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Recent Trends on Quantum Fluctuations in Ultra-Cold Quantum Gases

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A special issue of Atoms (ISSN 2218-2004).

Deadline for manuscript submissions: closed (15 May 2023) | Viewed by 13210

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Special Issue Editors

Dr. Luis Aldemar Peña Ardila

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Guest Editor

Institut für Theoretische Physik, Leibniz Universität, 30167 Hannover, Germany
Interests: ultracold quantum gases and liquids; impurity and polaron physics; quantum optics; many-body physics

Dr. Cesar Cabrera

E-Mail Website
Guest Editor

Max-Planck-Institut für Quantenoptik, D-85748 Garching, Germany
Interests: quantum droplets; polaron physics; topological systems in optical lattices; quantum gas microscopy

Special Issue Information

Dear Colleagues,

Quantum fluctuations (QF) lie at the heart of the Heisenberg uncertainty principle, and they represent a tiny shift in the ground-state properties of a simple quantum system. For instance, the beyond-mean-field energy corrections of a Bose gas, were found at the end of the fifties by Lee, Huang and Yang (LHY). This term stems from QF, and it is usually neglected in the weakly interacting regime. However, as the complexity of the quantum system increases, these fluctuations can be sizable with respect to other energy scales, bringing thus nontrivial consequences.

Ultracold quantum gases serve as a platform to create and engineer complex systems formed by different kinds of atoms. In these systems interactions can be tuned at will, allowing thus a high control of desirable energy scales. Prominent candidates are atoms with a high magnetic dipolar moment and quantum mixtures. QF has given rise to new states of matter with fined-tune interactions such as ultra dilute-liquid self-bound droplets, bubbles, supersolids, and polaronic effects beyond the mean-field paradigm. For instance, in highly magnetic atomic species such as Dysprosium or Erbium, a competition between the mean-field energy scales associated with the short and long-range interaction yields an enhancement of a repulsive beyond the mean-field term, forming ultra dilute quantum droplets, arresting the mean-field collapse. A similar phenomenon is observed in non-dipolar mixtures, yet a two-component mixture is needed to trigger a competition between the net mean-field energy and the QF term. Likewise to the dipolar case and for specific parameters, the system stabilizes, forming droplets and bubbles or even a gas ruled only by QF-the LHY gas. On the other hand, supersolidity also appears as a direct manifestation of the enhancement of QF in the neighborhood of phase a transition. Here a global phase-coherent array of dipolar quantum droplets acquires a spatial modulation similar to a crystal. Highly imbalanced atomic species permit the formation of polarons, where single particles are screened by the low-energy excitation of a quantum gas. These quasiparticles serve as a non-demolition probe of QF in their host environment, for different impurity-bath couplings. Consequences of beyond mean-field terms are the formation of many-body bound states, the interplay between few and many-body physics.

Understanding QF and how they arise in different quantum systems is essential for future technologies, such as gravitational wave detectors where these fluctuations are unavoidable in experimental measurements.

The special issue aims to bring together contributions from the theoretical and experimental points of view related to quantum fluctuations in ultracold gases. In this Special Issue, original research articles, communications and reviews are welcome. Research areas may include (but are not limited to) the following:

Quantum mixtures and polaron physics
Ultradilute quantum droplets and bubbles
superfluidity and supersolidty
Quantum sensing and metrology
Excitions and polariton
Casimir effect

We look forward to receiving your contributions.

Dr. Luis Aldemar Peña Ardila
Dr. Cesar Cabrera
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Atoms is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 1500 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

quantum droplets
polarons
quasiparticles
quantum mixtures
superfluidity
LHY
super solidity
quantum metrology

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Published Papers (7 papers)

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13 pages, 599 KiB

Open AccessArticle

Bose Polaron in a One-Dimensional Lattice with Power-Law Hopping

by G. A. Domínguez-Castro

Atoms 2023, 11(8), 110; https://doi.org/10.3390/atoms11080110 - 6 Aug 2023

Cited by 1 | Viewed by 1462

Abstract

Polarons, quasiparticles resulting from the interaction between an impurity and the collective excitations of a medium, play a fundamental role in physics, mainly because they represent an essential building block for understanding more complex many-body phenomena. In this manuscript, we study the spectral properties of a single impurity mixed with identical bosons in a one-dimensional lattice with power-law hopping. In particular, based on the so-called T-matrix approximation, we show the existence of well-defined quasiparticle branches for several tunneling ranges and for both repulsive and attractive impurity-boson interactions. Furthermore, we demonstrate the persistence of the attractive polaron branch when the impurity-boson bound state is absorbed into the two-body continuum and that the attractive polaron becomes more robust as the range of the hopping increases. The results discussed here are relevant for the understanding of the equilibrium properties of quantum systems with power-law interactions. Full article

(This article belongs to the Special Issue Recent Trends on Quantum Fluctuations in Ultra-Cold Quantum Gases)

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12 pages, 3988 KiB

Open AccessArticle

Decay of Persistent Currents in Annular Atomic Superfluids

by Klejdja Xhani, Giulia Del Pace, Francesco Scazza and Giacomo Roati

Atoms 2023, 11(8), 109; https://doi.org/10.3390/atoms11080109 - 27 Jul 2023

Cited by 9 | Viewed by 1383

Abstract

We investigate the role of vortices in the decay of persistent current states of annular atomic superfluids by solving numerically the Gross–Pitaevskii equation, and we directly compare our results with the

^{6}

Li experiment at LENS data. We theoretically model the optical phase-imprinting [...] Read more.

