are enhanced in low-dimensional systems, and in highly elongated superfluid systems, phase fluctuations are expected to cause power-law decay of long range order. We have investigated the effects of phase fluctuations in rings of ultracold fermionic superfluids, particularly how they affect preparation and deterministic control of quantized circulation states. We have observed increasing thermal phase fluctuations in narrow superfluid rings as the effective transverse width is decreased. In addition to the evidence of changing thermal correlation length within each ring, we also see a reduction in the strength of fluctuations when there is sufficiently strong coherent coupling between concentric rings.   

Interaction induced 1D-3D dimensional crossover

We have generated a one-dimensional quantum gas confined in an elongated dipole trap insteadof 2D optical lattices, and the sample, comprising thousands of atoms, spans several hundred mi-crometers and allows for independent control of temperature and chemical potential. This allows usto directly observe and investigate the spatial distribution and associated excitation of 1D quantumgas without any ensemble averaging. Utilizing Feshbach resonance, we observed that the dimensionof 1D gas will be popped up into 3D due to strong interaction without changing any trapping con-finement. During the dimensional crossover, we found that increasing the scattering length leads tothe failure of 1D theories, including 1D mean field, Yang-Yang equation, and 1D hydrodynamics.Specifically, the Yang-Yang equation effectively describes this 1D system at temperatures beyondthe 1D threshold, but it does not account for the effects of stronger interactions. Meanwhile, weobserve two quantized plateaus of breathing-mode oscillation frequencies predicted by 1D and 3Dhydrodynamics, corresponding to weak and strong interactions respectively. And there is also auniversal crossover connecting two different regimes where both hydrodynamics fail.   

Dynamics of a pseudo-2D Bose–Einstein condensate in an optical toroidal shell trap

Two dimensional quantum gases in curved geometries are of great interest and may be realizated using Bose–Einstein condensates (BECs) in microgravity. Here we investigate the dynamics of a pseudo-2D BEC confined to the surface of a torus. In our simulations, we consider ⁸⁷Rb atoms held in an optical dipole trap formed by Laguerre-Gaussian beams and an RF or MW field. The usual second-order contribution of the fields to the effective Hamiltonian is carefully canceled, which leads the third-order contribution to dominate and allows us to realize a potential in the shape of a toroidal shell. Using the Gross–Pitaevskii equation, we study how the local curvature of the trap, periodic boundary conditions, and nontrivial topology affect the evolution of vortices in the BEC. Finally, we discuss how this technique could also be used to realize a BEC confined to the surface of a trefoil knot.   

Observation of spatial first-order coherence in an optical quantum gas in a box

The emergence of long-range phase correlations is a key signature for phase transitions to ordered states of matter. Exploring the first-order spatial coherence in large samples with uniform density provides direct insight into such correlations insensitive to the local density. Here we report measurements of the spatial phase correlations in a two-dimensional photon gas inside a box potential trap realized in a nanostructured dye-filled microcavity. The correlations of the uniform optical quantum gas are determined both from the momentum space distribution and by interferometry of the microcavity emission. While in the normal gas the coherence length is governed by short-range thermal correlations, the quantum degenerate gas is observed to build up quasi-long-range correlations that decay exponentially. As the coherence length exceeds the finite size of the box trap, the optical quantum gas condenses, as verified for different system sizes.