Bose-Einstein Condensate Research Articles

Defect-induced discrete breather in dissipative optical lattices with weak nonlinearity

It is widely believed that the discrete breather (DB) can only be created when the nonlinearity is strong in nonlinear systems. However, we here establish that this belief is incorrect. In this work, we systemically investigate the generation of DBs induced by coupling of the defects and nonlinearity for Bose-Einstein condensates in dissipative optical lattices. The results show that, only in a clean lattice is strong nonlinearity a necessary condition for generating of DB; whereas, if the lattice has a defect, the DBs can also be discovered even in weak nonlinearity, and its generation turns out to be controllable. In addition, we further reveal a critical interval of the defect in weak nonlinearity, within which DBs can be found, while outside DBs do not exist. Furthermore, we also explore the impact of multiple defects on the generation of DBs, and analyze the underlying physical mechanisms of these interesting phenomena. The results not only have the potential to be used for more precise engineering in the DB experiments, but also suggest that the DB may be ubiquitous since the defects and dissipation are unavoidable in real physics.

Energy spectrum and superfluidity breakdown of Bose–Einstein condensates in optical lattice under density-dependent artificial gauge field

Nonlinear feedback between the gauge field and the material field can yield novel quantum phenomena. Here, the interplay between a density-dependent artificial gauge field and Bose–Einstein condensates (BECs) trapped in an optical lattice is studied. The energy spectrum and superfluidity represented by energetic and dynamical stabilities of the system are systematically discussed. A density-dependent artificial gauge field with a back-action between the BECs dynamics and the gauge field induces an effective atomic interaction that depends on the quasi-momentum and density of the condensates, resulting in a symmetry-broken energy spectrum and exotic stability phase diagram, that is, the system is only stable in a certain range of atoms density and under a limited lattice strength. The density-dependent artificial gauge field changes the sequence for the emergence of energetic and dynamical instability and the regimes of the energetic and dynamical instabilities are significantly separated, offering an efficient way to examine the energetic and dynamical instabilities of superfluids separately. In particular, the density-dependent artificial gauge field, as a mechanism for transferring momentum to the fluid, results in dynamic instability of the condensates even in free space. Our results provide deep insights into the dynamical response of superfluid systems to gauge fields and have potential applications for the coherent control of exotic superfluid states.

An electric-field-assisted magnetic trap for cold molecular evaporative cooling: Toward molecular Bose–Einstein condensation

Ultracold molecules provide fundamental new insights into molecular interaction dynamics in the quantum regime and represent a new platform for chemical physics where quantum behaviors play a dominant role in molecular interaction and dynamics. An electric-field-assisted magnetic trap for trapping and further evaporative cooling of cold molecules to ultracold regime is proposed utilizing the perturbation of the hyperfine levels in a mixed field, and the depth of the trap can be tuned by adjusting the assisted electric field rapidly. Thus, the evaporative cooling of [Formula: see text]I[Formula: see text]Br molecules in the [Formula: see text] state of the rovibronic ground state is simulated. It shows that IBr molecules would be cooled from 26[Formula: see text][Formula: see text]K to 556[Formula: see text]nK within 598.9[Formula: see text]ms and eventually we would obtain a number of [Formula: see text] molecules in a volume of [Formula: see text][Formula: see text]cm3 with the number density of [Formula: see text][Formula: see text]cm[Formula: see text]. The Bose–Einstein condensation of the alkali diatomic molecules would probably be realized in the proposed trap if the present experimentally available samples are loaded.

Six types of spin solitons in three-component Bose–Einstein condensates

Exact analytical solutions are good candidates for studying and explaining the dynamics of solitons in nonlinear systems. We further extend the region of existence of spin solitons in the nonlinearity coefficient space for the spin-1 Bose–Einstein condensate. Six types of spin soliton solutions can be obtained, and they exist in different regions. Stability analysis and numerical simulation results indicate that three types of spin solitons are stable against weak noise. The non-integrable properties of the model can induce shape oscillation and increase in speed after the collision between two spin solitons. These results further enrich the soliton family for non-integrable models and can provide theoretical references for experimental studies.

