Quantum-Fluctuation-Driven Crossover from a Dilute Bose-Einstein Condensate to a Macrodroplet in a Dipolar Quantum Fluid

Abstract

In a joint experimental and theoretical effort, we report on the formation of a macro-droplet state in an ultracold bosonic gas of erbium atoms with strong dipolar interactions. By precise tuning of the s-wave scattering length below the so-called dipolar length, we observe a smooth crossover of the ground state from a dilute Bose-Einstein condensate (BEC) to a dense macro-droplet state of more than 10 4 atoms. Based on the study of collective excitations and loss features, we quantitative prove that quantum fluctuations stabilize the ultracold gas far beyond the instability threshold imposed by mean-field interactions. Finally, we perform expansion measurements, showing the evolution of the normal BEC towards a three-dimensional self-bound state and show that the interplay between quantum stabilization and three-body losses gives rise to a minimal expansion velocity at a finite scattering length.

Classical and quantum filaments in the ground state of trapped dipolar Bose gases

“…A different outcome was recently reported [12], however, in a similar experiment involving dysprosium (Dy), in which collapse was replaced by the formation of ordered arrays of droplets, reminiscent of the Rosenweig instability in classical ferrofluids [13,14]. In a subsequent experiment with erbium (Er), a smooth evolution of the system from a trapped BEC to a dense macrodroplet was observed, as the s-wave scattering length of the atomic interaction was varied [15].…”

“…15. In the intermediate regime, henceforth referred to as "of quantum filaments", exchanges of particles across different filaments underlie a global superfluid response, similarly to what observed in supersolid droplet crystals [27].…”

Dissociation of One-Dimensional Matter-Wave Breathers due to Quantum Many-Body Effects

“…The resulting state constitutes an NLS breather with an aphelion density profile proportional to sech 2 ðx=l breather Þ and width l breather ¼ 8ℏ 2 =ðmgNÞ ¼ 36 μm [21,22]. Assuming that the splitting of the breather into two solitons becomes apparent when the distance between their centers-ofmasses, after evolution time τ, Δx ¼ Δv · τ, becomes comparable to the breather's width l breather , and using extrapolation (6) for the relative velocity of the solitons, we obtain τ ≃ 3 s for the time necessary to certainly observe the splitting of the breather caused by the quantum dynamics.…”

“…The observability of microscopic quantum effects involving a substantial fraction of the particles in a coherent macroscopic setting generally requires going beyond MF, for example, at low density in one dimension (1D) [1,2] or high density in three dimensions (3D). In 3D systems, the high-density Lee-Huang-Yang corrections, which are induced by quantum correlations, were realized experimentally using the Feshbach resonance [3] and in the spectacular form of "quantum droplets" in dipolar [4][5][6] and isotropic [7] bosonic gases, i.e., as self-trapped states stabilized against the collapse by the beyond-MF selfrepulsion. This stabilization was predicted in Refs.…”

Excitations and stability of weakly interacting Bose gases with multibody interactions

“…On the other hand, the presence of an asymmetric harmonic trapping in combination with short-range repulsive interactions can eliminate such instabilities, opening the way to the study of interesting many-body physics with long-range interactions [59,60]. Recent experiments with dipolar BECs showed that under certain conditions where instability is expected from a standard Bogoliubov approach, dense clusters with many atoms can occur [61][62][63][64][65], which are expected to be superfluid [66]. Two interpretations have been proposed to explain the stabilization of this phase, namely the presence of weak 3-body interactions [11,12] and beyond mean-field effects (Lee-Huang-Yang type corrections) [67][68][69].…”