Coupled Gross-Pitaevskii Equations Research Articles

Dynamical evolution of exciton-polariton Bose-Einstein condensate under coupled interaction

Abstract We studied the exciton-polariton Bose-Einstein condensation (BEC) under light field manipulation by considering the coupled interaction of the system between exciton-polariton and light. Based on the coupled Gross-Pitaevskii equation (GPE) model and with modified variational method, we analytically derived the solutions of the excitonic system under the polar angle direction perturbation and system rotation. We identified that for different rotational angular velocities and different coupled strengths, the system evolves from a meta-stable oscillation state to monotonically decaying state, with potential for the study of quantum vortex identification for such kind of coupled systems. Our theoretical results can be used to guide the study of the evolution mode of the exciton-polariton system involving coupled interaction.

Dynamics of localized waves in quasi-one-dimensional imbalanced binary Bose-Einstein condensates

In the framework of coupled Gross-Pitaevskii equations, we explore the dynamics of localized waves in quasi-1D imbalanced binary Bose-Einstein condensates where the intra-component interaction is repulsive, while the inter-component one is attractive. The existence regimes of stable self-trapped localized states in the form of symbiotic solitons have been analyzed. Imbalanced mixtures, where the number of atoms in one component exceeds the number of atoms in the other component, are considered in parabolic potential and box-like trap. A variational approach has been developed which allows us to find the stationary state of the system and frequency of small amplitude oscillations near the equilibrium. It is shown that the strength of inter-component coupling can be retrieved from the frequency of the localized state's vibrations. When all the intra-species and inter-species interactions are repulsive, we numerically find a new type of symbiotic solitons resembling dark-bright solitons. The motion of the minority component in the surrounding gas of the majority component with different velocities reveals its superfluid properties.

Stability analysis of nonlinear localized modes in the coupled Gross-Pitaevskii equations with [Formula: see text] -symmetric Scarf-II potential.

We study the nonlinear localized modes in two-component Bose-Einstein condensates with parity-time-symmetric Scarf-II potential, which can be described by the coupled Gross-Pitaevskii equations. Firstly, we investigate the linear stability of the nonlinear modes in the focusing and defocusing cases, and get the stable and unstable domains of nonlinear localized modes. Then we validate the results by evolving them with 5% perturbations as an initial condition. Finally, we get stable solitons by considering excitations of the soliton via adiabatical change of system parameters. These findings of nonlinear modes can be potentially applied to physical experiments of matter waves in Bose-Einstein condensates.

Dynamical self-trapping of two-dimensional binary solitons in cross-combined linear and nonlinear optical lattices.

Dynamical and self-trapping properties of two-dimensional (2D) binary mixtures of Bose-Einstein condensates in cross-combined lattices, consisting of a one-dimensional (1D) linear optical lattice (LOL) in the x direction for the first component and a 1D nonlinear optical lattice (NOL) in the y direction for the second component, are analytically and numerically investigated. The existence and stability of 2D binary matter wave solitons in these settings are demonstrated both by variational analysis and by direct numerical integration of the coupled Gross-Pitaevskii equations. We find that in the absence of the NOL, binary solitons, stabilized by the action of the 1D LOL and by the attractive intercomponent interaction, can freely move in the y direction. In the presence of the NOL, we find, quite remarkably, the existence of threshold curves in the parameter space separating regions where solitons can move from regions where the solitons become dynamically self-trapped. The mechanism underlying the dynamical self-trapping phenomenon (DSTP) is qualitatively understood in terms of a dynamical barrier induced by the NOL, similar to the Peirls-Nabarro barrier of solitons in discrete lattices. DSTP is numerically demonstrated for binary solitons that are put in motion both by phase imprinting and by the action of external potentials applied in the y direction. In the latter case, we show that the trapping action of the NOL allows one to maintain a 2D binary soliton at rest in a nonequilibrium position of a parabolic trap or to prevent it from falling under the action of gravity. Possible applications of the results are also briefly discussed.