Bose and Fermi Gases in Metric-Affine Gravity and Linear Generalized Uncertainty Principle

Palatini-like theories of gravity have a remarkable connection to models incorporating linear generalized uncertainty principles. Considering this, we delve into the thermodynamics of systems comprising both Bose and Fermi gases. Our analysis encompasses the equations of state for various systems, including general Fermi gases, degenerate Fermi gases, Boltzmann gases, and Bose gases such as phonons and photons, as well as Bose–Einstein condensates and liquid helium.

Stable stripe and vortex solitons in two-dimensional spin-orbit coupled Bose–Einstein condensates

We present a flexible manipulation and control of solitons via Bose–Einstein condensates. In the presence of Rashba spin–orbit coupling and repulsive interactions within a harmonic potential, our investigation reveals the numerical local solutions within the system. By manipulating the strength of repulsive interactions and adjusting spin–orbit coupling while maintaining a zero-frequency rotation, diverse soliton structures emerge within the system. These include plane-wave solitons, two distinct types of stripe solitons, and odd petal solitons with both single and double layers. The stability of these solitons is intricately dependent on the varying strength of spin–orbit coupling. Specifically, stripe solitons can maintain a stable existence within regions characterized by enhanced spin–orbit coupling while petal solitons are unable to sustain a stable existence under similar conditions. When rotational frequency is introduced to the system, solitons undergo a transition from stripe solitons to a vortex array characterized by a sustained rotation. The rotational directions of clockwise and counterclockwise are non-equivalent owing to spin–orbit coupling. As a result, the properties of vortex solitons exhibit significant variation and are capable of maintaining a stable existence in the presence of repulsive interactions.

Understanding dynamical phase transitions in a spinor Bose-Einstein condensate via quantum and semiclassical analyses

Abstract Phase transitions in nonequilibrium dynamics of many body quantum systems, 
known as dynamical phase transitions (DPTs), play an important role for understanding
various dynamical phenomena observed in different branches of physics.
In general, there are two types of DPTs, the first one is characterized by 
distinct evolutionary behaviors of a physical observable, while the second one is marked by 
the vanishing overlap between the time-evolved and initial states. 
Here, we focus on exploring such DPTs from both quantum and semiclassical perspectives 
in a spinor Bose-Einstein condensate (BEC), an ideal platform for
investigating nonequilibrium dynamics.
Utilizing the sudden quench process, we demonstrate that the system exhibits 
both types of DPTs as the control parameter is quenched through
the critical value, referring to as the critical quenching. 
We show analytically how to determine the critical quenching via
the semiclassical approach and carry out a detailed examination of both semiclassical 
and quantum signatures of DPTs. 
In particular, we reveal that the occurrence of DPTs is triggered by
the separatrix in the underlying semiclassical system.
Our findings offer deeper insights into the properties of DPTs and verify the usefulness 
of semiclassical analysis for studying DPTs in quantum systems 
with well-defined semiclassical limit. 

Composite cores of monopoles and Alice rings in spin-2 Bose-Einstein condensates

We show that energy relaxation causes a point defect in the uniaxial-nematic phase of a spin-2 Bose-Einstein condensate to deform into a spin-Alice ring that exhibits a composite core structure with distinct topology at short and long distances from the singular line. An outer biaxial-nematic core exhibits a spin half-quantum vortex structure with a uniaxial-nematic inner core. By numerical simulation, we demonstrate a dynamical oscillation between the spin-Alice ring and a split-core hedgehog configuration via the appearance of ferromagnetic rings with associated vorticity inside an extended core region. We further show that a similar dynamics is exhibited by a spin-Alice ring surrounding a spin-vortex line resulting from the relaxation of a monopole situated on a spin-vortex line in the biaxial-nematic phase. In the cyclic phase, similar states are shown instead to form extended phase-mixing cores containing rings with fractional mass circulation or cores whose spatial shape reflects the order-parameter symmetry of a cyclic inner core, depending on the initial configuration. Published by the American Physical Society 2024

Stable-fixed-point description of square-pattern formation in driven two-dimensional Bose-Einstein condensates

Stable-fixed-point description of square-pattern formation in driven two-dimensional Bose-Einstein condensates

