Inhomogeneous Bose-Einstein Condensates Research Articles

Two-dimensional superfluid flows in inhomogeneous Bose-Einstein condensates

We report an algorithm of constructing linear and nonlinear potentials in the two-dimensional Gross-Pitaevskii equation subject to given boundary conditions, which allow for exact analytic solutions. The obtained solutions represent superfluid flows in inhomogeneous Bose-Einstein condensates. The method is based on the combination of the similarity reduction of the two-dimensional Gross-Pitaevskii equation to the one-dimensional nonlinear Schrödinger equation, the latter allowing for exact solutions, with the conformal mapping of the given domain, where the flow is considered, to a half space. The stability of the obtained flows is addressed. A number of stable and physically relevant examples are described.

CONSTRUCTION OF MODULATED AMPLITUDE WAVES VIA AVERAGING IN COLLISIONALLY INHOMOGENEOUS BOSE–EINSTEIN CONDENSATES

We apply the averaging method to analyze spatio-temportal structures in nonlinear Schrödinger equations and thereby study the dynamics of quasi-one-dimensional collisionally inhomogeneous Bose–Einstein condensates with the scattering length varying periodically in space and crossing zero. Infinitely many modulated amplitude waves with nontrivial phases are shown.

Nonlinear optical effects in an inhomogeneous Bose—Einstein condensate

A variety of nonlinear optical effects arising upon the interaction of a signal beam with a strong pump beam at the second harmonic frequency in a nonlinear Bose-Einstein condensate is studied. Along with the ratio of signal and pump beam widths, the influence of the mutual geometry of the trap and the pump beam profile on the studied processes is investigated.

Variational treatment of Faraday waves in inhomogeneous Bose–Einstein condensates

Motivated by the recent experimental progress on the collective modes of a Bose–Einstein condensate whose atomic scattering length is tuned via Feshbach resonances, we analyze by variational means the dynamics of Faraday waves in trapped Bose–Einstein condensates. These waves can be excited by modulating periodically either the strength of the magnetic trap or the atomic scattering length. To study their dynamics, we develop a variational model that describes consistently both the bulk part of an inhomogeneous, low-density, cigar-shaped condensate and small-amplitude, small-wavelength Faraday waves. The main ansatz used in the variational treatment is tailored around a set of Gaussian envelopes and we show extensions for the high-density regime using a q-Gaussian function. Finally, we show explicitly that for drives of small amplitude, the two methods of obtaining Faraday waves are equivalent, and we discuss the existing experimental results.

Soliton dynamics in an inhomogeneous Bose–Einstein condensate driven by linear potential

In this paper we study the propagation of solitons in a Bose–Einstein condensate governed by the time dependent one dimensional Gross–Pitaevskii equation managed by Feshbach resonance in a linear external potential. We give the Lax pair of the Gross–Pitaevskii equation in Bose–Einstein condensates and obtain exact N-soliton solution by employing the simple, straightforward Darboux transformation. As an example, we present exact one and two-soliton solution and discuss their transmission, interaction and dynamic properties. We further calculate the particle number, momentum and energy of the solitons and discuss their conservation laws. Knowledge of soliton dynamics helps us in understanding the physical nature of the condensate and in the calculation of the thermodynamic properties.

Localization of collisionally inhomogeneous condensates in a bichromatic optical lattice

By direct numerical simulation and variational solution of the Gross-Pitaevskii equation, we studied the stationary and dynamic characteristics of a cigar-shaped, localized, collisionally inhomogeneous Bose-Einstein condensate trapped in a one-dimensional bichromatic quasiperiodic optical-lattice potential, as used in a recent experiment on the localization of a Bose-Einstein condensate [Roati et al., Nature (London) 453, 895 (2008)]. The effective potential characterizing the spatially modulated nonlinearity is obtained. It is found that the collisional inhomogeneity has influence not only on the central region but also on the tail of the Bose-Einstein condensate. The influence depends on the sign and value of the spatially modulated nonlinearity coefficient. We also demonstrate the stability of the stationary localized state by performing a standard linear stability analysis. Where possible, the numerical results are shown to be in good agreement with the variational results.

Dynamics of inhomogeneous condensates in contact with a surface

We show that interplay of linear attractive (repulsive) boundary with inhomogeneous repulsive (attractive) interatomic interactions results in nonlinear localized surface modes (surface solitons), some of which are stable. We consider several example systems describing interaction of inhomogeneous Bose-Einstein condensates with rigid surfaces and allowing for exact solutions. The stability of the obtained modes is analyzed analytically and numerically. Stable localized surface modes are found and dynamics of the unstable modes is described.

Nonlinear interaction of optical beams in a Bose-Einstein condensate

The results of simulation of the interaction of optical beams with different frequencies in an inhomogeneous Bose-Einstein condensate are given. The potential well has a parabolic profile, and the nonlinearity belongs to a defocusing class. The total reflection of a signal wave induced by a pump beam at a negative inhomogeneity during the wave capture in the potential well is considered.

Collisionally inhomogeneous Bose–Einstein condensates in double-well potentials

In this work, we consider quasi-one-dimensional Bose–Einstein condensates (BECs), with spatially varying collisional interactions, trapped in double-well potentials. In particular, we study a setup in which such a “collisionally inhomogeneous” BEC has the same (attractive–attractive or repulsive–repulsive) or different (attractive–repulsive) types of interparticle interactions. