Recent Trends on Quantum Fluctuations in Ultra-Cold Quantum Gases

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

Deadline for manuscript submissions: closed (15 May 2023) | Viewed by 10876

Special Issue Editors


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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

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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

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Keywords

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

Published Papers (7 papers)

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13 pages, 599 KiB  
Article
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 1164
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 [...] Read more.
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  
Article
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 5 | Viewed by 976
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 6Li experiment at LENS data. We theoretically model the optical phase-imprinting [...] Read more.
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 6Li 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 wmax in the superfluid ring. We also investigate the effect of a point-like obstacle with variable potential height V0 on the decay of circulating supercurrents. For a given obstacle height, a critical circulation wc exists, such that for an initial circulation w0 larger than wc the supercurrent decays through the emission of vortices, which cross the superflow and thus induce phase slippage. Higher values of the obstacle height V0 further favor the entrance of vortices, thus leading to lower values of wc. Furthermore, the stronger vortex-defect interaction at higher V0 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 w0, 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  
Article
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 2 | Viewed by 1195
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 [...] Read more.
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  
Article
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 917
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 [...] Read more.
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  
Article
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 4 | Viewed by 1873 | 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 [...] Read more.
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  
Article
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 1814
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 [...] Read more.
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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2 pages, 1808 KiB  
Correction
Correction: Cárdenas-Castillo, L.F.; Camacho-Guardian, A. Strongly Interacting Bose Polarons in Two-Dimensional Atomic Gases and Quantum Fluids of Polaritons. Atoms 2023, 11, 3
by Luis Fernando Cárdenas-Castillo and Arturo Camacho-Guardian
Atoms 2023, 11(11), 143; https://doi.org/10.3390/atoms11110143 - 7 Nov 2023
Cited by 1 | Viewed by 1112
Abstract
The authors wish to make the following corrections to their paper [...] Full article
(This article belongs to the Special Issue Recent Trends on Quantum Fluctuations in Ultra-Cold Quantum Gases)
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