Quantum liquid droplets in a mixture of Bose-Einstein condensates

TL;DR

This paper reports the discovery and characterization of a new type of quantum liquid droplets in a mixture of Bose-Einstein condensates, stabilized by contact interactions and exhibiting unique stability and phase transition properties.

Contribution

It introduces a novel class of quantum droplets stabilized solely by contact interactions in a BEC mixture, expanding understanding of self-bound quantum liquids.

Findings

Droplets are more dilute than liquid helium.

Quantum many-body effects prevent collapse.

A minimum atom number is required for stability.

Abstract

Quantum droplets are small clusters of atoms self-bound by the balance of attractive and repulsive forces. Here we report on the observation of a novel type of droplets, solely stabilized by contact interactions in a mixture of two Bose-Einstein condensates. We demonstrate that they are several orders of magnitude more dilute than liquid helium by directly measuring their size and density via in situ imaging. Moreover, by comparison to a single-component condensate, we show that quantum many-body effects stabilize them against collapse. We observe that droplets require a minimum atom number to be stable. Below, quantum pressure drives a liquid-to-gas transition that we map out as a function of interaction strength. These ultra-dilute isotropic liquids remain weakly interacting and constitute an ideal platform to benchmark quantum many-body theories.