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Atoms
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Physics of Impurities in Quantum Gases
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Physics of Impurities in Quantum Gases

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

Deadline for manuscript submissions: closed (31 March 2022) | Viewed by 43669

Printed Edition Available!
A printed edition of this Special Issue is available here.

Special Issue Editors

Dr. Simeon Mistakidis

E-Mail Website
Guest Editor
ITAMP, Center for Astrophysics | Harvard & Smithsonian, Cambridge, MA 02138, USA
Interests: ultracold quantum gases; quantum impurities; quasiparticles; correlated nonequilibrium dynamics of atomic mixtures and transport phenomena; impurity-impurity correlations; entanglement between impurity atoms and bath; nonlinear and topological excitations in multicomponent quantum systems; spinor gases; few-body two-component systems; strongly correlated quantum systems; physics of optical lattices; quench dynamics; driven quantum systems; few to many-body physics
Dr. Artem Volosniev

E-Mail Website
Guest Editor
IST Austria, am Campus 1, 3400 Klosterneuburg, Austria
Interests: ultracold quantum gases; systems with impurities; strongly correlated quantum systems

Special Issue Information

Dear Colleagues,

A problem of a mobile impurity in a quantum many-body environment is often reduced to an effective one-body model. This reduction is connected to the concept of a quasiparticle such as a polaron. The quasiparticles enjoy exciting physics that go beyond a minor change of the corresponding bare impurities. The underlying dressing mechanism strongly modifies single-particle properties of the impurity, such as its mass. Remarkably, it also affects impurity-impurity interactions, which usually become more attractive and might even support different types of additional bound states.

Recently, quantum systems with impurities have been realized in cold-atom experiments. These experiments offer a unique possibility to study, for instance, the spectra, the residua, and the effective mass of the generated quasiparticles, to monitor the trajectories of the impurities and even to witness induced impurity-impurity correlations. They spur a wave of theoretical works on the dynamical and static properties of quasiparticles, with particular focus on emergent correlation effects. Notable phenomena, beyond simple single-particle characteristics, include nonlinear pattern formation, induced impurity-impurity correlations, relaxation processes, and peculiar transport properties. All of the above-mentioned activities are a part of an exciting discussion that shapes our understanding of quasiparticles.

The aim of this Special Issue of Atoms is to contribute to this discussion by highlighting recent advances regarding the physics of impurities in quantum gases. The Special Issue will collect theoretical and experimental works dedicated to fundamental properties and universal aspects of quasiparticles, as well as to applications of the quasiparticle concept in different contexts such as trapped mixtures of bosons or fermions, Rydberg systems, cavity settings, etc. The Special Issue will contain studies of multicomponent setups and quantum mixtures, ranging from the many-body correlated dynamics to ground state properties of few particles, and few-body processes that play a role in many-body systems with impurities. We invite authors to submit original research as well as short pedagogical reviews and communications with technical details that can lead to a substantially improved understanding of the existing theoretical and experimental results. 

Dr. Simeon Mistakidis
Dr. Artem Volosniev
Guest Editors

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Keywords

  • Quantum gases and mixtures;
  • Systems with impurities;
  • Polarons;
  • Rydberg polarons;
  • Polaritons;
  • Induced interactions;
  • Bipolarons;
  • Pattern formation;
  • Multicomponent few-body systems;
  • Engineering of Hamiltonians (correlations);
  • Few-body scattering processes;
  • Experimental techniques in quasiparticle and few-body physics

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Published Papers (12 papers)