A mass‐ and energy‐preserving numerical scheme for solving coupled Gross–Pitaevskii equations in high dimensions

AbstractThis article is concerned with numerical study of a coupled system of Gross–Pitaevskii equations which describes the spin‐orbit‐coupled Bose–Einstein condensates. Due to the fact that this system possesses the total mass and energy conservation property and often appears in high dimensions, it brings a significant burden in designing and analyzing a suitable numerical scheme for solving the coupled Gross–Pitaevskii equations (CGPEs). In this article, an implicit finite difference scheme is proposed to solve the CGPEs, which is proved to be uniquely solvable, mass‐ and energy‐conservative in the discrete sense. In particular, it is proved in a rigorous way that, without any grid‐ratio restriction, the scheme is stable and convergent at the rate of with time step and mesh size in the maximum norm, while previous works often require certain restriction on the grid ratio and only give the error estimates in the discrete norm or norm which could not imply the maximum error estimate. Numerical results are carried out to underline the error estimate and conservation laws, and investigate several dynamics of the CGPEs.

Distributed vorticity model for vortex molecule dynamics

We analyze the effect of a hard wall trapping potential on the dynamics of a vortex molecule in a two-component Bose-Einstein condensate with linear coherent coupling. A vortex molecule consists of a vortex of the same charge in each component condensate connected by a domain wall of the relative phase. In a previous paper [S. Choudhury and J. Brand, Phys. Rev. A 106, 043319 (2022)] we described the interaction of a vortex molecule with the boundary using the method of images by separately treating each component vortex as a point vortex, in addition to a Magnus force effect from the surface tension of the domain wall. Here we extend the model by considering a continuous distribution of image vorticity reflecting the effect of the domain wall on the vortex molecule phase structure. In the case of a precessing centered vortex molecule in an isotropic trap, distributing the image vorticity weakens its contribution to the precession frequency. We test the model predictions against numerical simulations of the coupled Gross-Pitaevskii equations in a two-dimensional circular disk and find support for the improved model.

Massive Quantum Vortices in Superfluids

We consider the dynamical properties of quantum vortices with filled massive cores, hence the term “massive vortices”. While the motion of massless vortices is described by first-order motion equations, the inclusion of core mass introduces a second-order time derivative in the motion equations and thus doubles the number of independent dynamical variables needed to describe the vortex. The simplest possible system where this physics is present, i.e. a single massive vortex in a circular domain, is thoroughly discussed. We point out that a massive vortex can exhibit various dynamical regimes, as opposed to its massless counterpart, which can only precess at a constant rate. The predictions of our analytical model are validated by means of numerical simulations of coupled Gross-Pitaevskii equations, which indeed display the signature of the core inertial mass. Eventually, we discuss a nice formal analogy between the motion of massive vortices and that of massive charges in two-dimensional domains pierced by magnetic fields.

Compacton existence and spin-orbit density dependence in Bose-Einstein condensates.

We demonstrate the existence of compactons matter waves in binary mixtures of Bose-Einstein condensates (BEC) trapped in deep optical lattices (OL) subjected to equal contributions of intraspecies Rashba and Dresselhaus spin-orbit coupling (SOC) under periodic time modulations of the intraspecies scattering length. We show that these modulations lead to a rescaling of the SOC parameters that involves the density imbalance of the two components. This gives rise to density dependent SOC parameters that strongly influence the existence and the stability of compacton matter waves. The stability of SOC-compactons is investigated both by linear stability analysis and by time integrations of the coupled Gross-Pitaevskii equations. We find that SOC restricts the parameter ranges for stable stationary SOC-compacton existence but, on the other side, it gives a more stringent signature of their occurrence. In particular, SOC-compactons should appear when the intraspecies interactions and the number of atoms in the two components are perfectly balanced (or close to being balanced for the metastable case). The possibility to use SOC-compactons as a tool for indirect measurements of the number of atoms and/or the intraspecies interactions is also suggested.

Three-dimensional quantum droplets in spin-orbit-coupled Bose-Einstein condensates

Quantum droplets (QDs) are a recently discovered new state of matter in ultracold atoms. We study three-dimensional (3D) self-trapped modes in spinor Bose-Einstein condensates with spin-orbit coupling (SOC), described by coupled Gross-Pitaevskii equations including beyond-mean-field Lee-Huang-Yang terms. The 3D QDs, semi-vortex and mixed-mode states, of a large size with an anisotropic density profile, exist with a wide large values of the norm as the Lee-Huang-Yang terms eliminate the collapse. The effect of the intra- and inter-component of the nonlinearity and SOC on characteristics of the QDs is systematically addressed. Using the linear-stability analysis and direct simulations, we have checked the stability of all the 3D QDs states, stressing they are stable against small perturbations in propagation in limited scales. The present analysis opens a new way for creating multidimensional solitary waves, and may be developed in other directions, including nonlinear optics, not only in conservative, but also in dissipative systems.

