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Quantum droplets and supersolids in dipolar Bose-Einstein condensates

R. N. Bisset

Published Jan 1, 2025• 0 citations
OpenAlex

Abstract

This habilitation thesis pertains to the theoretical and computational investigation of dilute dipolar Bose-Einstein condensates in the bulk, where atoms interact through both short-range and dipole-dipole interactions. Twelve papers are summarized that were published over the period from 2015 to 2024, with four based on theory-experiment collaborations, and the remaining eight papers being purely theoretical. The papers are grouped thematically into three chapters: the first focuses on quantum droplets, which are dilute liquids resulting from a balance between attractive mean-field interactions and beyond-mean-field quantum fluctuations; the second pertains to the exploration and realization of the long-sought supersolid phase of matter, which simultaneously exhibits properties of both a solid and a superuid; and the third addresses binary supersolids in two-component dipolar Bose-Einstein condensates, whose additional degrees of freedom offer the potential for novel physics and provide a pathway to realizing bulk supersolids.

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•Jan 1, 2025•0 citations
This habilitation thesis pertains to the theoretical and computational investigation of dilute dipolar Bose-Einstein condensates in the bulk, where atoms interact through both short-range and dipole-dipole interactions. Twelve papers are summarized that were published over the period from 2015 to 2024, with four based on theory-experiment collaborations, and the remaining eight papers being purely theoretical. The papers are grouped thematically into three chapters: the first focuses on quantum droplets, which are dilute liquids resulting from a balance between attractive mean-field interactions and beyond-mean-field quantum fluctuations; the second pertains to the exploration and realization of the long-sought supersolid phase of matter, which simultaneously exhibits properties of both a solid and a superuid; and the third addresses binary supersolids in two-component dipolar Bose-Einstein condensates, whose additional degrees of freedom offer the potential for novel physics and provide a pathway to realizing bulk supersolids.
We numerically study the many-body physics of molecular Bose-Einstein condensates with strong dipole-dipole interactions. We observe the formation of self-bound droplets, and explore phase diagrams that feature a variety of exotic supersolid states. In all of these cases, the large and tunable molecular dipole moments enable the study of unexplored regimes and phenomena, including liquidlike density saturation and universal stability scaling laws for droplets, as well as pattern formation and the limits of droplet supersolidity. We discuss a realistic experimental approach to realize both the required collisional stability of the molecular gases and the independent tunability of their contact and dipolar interaction strengths. Our work provides both a blueprint and a benchmark for near-future experiments with bulk molecular Bose-Einstein condensates.
Quantum fluctuations can stabilize Bose-Einstein condensates (BEC) against the mean-field collapse. Stabilization of the condensate has been observed in quantum degenerate Bose-Bose mixtures and dipolar BECs. The fine-tuning of the interatomic interactions can lead to the emergence of two new states of matter: liquid-like self-bound quantum droplets and supersolid crystals formed from these droplets. We review the properties of these exotic states of matter and summarize the experimental progress made using dipolar quantum gases and Bose-Bose mixtures. We conclude with an outline of important open questions that could be addressed in the future.
Abstract In this mini-review, we briefly summarize some of the main concepts and ideas behind highly dilute self-bound quantum droplets of both binary and dipolar character. The latter type of systems has more recently led to the experimental discovery of a dipolar supersolid state that allows entirely new insights on this long-sought purely quantum state of matter, with exciting prospects for fundamental research as well as future applied quantum sensing technologies. The first half of the review provides a brief history of droplets and supersolidity in various settings and also discusses the self-binding in binary quantum gases, and the second half of the review summarizes our own recent work in the field, presented at the 2022 FQMT conference in Prague.
These lecture notes contain an introduction to quantum simulation of bosonic systems in the continuum, focusing on weakly interacting Bose-Bose mixtures with competing mean-field interactions. When the values of such interactions are fine-tuned to almost completely cancel the mean-field energy, quantum fluctuations become apparent and dominate the behavior of the system, stabilizing an ultradilute quantum liquid phase. An analogous situation appears in single-component dipolar quantum gases. We review the mechanism that gives rise to this exotic quantum liquid, which can form droplets that are self-bound in the absence of any external confinement, and discuss their properties and dynamics in both the mixture and the dipolar cases. In dipolar gases, arrays of dipolar droplets stabilized by quantum fluctuations can establish global phase coherence and form supersolids. In bosonic mixtures, supersolidity can emerge already at the mean-field level through spin-orbit coupling. We discuss the properties of such spin-orbit-coupled supersolids, comparing them to their dipolar counterparts. Specifically, we focus on their periodic density modulation, phase coherence, and peculiar excitation spectrum, which hosts both superfluid and crystal excitations. Finally, we conclude by discussing open research directions in the areas of quantum liquid droplets and spin-orbit-coupled supersolids, in particular at the interface of the two research topics.
We study theoretically and experimentally the emergence of supersolid properties in a dipolar Bose-Einstein condensate.The theory reveals a ground state phase diagram with three distinct regimes-a regular Bose-Einstein condensate and incoherent and coherent arrays of quantum droplets.The coherent droplets are connected by a background condensate, which leads-in addition to the periodic density modulation-to a robust phase coherence throughout the whole system.We further theoretically demonstrate that we are able to dynamically approach the ground state in our experiment and that its lifetime is limited only by three-body losses.Experimentally we probe and confirm the signatures of the phase diagram by observing the in situ density modulation as well as the phase coherence using matter wave interference.
This thesis discusses the experiments on 2D dipolar Bose gas of ultracold erbium atoms. Long-range and anisotropic dipole-dipole interaction induces complex orders in quantum many-body systems. Highly magnetic lanthanide atoms such as dysprosium and erbium have the largest magnetic moment in the periodic table, which has attracted significant attention in the past decades. Starting from quantum degenerate dysprosium and erbium atoms, exotic phases like quantum droplets, supersolid, dipolar quantum solid in the Hubbard regime have been studied. However, the 2D dipolar superfluid still remains understudied, despite being theoretically discussed for over two decades. Although tremendous theoretical proposals predict different phenomena in 2D dipolar superfluids, our first interest is to know how the superfluidity emerges in the 2D dipolar gases at finite temperatures. Our observations suggest that the superfluid transition can be well described by the Berezinskii-Kosterlitz-Thouless transition with an effective contact interaction as a function of dipolar orientation. With the 2D dipolar superfluid with strong DDI, we characterized the elementary excitation of the matter: the collective breathing mode. Our measurements establish how the competition between local and nonlocal interaction in strongly dipolar 2D superfluid retains quasi-scale invariance or strongly breaks scale invariance. By further searching the interaction parameters, we created stripes with global coherence, which are the first supersolid stripes in 2D. Experiments including measuring density fluctuation, coherence, and collective mode have been performed to understand the 2D supersolid. At last, we also observe unconventional light scattering from round-disk dipolar BECs, which provides new possibilities for atom optics.