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Research

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10 pages, 1125 KiB  
Article
Ultra-Dilute Gas of Polarons in a Bose–Einstein Condensate
by Luis A. Peña Ardila
Atoms 2022, 10(1), 29; https://doi.org/10.3390/atoms10010029 - 2 Mar 2022
Cited by 7 | Viewed by 3693
Abstract
We investigate the properties of a dilute gas of impurities embedded in an ultracold gas of bosons that forms a Bose–Einstein condensate (BEC). This work focuses mainly on the equation of state (EoS) of the impurity gas at zero temperature and the induced [...] Read more.
We investigate the properties of a dilute gas of impurities embedded in an ultracold gas of bosons that forms a Bose–Einstein condensate (BEC). This work focuses mainly on the equation of state (EoS) of the impurity gas at zero temperature and the induced interaction between impurities mediated by the host bath. We use perturbative field-theory approaches, such as Hugenholtz–Pines formalism, in the weakly interacting regime. In turn, for strong interactions, we aim at non-perturbative techniques such as quantum–Monte Carlo (QMC) methods. Our findings agree with experimental observations for an ultra dilute gas of impurities, modeled in the framework of the single impurity problem; however, as the density of impurities increases, systematic deviations are displayed with respect to the one-body Bose polaron problem. Full article
(This article belongs to the Special Issue Physics of Impurities in Quantum Gases)
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12 pages, 389 KiB  
Article
Static Impurities in a Weakly Interacting Bose Gas
by Galyna Panochko and Volodymyr Pastukhov
Atoms 2022, 10(1), 19; https://doi.org/10.3390/atoms10010019 - 8 Feb 2022
Cited by 10 | Viewed by 2490
Abstract
We present a comprehensive discussion of the ground-state properties of dilute D-dimensional Bose gas interacting with a few static impurities. Assuming the short-ranged character of the boson-impurity interaction, we calculated the energy of three- and two-dimensional Bose systems with one and two [...] Read more.
We present a comprehensive discussion of the ground-state properties of dilute D-dimensional Bose gas interacting with a few static impurities. Assuming the short-ranged character of the boson-impurity interaction, we calculated the energy of three- and two-dimensional Bose systems with one and two impurities immersed. Full article
(This article belongs to the Special Issue Physics of Impurities in Quantum Gases)
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29 pages, 3571 KiB  
Article
Pattern Formation in One-Dimensional Polaron Systems and Temporal Orthogonality Catastrophe
by Georgios M. Koutentakis, Simeon I. Mistakidis and Peter Schmelcher
Atoms 2022, 10(1), 3; https://doi.org/10.3390/atoms10010003 - 28 Dec 2021
Cited by 10 | Viewed by 2808
Abstract
Recent studies have demonstrated that higher than two-body bath-impurity correlations are not important for quantitatively describing the ground state of the Bose polaron. Motivated by the above, we employ the so-called Gross Ansatz (GA) approach to unravel the stationary and dynamical properties of [...] Read more.
Recent studies have demonstrated that higher than two-body bath-impurity correlations are not important for quantitatively describing the ground state of the Bose polaron. Motivated by the above, we employ the so-called Gross Ansatz (GA) approach to unravel the stationary and dynamical properties of the homogeneous one-dimensional Bose-polaron for different impurity momenta and bath-impurity couplings. We explicate that the character of the equilibrium state crossovers from the quasi-particle Bose polaron regime to the collective-excitation stationary dark-bright soliton for varying impurity momentum and interactions. Following an interspecies interaction quench the temporal orthogonality catastrophe is identified, provided that bath-impurity interactions are sufficiently stronger than the intraspecies bath ones, thus generalizing the results of the confined case. This catastrophe originates from the formation of dispersive shock wave structures associated with the zero-range character of the bath-impurity potential. For initially moving impurities, a momentum transfer process from the impurity to the dispersive shock waves via the exerted drag force is demonstrated, resulting in a final polaronic state with reduced velocity. Our results clearly demonstrate the crucial role of non-linear excitations for determining the behavior of the one-dimensional Bose polaron. Full article
(This article belongs to the Special Issue Physics of Impurities in Quantum Gases)
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15 pages, 1290 KiB  
Article
Asymmetric Lineshapes of Efimov Resonances in Mass-Imbalanced Ultracold Gases
by Panagiotis Giannakeas and Chris H. Greene
Atoms 2021, 9(4), 110; https://doi.org/10.3390/atoms9040110 - 7 Dec 2021
Cited by 2 | Viewed by 2491
Abstract
The resonant profile of the rate coefficient for three-body recombination into a shallow dimer is investigated for mass-imbalanced systems. In the low-energy limit, three atoms collide with zero-range interactions, in a regime where the scattering lengths of the heavy–heavy and the heavy–light subsystems [...] Read more.