Modulational instability and quantum droplets in a two-dimensional Bose-Einstein condensate

Modulational instability of a uniform two-dimensional binary Bose-Einstein condensate (BEC) in the presence of quantum fluctuations is studied. The analysis is based on the coupled Gross-Pitaevskii equations. It is shown that quantum fluctuations can induce instability when the BEC density is below a threshold. The dependence of the growth rate of modulations on the BEC parameters is found. It is observed that an asymmetry of the interaction parameters and/or initial densities of the components typically decreases the growth rate. Further development of the instability results in the BEC breaking up into a set of quantum droplets. These droplets merge dynamically with each other, so the total number of droplets decreases rapidly. The rate of this decrease is evaluated numerically for different initial parameters.

FORTRESS: FORTRAN programs to solve coupled Gross-Pitaevskii equations for spin-orbit coupled spin-f Bose-Einstein condensate with spin f = 1 or 2

We provide here the updated versions of OpenMP parallelized FORTRAN 90/95 programs to numerically study the ground states and/or the dynamics of homogeneous or trapped spin-1 or spin-2 Bose-Einstein condensates (BECs) with anisotropic spin-orbit (SO) coupling. The coupled sets of three or five Gross-Pitaevskii (GP) equations, respectively, for a spin-1 or a spin-2 Bose-Einstein condensate (BEC) are solved using a time-splitting Fourier spectral method. New version program summaryProgram Title: FORTRESSCPC Library link to program files:https://doi.org/10.17632/st7md3ss85.2Developer's repository link:https://github.com/arko87/FORTRESSLicensing provisions: MITProgramming language: OpenMP Fortran 90/95Supplementary material: The key details to solve the five coupled Gross-Pitaevskii equations for an SO-coupled spin-2 BEC are presented in the supplementary file. In addition, some sample numerically obtained ground state solutions of SO-coupled spin-2 BECs and OpenMP performance results of the software with spin f=2 are provided.Journal reference of previous version: Comput. Phys. Commun. 259 (2021) 107671Does the new version supersede the previous version?: YesReasons for the new version: The previous version can be used to numerically study an SO-coupled spin-1 BEC only, whereas the new version extends the applicability of the software to the SO-coupled spin-2 BECs.Summary of revisions: The new features and augmentations in the package are as follows:(i) Input parameters relevant to the problem under investigation are now read via an input file. Three sample input files corresponding to quasi-one-dimensional (q1D), quasi-two-dimensional (q2D), and three-dimensional (3D) spinor BECs have been provided. The user can edit these files as per the requirement of the problem. Details of the parameters in the sample input files are described in the README.md file provided with the software.(ii) The software has been augmented to solve the five coupled Gross-Pitaevskii equations for SO-coupled spin-2 BECs. The users can now employ the software to study SO-coupled spin-2 [1] as well as spin-1 [2,3] BECs by merely providing the integer ‘SPIN’ parameter equal to 1 for a spin-1 or 2 for a spin-2 BEC in the input file.(iii) A Makefile has been provided to compile the programs with multiple (intel, GNU, and f95) compiler options. Depending on the availability or the choice of user, any of the above compiler options can be used. The user can furthermore choose the variable DIMENSION = 1D, 2D or 3D in the Makefile to simulate, respectively, q1D, q2D or 3D spinor BECs. Depending on the value of DIMENSION, the relevant input file will be read. All the details about using the Makefile are given in the README.md file.(iv) The double precision reals and arrays have been declared in a processor-independent manner.(v) The various arrays have been declared with ALLOCATABLE attribute to assign the memory dynamically and thus more efficiently manage the memory requirement of the software. For a particular allocatable array, memory is allocated using an ALLOCATE statement and deallocated by using a DEALLOCATE statement.(vi) OpenMP parallelization has been improved further.(vii) A few other minor modifications to the codes, e.g. inclusion of convergence criterion based on energy in addition to the criterion based on the absolute value of the wavefunctions in the previous version, directing error messages to a file, etc.Nature of problem: To solve the coupled Gross-Pitaevskii equations (CGPEs) for an SO-spin-f BEC with f=1 or 2.Solution method: We use a time-splitting Fourier spectral method to solve the CGPEs. The resulting equations are evolved in imaginary time to obtain the ground state of the system or in real time to study the dynamics. Additional details of the solution method applicable to spin-2 BEC are provided in the supplementary material.

