Abstract
In a joint experimental and theoretical effort, we report on the formation of a macrodroplet state in an ultracold bosonic gas of erbium atoms with strong dipolar interactions. By precise tuning of the -wave scattering length below the so-called dipolar length, we observe a smooth crossover of the ground state from a dilute Bose-Einstein condensate to a dense macrodroplet state of more than . Based on the study of collective excitations and loss features, we prove that quantum fluctuations stabilize the ultracold gas far beyond the instability threshold imposed by mean-field interactions. Finally, we perform expansion measurements, showing that although self-bound solutions are prevented by losses, the interplay between quantum stabilization and losses results in a minimal time-of-flight expansion velocity at a finite scattering length.
- Received 22 July 2016
DOI:https://doi.org/10.1103/PhysRevX.6.041039
Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Synopsis
Quantum Droplets Swell to a Macrodrop
Published 22 November 2016
Experiments with ultracold magnetic atoms reveal liquid-like quantum droplets that are 20 times larger than previously observed droplets.
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Popular Summary
Recent experiments of magnetic atoms at extremely low temperatures have revealed a surprising phase of matter: The atoms, behaving like tiny magnets, form a qualitatively new type of quantum liquid that is much more dilute than standard liquids. These so-called “quantum droplets” can preserve their form in the absence of external confinement, because of quantum effects, paving the way for completely novel research in the active field of ultracold gases. Here, we have, for the first time, realized a controlled crossover from an ordinary Bose-Einstein condensate, which behaves as a superfluid gas, into a single giant quantum droplet. Our joint experimental and theoretical results conclusively demonstrate the key role that quantum many-particle physics plays in the droplet regime.
Typical experiments relating to Bose-Einstein condensates can be well described within the so-called “mean-field approximation.” Our combined analysis, performed with a condensate of roughly 100,000 erbium atoms, focuses on the density distribution, elementary excitations, atom losses, and expansion dynamics after a precise tuning of the scattering length of the gas. Our results prove precisely and quantitatively that the mere existence of the droplet regime and its properties are crucially determined by quantum fluctuations beyond the mean-field scenario. These fluctuations result in an effective repulsion that provides the necessary surface tension to stabilize the quantum droplet against collapse. Our work shows convincingly that quantum droplets, far from being specific to any given species, constitute instead a general feature of strongly dipolar gases.
Our work, which reveals an effect common to dipolar gases, paves the way for complementary follow-up studies characterizing the new superfluid droplet state.