The resonant profile of the rate coefficient for three-body recombination into a shallow dimer is investigated for mass-imbalanced systems. In the low-energy limit, three atoms collide with zero-range interactions, in a regime where the scattering lengths of the heavy–heavy and the heavy–light subsystems are positive and negative, respectively. For this physical system, the adiabatic hyperspherical representation is combined with a fully semi-classical method and we show that the shallow dimer recombination spectra display an asymmetric lineshape that originates from the coexistence of Efimov resonances with Stückelberg interference minima. These asymmetric lineshapes are quantified utilizing the Fano profile formula. In particular, a closed-form expression is derived that describes the width of the corresponding Efimov resonances and the Fano lineshape asymmetry parameter q. The profile of Efimov resonances exhibits a q-reversal effect as the inter- and intra-species scattering lengths vary. In the case of a diverging asymmetry parameter, i.e., |q|→∞, we show that the Efimov resonances possess zero width and are fully decoupled from the three-body and atom–dimer continua, and the corresponding Efimov metastable states behave as bound levels. Full article
(This article belongs to the Special Issue Physics of Impurities in Quantum Gases)
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12 pages, 296 KiB  
Article
Emergence of Anyons on the Two-Sphere in Molecular Impurities
by Morris Brooks, Mikhail Lemeshko, Douglas Lundholm and Enderalp Yakaboylu
Atoms 2021, 9(4), 106; https://doi.org/10.3390/atoms9040106 - 2 Dec 2021
Cited by 1 | Viewed by 2253
Abstract
Recently it was shown that anyons on the two-sphere naturally arise from a system of molecular impurities exchanging angular momentum with a many-particle bath (Phys. Rev. Lett. 126, 015301 (2021)). Here we further advance this approach and rigorously demonstrate that in the experimentally [...] Read more.
Recently it was shown that anyons on the two-sphere naturally arise from a system of molecular impurities exchanging angular momentum with a many-particle bath (Phys. Rev. Lett. 126, 015301 (2021)). Here we further advance this approach and rigorously demonstrate that in the experimentally realized regime the lowest spectrum of two linear molecules immersed in superfluid helium corresponds to the spectrum of two anyons on the sphere. We develop the formalism within the framework of the recently experimentally observed angulon quasiparticle. Full article
(This article belongs to the Special Issue Physics of Impurities in Quantum Gases)
25 pages, 404 KiB  
Article
Fragmentation of Identical and Distinguishable Bosons’ Pairs and Natural Geminals of a Trapped Bosonic Mixture
by Ofir E. Alon
Atoms 2021, 9(4), 92; https://doi.org/10.3390/atoms9040092 - 2 Nov 2021
Cited by 3 | Viewed by 5047
Abstract
In a mixture of two kinds of identical bosons, there are two types of pairs: identical bosons’ pairs, of either species, and pairs of distinguishable bosons. In the present work, the fragmentation of pairs in a trapped mixture of Bose–Einstein condensates is investigated [...] Read more.
In a mixture of two kinds of identical bosons, there are two types of pairs: identical bosons’ pairs, of either species, and pairs of distinguishable bosons. In the present work, the fragmentation of pairs in a trapped mixture of Bose–Einstein condensates is investigated using a solvable model, the symmetric harmonic-interaction model for mixtures. The natural geminals for pairs made of identical or distinguishable bosons are explicitly contracted by diagonalizing the intra-species and inter-species reduced two-particle density matrices, respectively. Properties of pairs’ fragmentation in the mixture are discussed, the role of the mixture’s center-of-mass and relative center-of-mass coordinates is elucidated, and a generalization to higher-order reduced density matrices is made. As a complementary result, the exact Schmidt decomposition of the wave function of the bosonic mixture is constructed. The entanglement between the two species is governed by the coupling of their individual center-of-mass coordinates, and it does not vanish at the limit of an infinite number of particles where any finite-order intra-species and inter-species reduced density matrix per particle is 100% condensed. Implications are briefly discussed. Full article
(This article belongs to the Special Issue Physics of Impurities in Quantum Gases)
16 pages, 1507 KiB  
Article
Polaritons in an Electron Gas—Quasiparticles and Landau Effective Interactions
by Miguel Angel Bastarrachea-Magnani, Jannie Thomsen, Arturo Camacho-Guardian and Georg M. Bruun
Atoms 2021, 9(4), 81; https://doi.org/10.3390/atoms9040081 - 16 Oct 2021
Cited by 7 | Viewed by 3416
Abstract
Two-dimensional semiconductors inside optical microcavities have emerged as a versatile platform to explore new hybrid light–matter quantum states. A strong light–matter coupling leads to the formation of exciton-polaritons, which in turn interact with the surrounding electron gas to form quasiparticles called polaron-polaritons. Here, [...] Read more.
Two-dimensional semiconductors inside optical microcavities have emerged as a versatile platform to explore new hybrid light–matter quantum states. A strong light–matter coupling leads to the formation of exciton-polaritons, which in turn interact with the surrounding electron gas to form quasiparticles called polaron-polaritons. Here, we develop a general microscopic framework to calculate the properties of these quasiparticles, such as their energy and the interactions between them. From this, we give microscopic expressions for the parameters entering a Landau theory for the polaron-polaritons, which offers a simple yet powerful way to describe such interacting light–matter many-body systems. As an example of the application of our framework, we then use the ladder approximation to explore the properties of the polaron-polaritons. Furthermore, we show that they can be measured in a non-demolition way via the light transmission/reflection spectrum of the system. Finally, we demonstrate that the Landau effective interaction mediated by electron-hole excitations is attractive leading to red shifts of the polaron-polaritons. Our work provides a systematic framework to study exciton-polaritons in electronically doped two-dimensional materials such as novel van der Waals heterostructures. Full article
