Quantum-fluctuation-driven crossover from a dilute Bose-Einstein condensate to a macro-droplet in a dipolar quantum fluid

TL;DR

This paper demonstrates experimentally and theoretically how quantum fluctuations induce a smooth transition from a dilute Bose-Einstein condensate to a dense macro-droplet in a dipolar quantum fluid, stabilized beyond mean-field predictions.

Contribution

It provides the first combined experimental and theoretical evidence of quantum-fluctuation-driven stabilization of a dipolar Bose gas in the macro-droplet regime.

Findings

Observation of a smooth BEC to macro-droplet crossover

Quantum fluctuations stabilize the droplet state beyond mean-field instability

Identification of a minimal expansion velocity at finite scattering length

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.