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Laser Physics

Stability of Bose-Einstein condensates in a double-well potential

Ryoko Ichihara, Ippei Danshita, K. Egawa, Tetsuro Nikuni

Published Mar 1, 2008• 0 citations
Publisher

Abstract

We investigate the stability of the first excited state, the so-called “π-state,” of Bose-Einstein condensates in a double-well potential. From the condition of complex excitation energies, we determine the critical barrier height, above which the π-state is dynamically unstable. We find that the critical barrier height decreases monotonically as the number of condensate atoms increases. We also simulate the dynamics of the π-state by solving the time-dependent Gross-Pitaevskii equation. Our simulation results show that the π-state in the dynamically unstable region exhibits distinctively different behavior from that in the dynamically stable region.

DOI
10.1134/s1054660x08030201

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We investigate the stability of the first excited state, the so-called “π-state,” of Bose-Einstein condensates in a double-well potential. From the condition of complex excitation energies, we determine the critical barrier height, above which the π-state is dynamically unstable. We find that the critical barrier height decreases monotonically as the number of condensate atoms increases. We also simulate the dynamics of the π-state by solving the time-dependent Gross-Pitaevskii equation. Our simulation results show that the π-state in the dynamically unstable region exhibits distinctively different behavior from that in the dynamically stable region.
We study stability of the first excited state of quasi-one-dimensional Bose-Einstein condensates in a double-well potential, which is called ``$\ensuremath{\pi}$ state.'' The density notch in the $\ensuremath{\pi}$ state can be regarded as a standing dark soliton. From the excitation spectrum, we determine the critical barrier height, above which the $\ensuremath{\pi}$ state is dynamically unstable. We find that the critical barrier height decreases monotonically as the number of condensate atoms increases. We also simulate the dynamics of the $\ensuremath{\pi}$ state by solving the time-dependent Gross-Pitaevskii equation. We show that due to the dynamical instability the dark soliton starts to move away from the trap center and exhibits a large-amplitude oscillation.
We study dynamical behaviors of the weakly interacting Bose-Einstein condensate in the one-dimensional optical lattice with an overall double-well potential by solving the time-dependent Gross-Pitaevskii equation. It is observed that the double-well potential dominates the dynamics of such a system even if the lattice depth is several times larger than the height of the double-well potential. This result suggests that the condensate flows without resistance in the periodic lattice just like the case of a single particle moving in periodic potentials. Nevertheless, the effective mass of atoms is increased, which can be experimentally verified since it is connected to the Josephson oscillation frequency. Moreover, the periodic lattice enhances the nonlinearity of the double-well condensate, making the condensate more “self-trapped” in the π -mode self-trapping regime.
In this Thesis we apply the Gross-Pitaevskii equation (GPE) to describe properties of a dilute, near zero temperature Bose gas for various confining geometries. We start by reviewing some basic information about the density, the chemical potential and elementary excitations of a dilute atomic condensate confined in a single harmonic trap for a Bose condensate with repulsive and attractive interactions and we also discuss the stability in the case of attractive interactions. We extend our study to a one and three dimensional double-well trap. We investigate the eigenenergy levels and show that the nonlinearity leads to triangular structures which appear either in the ground or excited states for the case of a Bose condensate with attractive or repulsive interactions respectively. We apply the eigenenergy level picture to analyse Josephson effects induced when the barrier IS moved at a constant velocity across the trapping potential or by the application of a time-dependent potential gradient. The GPE simulations are compared to the predictions of a nonlinear two state model. Above a critical velocity there is a transition to a superposition of ground and excited states which leads to sudden changes in the population difference. The direction of Josephson flow depends critically on the initial state of the system and we discuss the feasibility of experimental control of the atomic flow using phase-imprinting. The stability of a low temperature Bose-Einstein condensate with attract interactions in one and three dimensional double-well potentiate is discussed. The condensate is shown to collapse at a critical potential gradient which corresponds to a critical number of atoms in one of the two wells. Finally we investigate the stability and tunnelling effects in a multi-well system.
The dc and ac Josephson dynamics of a Bose-Einstein condensate in a double-well potential is analyzed in terms of transitions between energy eigenstates. By solving the time-independent and time-dependent Gross-Pitaevskii equations in one dimension, we identify population transfer associated with level crossings, and determine the critical velocity that characterizes the transition. We test the validity of a nonlinear two-state model, and show that for the experimentally interesting case, where the critical velocity is large, the influence of higher-lying states is important.
We study the dynamics of merging two initially independent Bose–Einstein condensates by adiabatically deforming the trap potential from a double well into a single well. The process of slowly decreasing the height of the central barrier of the double well is simulated by a full 3D calculation of the Gross–Pitaevskii equation. We investigate the dynamics by considering some factors relevant to experiments, e.g., the atom number, the initial state in each well, the initial relative phase between the two condensates. For a small number of 4400 atoms in each well and the same initial phase, we find for both condensates in the ground state or the p-wave soliton state that the final state is the ground state or the p-wave soliton state of the final trap potential. For cases in which both condensates are vortex states, we find much richer dynamics with the appearance of an extra vortex. We also discuss the cases for different initial relative phases. For comparison, we present results for 44 000 atoms corresponding to a larger nonlinear term in the Gross–Pitaevskii equation. In support of future experimental observations, we show both probability density and current maps.
We investigate dipolar Bose-Einstein condensates in a complex external double-well potential that features combined parity and time-reversal symmetry. On the basis of the Gross-Pitaevskii equation we study the effects of the long-ranged anisotropic dipole-dipole interaction on ground and excited states by the use of a time-dependent variational approach. We show that the property of a similar nondipolar condensate of possessing real energy eigenvalues in certain parameter ranges is preserved despite the inclusion of this nonlinear interaction. Furthermore, we present states that break the PT\mathcal{PT}PT symmetry and investigate the stability of the distinct stationary solutions. In our dynamical simulations we reveal a complex stabilization mechanism for PT\mathcal{PT}PT-symmetric as well as for PT\mathcal{PT}PT-broken states which are, in principle, unstable with respect to small perturbations.