(This article belongs to the Special Issue Physics of Impurities in Quantum Gases)
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9 pages, 396 KiB  
Article
Initial Dynamics of Quantum Impurities in a Bose–Einstein Condensate
by Magnus G. Skou, Thomas G. Skov, Nils B. Jørgensen and Jan J. Arlt
Atoms 2021, 9(2), 22; https://doi.org/10.3390/atoms9020022 - 27 Mar 2021
Cited by 5 | Viewed by 2651
Abstract
An impurity immersed in a medium constitutes a canonical scenario applicable in a wide range of fields in physics. Though our understanding has advanced significantly in the past decades, quantum impurities in a bosonic environment are still of considerable theoretical and experimental interest. [...] Read more.
An impurity immersed in a medium constitutes a canonical scenario applicable in a wide range of fields in physics. Though our understanding has advanced significantly in the past decades, quantum impurities in a bosonic environment are still of considerable theoretical and experimental interest. Here, we discuss the initial dynamics of such impurities, which was recently observed in interferometric experiments. Experimental observations from weak to unitary interactions are presented and compared to a theoretical description. In particular, the transition between two initial dynamical regimes dominated by two-body interactions is analyzed, yielding transition times in clear agreement with the theoretical prediction. Additionally, the distinct time dependence of the coherence amplitude in these regimes is obtained by extracting its power-law exponents. This benchmarks our understanding and suggests new ways of probing dynamical properties of quantum impurities. Full article
(This article belongs to the Special Issue Physics of Impurities in Quantum Gases)
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25 pages, 1407 KiB  
Article
Polaron Problems in Ultracold Atoms: Role of a Fermi Sea across Different Spatial Dimensions and Quantum Fluctuations of a Bose Medium
by Hiroyuki Tajima, Junichi Takahashi, Simeon I. Mistakidis, Eiji Nakano and Kei Iida
Atoms 2021, 9(1), 18; https://doi.org/10.3390/atoms9010018 - 9 Mar 2021
Cited by 23 | Viewed by 4851
Abstract
The notion of a polaron, originally introduced in the context of electrons in ionic lattices, helps us to understand how a quantum impurity behaves when being immersed in and interacting with a many-body background. We discuss the impact of the impurities on the [...] Read more.
The notion of a polaron, originally introduced in the context of electrons in ionic lattices, helps us to understand how a quantum impurity behaves when being immersed in and interacting with a many-body background. We discuss the impact of the impurities on the medium particles by considering feedback effects from polarons that can be realized in ultracold quantum gas experiments. In particular, we exemplify the modifications of the medium in the presence of either Fermi or Bose polarons. Regarding Fermi polarons we present a corresponding many-body diagrammatic approach operating at finite temperatures and discuss how mediated two- and three-body interactions are implemented within this framework. Utilizing this approach, we analyze the behavior of the spectral function of Fermi polarons at finite temperature by varying impurity-medium interactions as well as spatial dimensions from three to one. Interestingly, we reveal that the spectral function of the medium atoms could be a useful quantity for analyzing the transition/crossover from attractive polarons to molecules in three-dimensions. As for the Bose polaron, we showcase the depletion of the background Bose-Einstein condensate in the vicinity of the impurity atom. Such spatial modulations would be important for future investigations regarding the quantification of interpolaron correlations in Bose polaron problems. Full article
(This article belongs to the Special Issue Physics of Impurities in Quantum Gases)
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19 pages, 1130 KiB  
Article
Dynamics of the Creation of a Rotating Bose–Einstein Condensation by Two Photon Raman Transition Using a Laguerre–Gaussian Laser Pulse
by Koushik Mukherjee, Soumik Bandyopadhyay, Dilip Angom, Andrew M. Martin and Sonjoy Majumder
Atoms 2021, 9(1), 14; https://doi.org/10.3390/atoms9010014 - 8 Feb 2021
Cited by 5 | Viewed by 3507
Abstract
We present numerical simulations to unravel the dynamics associated with the creation of a vortex in a Bose–Einstein condensate (BEC), from another nonrotating BEC using two-photon Raman transition with Gaussian (G) and Laguerre–Gaussian (LG) laser pulses. In particular, we consider BEC of Rb [...] Read more.
We present numerical simulations to unravel the dynamics associated with the creation of a vortex in a Bose–Einstein condensate (BEC), from another nonrotating BEC using two-photon Raman transition with Gaussian (G) and Laguerre–Gaussian (LG) laser pulses. In particular, we consider BEC of Rb atoms at their hyperfine ground states confined in a quasi two dimensional harmonic trap. Optical dipole potentials created by G and LG laser pulses modify the harmonic trap in such a way that density patterns of the condensates during the Raman transition process depend on the sign of the generated vortex. We investigate the role played by the Raman coupling parameter manifested through dimensionless peak Rabi frequency and intercomponent interaction on the dynamics during the population transfer process and on the final population of the rotating condensate. During the Raman transition process, the two BECs tend to have larger overlap with each other for stronger intercomponent interaction strength. Full article
(This article belongs to the Special Issue Physics of Impurities in Quantum Gases)
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Review