On-demand generation of dark-bright soliton trains in Bose-Einstein condensates

The controlled creation of dark-bright (DB) soliton trains in multi-component Bose-Einstein condensates (BECs) is a topic of ongoing interest. In this work we generalize earlier findings on the creation of dark soliton trains in single-component BECs [A. Romero-Ros et al., Phys. Rev. A 103, 023329 (2021)] to two-component BECs. By choosing suitable filled box-type initial configurations (FBTCs) and solving the direct scattering problem for the defocusing vector nonlinear Schr\"odinger equation with nonzero boundary conditions we obtain analytical expressions for the DB soliton solutions produced by a general FBTC. It is found that the size of the initial box and the amount of filling directly affects the number, size, and velocity of the solitons, while the initial phase determines the parity (even or odd) of the solutions. Our analytical results are compared to direct numerical integration of the coupled Gross-Pitaevskii equations, both in the absence and in the presence of a harmonic trap, and an excellent agreement between the two is demonstrated.

Instabilities of a vortex-ring-bright soliton in trapped binary three-dimensional Bose-Einstein condensates

Instabilities of vortex-ring-bright coherent structures in harmonically trapped two-component three-dimensional Bose-Einstein condensates are studied numerically within the coupled Gross-Pitaevskii equations and interpreted analytically. Interestingly, the filled vortex core with a sufficiently large amount of the bright component is observed to reduce the parametric interval of stability of the vortex ring. We have identified the mechanisms of several linear instabilities and one nonlinear parametric instability in this connection. Two of the linear instabilities are qualitatively different from ones reported earlier, to our knowledge, and are associated with azimuthal modes of $m=0$ and $m=1$, i.e., deviations of the vortex from the stationary ring shape. Our nonlinear parametric resonance instability occurs between the $m=0$ and $m=2$ modes and signals the exchange of energy between them.

Effect of Rashba spin-orbit and Rabi couplings on the excitation spectrum of binary Bose-Einstein condensates

We present the collective excitation spectrum analysis of binary Bose-Einstein condensates (BECs) with spin-orbit (SO) and Rabi couplings in a quasi-two-dimensional system. In particular, we investigate the role of SO and Rabi coupling strengths in determining the dynamical stability of the coupled BECs using Bogoliubov-de Gennes (BdG) theory. Using the eigenergy of BdG spectrum, we confirm the existence of phonon, roton, and maxon modes with weak repulsive intra- and inter-species contact interactions. The depth of the minimum corresponding to the roton mode depends strongly on the coupling strength. We find that the increase of the SO coupling leads to instability, while the increase in the Rabi coupling stabilizes the system. Also the eigenvectors of BdG spectrum indicates the presence of density like mode in the stable regime and spin like modes in unstable regimes. A phase diagram demonstrating the stability regime in the plane of SO and Rabi coupling strengths is obtained. Finally, we complement the observation of the excitation spectrum with the direct numerical simulation results of coupled Gross-Pitaevskii equations.

Dynamics of a degenerate Cs-Yb mixture with attractive interspecies interactions

We probe the collective dynamics of a quantum degenerate Bose-Bose mixture of Cs and $^{174}$Yb with attractive interspecies interactions. Specifically, we excite vertical center of mass oscillations of the Cs condensate, and observe significant damping for the Cs dipole mode, due to the rapid transfer of energy to the larger Yb component, and the ensuing acoustic dissipation. Numerical simulations based on coupled Gross-Pitaevskii equations provide excellent agreement, and additionally reveal the possibility of late-time revivals (beating) which are found to be highly sensitive to the Cs and Yb atom number combinations. By further tuning the interaction strength of Cs using a broad Feshbach resonance, we explore the stability of the degenerate mixture, and observe collapse of the Cs condensate mediated by the attractive Cs-Yb interaction when $a_{\mathrm{Cs}}<50 \, a_0$, well above the single-species collapse threshold, in good agreement with simulations.

