We consider the evolution of an interacting dilute Bose-Einstein condensate transferred from a harmonic trap to a perfect box of finite size. We study numerically the influence of the nonlinearities in the system on its dynamics. The parallelism between this problem and the Fermi-Pasta-Ulam problem in the context of matter waves is established. Criteria for the appearance of statistical behavior are discussed. We show that by increasing the nonlinearity we pass from a regime of collapses and revivals for the wave function to a regime of collapses and revivals of regular motion.
We consider the evolution of an interacting dilute Bose-Einstein condensate transferred from a harmonic trap to a perfect box of finite size. We study numerically the influence of the nonlinearities in the system on its dynamics. The parallelism between this problem and the Fermi-Pasta-Ulam problem in the context of matter waves is established. Criteria for the appearance of statistical behavior are discussed. We show that by increasing the nonlinearity we pass from a regime of collapses and revivals for the wave function to a regime of collapses and revivals of regular motion.
We consider, analytically and numerically, the dynamics of an atomic Bose–Einstein condensate soliton bouncing in a magnetic or optical trap between two ideal barriers oscillating harmonically with coinciding frequency and opposite phases. There are zones of parametric instability of the soliton’s central position in the trap for certain ranges of the frequency of the barriers’ oscillations. We demonstrate the Fermi–Pasta–Ulam recurrence in the Fermi–Ulam scheme for solitons: the soliton’s deviations from the trap’s center grow and decay quasi-periodically in time. The results allow one to excite and investigate various structures of matter waves in traps and to manipulate their localization and dynamics.
We consider the evolution of a dilute Bose-Einstein condensate in an optical trap formed by a doughnut laser mode. By solving a one-dimensional Gross-Pitaevskii equation and looking at the variance and the statistical entropy associated with the position of the system, we can study the dynamical behavior of the system. It is shown that for small condensates nonlinear revivals of the macroscopic wave function are expected. For sufficiently large and dense condensates irreversible dynamics takes place for which revivals of regular dynamics appear as predicted in P. Villain and M. Lewenstein, Phys. Rev. A 62, 043601 (2000). These results are confirmed by a two-dimensional simulation in which the scales of energy associated with the two different directions mimic the experimental situation.
The Fermi-Pasta-Ulam (FPU) model, which was proposed 50 years ago to examine thermalization in nonmetallic solids and develop "experimental" techniques for studying nonlinear problems, continues to yield a wealth of results in the theory and applications of nonlinear Hamiltonian systems with many degrees of freedom. Inspired by the studies of this seminal model, solitary-wave dynamics in lattice dynamical systems have proven vitally important in a diverse range of physical problems-including energy relaxation in solids, denaturation of the DNA double strand, self-trapping of light in arrays of optical waveguides, and Bose-Einstein condensates (BECs) in optical lattices. BECs, in particular, due to their widely ranging and easily manipulated dynamical apparatuses-with one to three spatial dimensions, positive-to-negative tuning of the nonlinearity, one to multiple components, and numerous experimentally accessible external trapping potentials-provide one of the most fertile grounds for the analysis of solitary waves and their interactions. In this paper, we review recent research on BECs in the presence of deep periodic potentials, which can be reduced to nonlinear chains in appropriate circumstances. These reductions, in turn, exhibit many of the remarkable nonlinear structures (including solitons, intrinsic localized modes, and vortices) that lie at the heart of the nonlinear science research seeded by the FPU paradigm.
Strong energy sharing is shown by numerically investigating coupled multi-component Bose–Einstein condensates (BECs) with a harmonic trap to simulate the Fermi–Pasta–Ulam model (FPU). For two-component BECs, the energy exchanging between each part, from regular, quantum beating to complete energy sharing, is explored by simulating their Husimi distributions, the time evolution of energies and the statistical entropy. Meanwhile, in the three-component case, a more complex energy sharing behavior is reported and a strong energy sharing is found.
Previously we numerically showed that thermalization can occur in an oscillating Bose-Einstein condensate (BEC) with a disordered harmonic trap when healing length ξ of the condensate is shorter than the correlation length σD of the disorder [see, for example, the experiment reported in Phys. Rev. A \textbf{82}, 033603 (2010)]. In this work, we investigate the weakly interacting or Anderson localization regime ξ>σD and show that the oscillating BEC can also exhibit a relaxation process from nonequilibrium to equilibrium. In such an isolated quantum system, energy and particle number are conserved and the irreversible evolution towards thermodynamic equilibrium is induced by the disorder. The thermodynamic equilibrium is evidenced by the maximized entropy S[nk] in which the waveaction spectrum nk follows the Rayleigh-Jeans distribution. Besides, unlike a monotonic irreversible process of thermalization to equilibrium, the Fermi-Pasta-Ulam-Tsingou recurrence arises in this system, manifested by the oscillation of the non-equilibrium entropy.
We numerically study the dynamics of Faraday waves in a pancake-shaped Bose--Einstein condensate (BEC) subject to periodic modulation of the interaction. After the modulation starts, Faraday waves appear and, thereafter, the BEC enters the "nonlinear regime", in which several collective modes are excited. By maintaining the modulation without dissipation, the kinetic energy that contributes to the density gradient causes quasiperiodic motion and increases monotonically. In the nonlinear regime, the dips in the density similar to dark solitons move around in the BEC, intersecting with each other and maintaining their shapes. On the other hand, even by turning off the modulation, Faraday waves and the nonlinear regime appear. The kinetic energy converges to the statistical steady state. The calculation of the modulation with the dissipation illustrates the collapse and revival of Faraday waves. When the dissipation is small, the appearance and collapse of the Faraday waves and the nonlinear regime occur again.