^{6}

Li experiment at LENS data. We theoretically model the optical phase-imprinting technique employed to experimentally excite finite-circulation states in the Bose–Einstein condensation regime, accounting for imperfections of the optical gradient imprinting profile. By comparing simulations of this realistic protocol to an ideal imprinting, we show that the introduced density excitations arising from imperfect imprinting are mainly responsible for limiting the maximum reachable winding number

w_{\max}

in the superfluid ring. We also investigate the effect of a point-like obstacle with variable potential height

V_{0}

on the decay of circulating supercurrents. For a given obstacle height, a critical circulation

w_{c}

exists, such that for an initial circulation

w_{0}

larger than

w_{c}

the supercurrent decays through the emission of vortices, which cross the superflow and thus induce phase slippage. Higher values of the obstacle height

V_{0}

further favor the entrance of vortices, thus leading to lower values of

w_{c}

. Furthermore, the stronger vortex-defect interaction at higher

V_{0}

leads to vortices that propagate closer to the center of the ring condensate. The combination of both these effects leads to an increase in the supercurrent decay rate for increasing

w_{0}

, in agreement with experimental observations. Full article

(This article belongs to the Special Issue Recent Trends on Quantum Fluctuations in Ultra-Cold Quantum Gases)

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12 pages, 1031 KiB

Open AccessArticle

Variational Approaches to Two-Dimensionally Symmetry-Broken Dipolar Bose–Einstein Condensates

by Yong-Chang Zhang and Fabian Maucher

Atoms 2023, 11(7), 102; https://doi.org/10.3390/atoms11070102 - 6 Jul 2023

Cited by 4 | Viewed by 1530

Abstract

It has been shown that quantum fluctuations in dipolar Bose–Einstein condensates (BECs) lead to a stabilisation against collapse, thereby providing access to a range of states with different symmetries. In this paper, we discuss variational approaches to approximately determine the phase diagrams for dipolar BECs that are trapped along the dipolar orientation and otherwise infinite in the perpendicular direction (thermodynamic limit). The two-dimensional symmetry breaking occurs in the plane perpendicular to the polarisation axis. We show in detail how to derive approximate expressions that are valid in a region where modulations to an otherwise unmodulated perfect superfluid emerge gradually with a small modulation amplitude and compare the results to rigorous numerics. Full article

(This article belongs to the Special Issue Recent Trends on Quantum Fluctuations in Ultra-Cold Quantum Gases)

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11 pages, 736 KiB

Open AccessArticle

Trapped Ideal Bose Gas with a Few Heavy Impurities

by Orest Hryhorchak and Volodymyr Pastukhov

Atoms 2023, 11(5), 77; https://doi.org/10.3390/atoms11050077 - 28 Apr 2023

Viewed by 1153

Abstract

In this article, we formulate a general scheme for the calculation of the thermodynamic properties of an ideal Bose gas with one or two immersed static impurities, when the bosonic particles are trapped in a harmonic potential with either a quasi-1D or quasi-2D configuration. The binding energy of a single impurity and the medium-induced Casimir-like forces between the two impurities are numerically calculated for a wide range of temperatures and boson–impurity interaction strengths. Full article

(This article belongs to the Special Issue Recent Trends on Quantum Fluctuations in Ultra-Cold Quantum Gases)

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16 pages, 1612 KiB

Open AccessArticle

Strongly Interacting Bose Polarons in Two-Dimensional Atomic Gases and Quantum Fluids of Polaritons

by Luis Fernando Cárdenas-Castillo and Arturo Camacho-Guardian

Atoms 2023, 11(1), 3; https://doi.org/10.3390/atoms11010003 - 29 Dec 2022

Cited by 5 | Viewed by 2278 | Correction

Abstract

Polarons are quasiparticles relevant across many fields in physics: from condensed matter to atomic physics. Here, we study the quasiparticle properties of two-dimensional strongly interacting Bose polarons in atomic Bose–Einstein condensates and polariton gases. Our studies are based on the non-self consistent T-matrix approximation adapted to these physical systems. For the atomic case, we study the spectral and quasiparticle properties of the polaron in the presence of a magnetic Feshbach resonance. We show the presence of two polaron branches: an attractive polaron, a low-lying state that appears as a well-defined quasiparticle for weak attractive interactions, and a repulsive polaron, a metastable state that becomes the dominant branch at weak repulsive interactions. In addition, we study a polaron arising from the dressing of a single itinerant electron by a quantum fluid of polaritons in a semiconductor microcavity. We demonstrate the persistence of the two polaron branches whose properties can be controlled over a wide range of parameters by tuning the cavity mode. Full article

(This article belongs to the Special Issue Recent Trends on Quantum Fluctuations in Ultra-Cold Quantum Gases)

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11 pages, 463 KiB

Open AccessArticle

Ultradilute Quantum Droplets in the Presence of Higher-Order Quantum Fluctuations

by Abdelaali Boudjemaa, Karima Abbas and Nadia Guebli

Atoms 2022, 10(2), 64; https://doi.org/10.3390/atoms10020064 - 17 Jun 2022

Cited by 6 | Viewed by 2091

Abstract

We investigate the effects of higher-order quantum fluctuations on the bulk properties of self-bound droplets in three-, two- and one-dimensional binary Bose mixtures using the Hartree–Fock–Bogoliubov theory. We calculate higher-order corrections to the equation of state of the droplet at both zero and finite temperatures. We show that our results for the ground-state energy are in a good agreement with recent quantum Monte Carlo simulations in any dimension. Our study extends to the finite temperature case where it is found that thermal fluctuations may destabilize the droplet state and eventually destroy it. In two dimensions, we reveal that the droplet occurs at temperatures well below the Berezinskii–Kosterlitz–Thouless transition temperature. Full article

(This article belongs to the Special Issue Recent Trends on Quantum Fluctuations in Ultra-Cold Quantum Gases)

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