Two-dimensional quantum droplets in binary quadrupolar condensates

Abstract We study the stability and characteristics of two-dimensional (2D) quasi-isotropic quantum droplets (QDs) of fundamental and vortex types, formed by binary Bose-Einstein condensate with magnetic quadrupole-quadrupole interactions (MQQIs). The magnetic quadrupoles are built as pairs of dipoles and antidipoles polarized along the x-axis. The MQQIs are induced by applying an external magnetic field that varies along the x-axis. The system is modeled by the Gross-Pitaevskii equations including the MQQIs and Lee-Huang-Yang correction to the mean-field approximation. Stable 2D fundamental QDs and quasi-isotropic vortex QDs with topological charges S ≤ 4 are produced by means of the imaginary-time-integration method for configurations with the quadrupoles polarized parallel to the system’s two-dimensional plane. Effects of the norm and MQQI strength on the QDs are studied in detail. Some results, including an accurate prediction of the effective area, chemical potential, and peak density of QDs, are obtained in an analytical form by means of the Thomas-Fermi approximation. Collisions between moving QDs are studied by means of systematic simulations.

Bright solitons and interaction in the higher-order Gross-Pitaevskii equation investigated with Hirota's bilinear method

In this paper, we investigate bright solitons and their interactions in trapless Bose-Einstein condensate described by a one-dimensional Gross-Pitaevskii equation in the presence of higher-order nonlinearity. By constructing appropriate Hirota bilinear form of the problem, we derive the one- and two-bright soliton solutions where the effects of the higher-order interaction term are discussed in details. The profile of different solutions obtained clearly presents the phenomenon of soliton interaction that embodies the properties of soliton. It also highlights the impact of higher-order interactions that plays the role of wave amplification/compression resulting from the condensate. The theoretical results confirm that Hirota bilinear method is an efficient and interesting tool for the analysis of nonlinearity in physical science. In addition, the higher-order interactions have significant effects on the stabilization process of Bose-Einstein condensates.

Ground-state phases and collective modes of supersolid spin-orbit-coupled Bose-Einstein condensates

Ground-state phases and collective modes of supersolid spin-orbit-coupled Bose-Einstein condensates

Quantum control of classical motion: piston dynamics in a Rabi-coupled Bose-Einstein condensate

Abstract We develop a model and explore the dynamics of a hybrid classical-quantum system consisting of a classical piston and a self-interacting pseudospin 1/2 Bose-Einstein condensate with a time-dependent Rabi coupling. We investigate the mechanical work produced by the piston moving as a result of the quantum pressure of the condensate. The time-dependent Rabi field redistributes the condensate density between the spin components and, as a result, causes a time-dependent pressure acting on the piston. Correspondingly, the motion of the piston produces quantum evolution of the condensate mass- and spin density profiles. We show how by optimised design of the time-dependent direction of the Rabi field based on a quasi-stationary quantum pressure approximation, one can control both the position and velocity of the piston.

Neural-network quantum states for ultra-cold Fermi gases

Ultra-cold Fermi gases exhibit a rich array of quantum mechanical properties, including the transition from a fermionic superfluid Bardeen-Cooper-Schrieffer (BCS) state to a bosonic superfluid Bose-Einstein condensate (BEC). While these properties can be precisely probed experimentally, accurately describing them poses significant theoretical challenges due to strong pairing correlations and the non-perturbative nature of particle interactions. In this work, we introduce a Pfaffian-Jastrow neural-network quantum state featuring a message-passing architecture to efficiently capture pairing and backflow correlations. We benchmark our approach on existing Slater-Jastrow frameworks and state-of-the-art diffusion Monte Carlo methods, demonstrating a performance advantage and the scalability of our scheme. We show that transfer learning stabilizes the training process in the presence of strong, short-ranged interactions, and allows for an effective exploration of the BCS-BEC crossover region. Our findings highlight the potential of neural-network quantum states as a promising strategy for investigating ultra-cold Fermi gases.

Nonequilibrium dynamics of two-component Bose gas with tunable Rydberg interactions: From ring dark soliton to self-structured pattern formation

We explore the nonequilibrium dynamics of ring dark solitons in a two-dimensional binary Bose–Einstein condensate with tunable Rydberg interactions by numerical simulation of the time-dependent Gross–Pitaevskii equation. Our results show that both the vortex dynamics and self-structured pattern formation show strong dependence on the periodic modulation of Rydberg interactions. For the cases without periodic modulation of the interactions, or the periodic modulation part is almost equal to the fixed part, the system exhibits exotic vortex dynamics without self-structured pattern formation. Finally, the effect of periodically varying Rydberg interactions on the lifetime of ring dark soliton is also considered.