Jump to: Research

30 pages, 2572 KiB  
Review
Repulsive Fermi and Bose Polarons in Quantum Gases
by Francesco Scazza, Matteo Zaccanti, Pietro Massignan, Meera M. Parish and Jesper Levinsen
Atoms 2022, 10(2), 55; https://doi.org/10.3390/atoms10020055 - 27 May 2022
Cited by 38 | Viewed by 4700
Abstract
Polaron quasiparticles are formed when a mobile impurity is coupled to the elementary excitations of a many-particle background. In the field of ultracold atoms, the study of the associated impurity problem has attracted a growing interest over the last fifteen years. Polaron quasiparticle [...] Read more.
Polaron quasiparticles are formed when a mobile impurity is coupled to the elementary excitations of a many-particle background. In the field of ultracold atoms, the study of the associated impurity problem has attracted a growing interest over the last fifteen years. Polaron quasiparticle properties are essential to our understanding of a variety of paradigmatic quantum many-body systems realized in ultracold atomic gases and in the solid state, from imbalanced Bose–Fermi and Fermi–Fermi mixtures to fermionic Hubbard models. In this topical review, we focus on the so-called repulsive polaron branch, which emerges as an excited many-body state in systems with underlying attractive interactions such as ultracold atomic mixtures, and is characterized by an effective repulsion between the impurity and the surrounding medium. We give a brief account of the current theoretical and experimental understanding of repulsive polaron properties, for impurities embedded in both fermionic and bosonic media, and we highlight open issues deserving future investigations. Full article
(This article belongs to the Special Issue Physics of Impurities in Quantum Gases)
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14 pages, 5876 KiB  
Review
One-Dimensional Disordered Bosonic Systems
by Chiara D’Errico and Marco G. Tarallo
Atoms 2021, 9(4), 112; https://doi.org/10.3390/atoms9040112 - 14 Dec 2021
Cited by 1 | Viewed by 2590
Abstract
Disorder is everywhere in nature and it has a fundamental impact on the behavior of many quantum systems. The presence of a small amount of disorder, in fact, can dramatically change the coherence and transport properties of a system. Despite the growing interest [...] Read more.
Disorder is everywhere in nature and it has a fundamental impact on the behavior of many quantum systems. The presence of a small amount of disorder, in fact, can dramatically change the coherence and transport properties of a system. Despite the growing interest in this topic, a complete understanding of the issue is still missing. An open question, for example, is the description of the interplay of disorder and interactions, which has been predicted to give rise to exotic states of matter such as quantum glasses or many-body localization. In this review, we will present an overview of experimental observations with disordered quantum gases, focused on one-dimensional bosons, and we will connect them with theoretical predictions. Full article
(This article belongs to the Special Issue Physics of Impurities in Quantum Gases)
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