An Energy-Preserving Scheme for the Coupled Gross-Pitaevskii Equations

An Energy-Preserving Scheme for the Coupled Gross-Pitaevskii Equations

Trapped Bose-Bose mixtures at finite temperature: A quantum Monte Carlo approach

We study thermal properties of a trapped Bose-Bose mixture in a dilute regime using quantum Monte Carlo methods. Our main aim is to investigate the dependence of the superfluid density and the condensate fraction on temperature, for the mixed and separated phases. To this end, we use the diffusion Monte Carlo method, in the zero-temperature limit, and the path-integral Monte Carlo method for finite temperatures. The results obtained are compared with solutions of the coupled Gross-Pitaevskii equations for the mixture at zero temperature. We notice the existence of an anisotropic superfluid density in some phase-separated mixtures. Our results also show that the temperature evolution of the superfluid density and condensate fraction is slightly different, showing noteworthy situations where the superfluid fraction is smaller than the condensate fraction.

FORTRESS: FORTRAN programs for solving coupled Gross–Pitaevskii equations for spin–orbit coupled spin-1 Bose–Einstein condensate

Here, we present simple and efficient numerical scheme to study static and dynamic properties of spin-1 Bose–Einstein condensates (BECs) with spin–orbit (SO) coupling by solving three coupled Gross–Pitaevskii equations (CGPEs) in three-, quasi-two and quasi-one dimensional systems. We provide a set of three codes developed in FORTRAN 90/95 programming language with user defined ‘option’ of imaginary and real-time propagation. We present the numerical results for energy, chemical potentials, and component densities for the ground state and compare with the available results from the literature. The results are presented for both the ferromagnetic and antiferromagnetic spin-1 BECs with and without SO coupling. To improve the computational speed, all the codes have the option of OpenMP parallelization. We have also presented the results for speedup and efficiency of OpenMP parallelization for the three codes with both imaginary and real-time propagation. Program summaryProgram Title: FORTRESSCPC Library link to program files:https://doi.org/10.17632/st7md3ss85.1Licensing provisions: MITProgramming language: (OpenMP) FORTRAN 90/95External routines/libraries: FFTW 3.3.8Nature of problem: To solve the coupled Gross–Pitaevskii equations for spin-1 BEC with anisotropic spin–orbit coupling using the time-splitting spectral method.Solution method: We use the time-splitting Fourier spectral method to solve the coupled Gross–Pitaevskii equations. The resulting equations are evolved in imaginary time to obtain the ground state of the system or in real-time to study the dynamics.

Kinetics and luminescence of the excitations of a nonequilibrium polariton condensate

We extend the description of the nonequilibrium Gross-Pitaevskii equation for microcavity polaritons by including not only the kinetics of the nonresonantly pumped exciton reservoir but also that of the collective excitations---called bogolons---above the condensate. Quasiequilibrium Hartree-Fock-Bogoliubov-Popov self-energies are used to describe the condensate and the bogolons. With nonequilibrium Green functions, the quantum Boltzmann equation of the bogolons in a frame moving with the condensate, together with the coupled Gross-Pitaevskii equation can be derived. The resulting scattering rates contain vertex corrections in form of symmetrized products of the $u$ and $v$ pairing functions of all involved scattering states. Corresponding results have been derived for ultracold atoms by A. Griffin, T. Nikuni, and E. Zaremba [Bose-Condensed Gases at Finite Temperatures, 1st ed. (Cambridge University Press, Cambridge, New York, 2009)] Furthermore, we show that the results of the quantum kinetics also determine the luminescence spectrum including the ghost branch. The kinetics is solved numerically, the resulting condensate, the bogolon distribution, and the luminescence are calculated temporally and spatially resolved after the switch-on of a $\mathit{cw}$ nonresonant pump beam with a Gaussian profile. It is studied in detail to which extent the bogolons approach a local thermal equilibrium.

Multisymplectic structure-preserving scheme for the coupled Gross–Pitaevskii equations

In this paper, we study numerically the dynamics of the rotating Bose–Einstein condensates (BECs) modelled by the coupled Gross–Pitaevskii (CGP) equations with angular momentum rotating terms. First, the multisymplectic structure of the CGP equations is investigated by introducing some canonical momenta which allows us to construct the corresponding multisymplectic schemes. Then applying the midpoint rule in both temporal and spatial directions, a multisymplectic scheme is proposed for the CGP equations. The conservative properties and the convergence analysis are discussed for the multisymplectic scheme. The cut-off function technique is utilized for the error estimation. Numerical examples are carried out to verify the conservative property and the convergence rate.