Superfluid fraction tensor of a two-dimensional supersolid

We investigate the superfluid fraction of crystalline stationary states within the framework of mean-field Gross–Pitaevskii theory. Our primary focus is on a two-dimensional Bose–Einstein condensate with a non-local soft-core interaction, where the superfluid fraction is described by a rank-2 tensor. We then calculate the superfluid fraction tensor for crystalline states exhibiting triangular, square, and stripe geometries across a broad range of interaction parameters. Factors leading to an anisotropic superfluid fraction tensor are also considered. We also refine the Leggett bounds for the superfluid fraction of the 2D system. We systematically compare these bounds to our full numerical results, and other results in the literature. This work is of direct relevance to other supersolid systems of current interest, such as supersolids produced using dipolar Bose–Einstein condensates.

Axion dark matter

Axions have a double motivation: they solve the ‘strong CP problem’ of the Standard Model of elementary particles and they are a candidate for the dark matter of the Universe. Dark matter axions can be detected on Earth by converting them to photons in an electromagnetic cavity permeated by a strong magnetic field. A signal in such a detector, called an axion haloscope, would immediately reveal the velocity spectrum of dark matter axions on Earth in great detail. This prospect motivated the study of the special properties that axions have in large scale structure formation. It is shown that cold dark matter axions thermalize through their gravitational self-interactions, and form a Bose-Einstein condensate. As a result, axion dark matter behaves differently from the other proposed forms of dark matter. The differences are observable.

Can machine learning efficiently predict symmetry breaking in physical problems like Bose-Einstein condensates?

In this work, our objective is to evaluate whether machine learning algorithms combined with computational methods used in physical problems such as spontaneous symmetry breaking in Bose-Einstein condensates are capable of efficiently predicting results obtained from solutions of nonlinear equations. Thus, we use K-means algorithms, support vector machines (SVM) and artificial neural network (ANN) multilayer perceptron to evaluate the efficiency of each algorithm in the process of predicting theoretical results in regions where computational calculations cannot be performed. either by numerical inaccuracy or computational cost. The results show that sophisticated machine learning algorithms can predict the behavior of represented physical systems with a satisfactory quality, where we especially highlight the correct interpretation of the physical problem in border regions and/or transitions.

Ground state of two-species spin–orbit-coupled in mass-imbalanced Bose condensates

In this paper, we study the ground state of two-component spin–orbit-coupled (SOC) Bose–Einstein condensates with mass imbalance. Our results are based on the framework of mean-field Gross–Pitaevskii theory. The effects of unequal atomic mass and SOC strength are studied. Different density structures such as heliciform stripes, linear stripes and string vortex chains are found. Different density structures such as heliciform stripes, linear stripes and string vortex chains are found. With the increase of SOC strength, the azimuthal phase separation is kept while radial phase separation is broken. In addition, increasing the mass ratio is unfavorable to vortex formation, whereas more vortices can be generated by increasing the SOC strength. We also discuss the physical quantities such as angular momentum per atom and spin polarisation, for a larger mass ratio, showing that angular momentum gets a little bigger as the SOC strength increases and the first-order phase transition does not exist.

Bose-Einstein condensation of non-ground-state caesium atoms

Bose-Einstein condensates of ultracold atoms serve as low-entropy sources for a multitude of quantum-science applications, ranging from quantum simulation and quantum many-body physics to proof-of-principle experiments in quantum metrology and quantum computing. For stability reasons, in the majority of cases the energetically lowest-lying atomic spin state is used. Here, we report the Bose-Einstein condensation of caesium atoms in the Zeeman-excited mf = 2 state, realizing a non-ground-state Bose-Einstein condensate with tunable interactions and tunable loss. We identify two regions of magnetic field in which the two-body relaxation rate is low enough that condensation is possible. We characterize the phase transition and quantify the loss processes, finding unusually high three-body losses in one of the two regions. Our results open up new possibilities for the mixing of quantum-degenerate gases, for polaron and impurity physics, and in particular for the study of impurity transport in strongly correlated one-dimensional quantum wires.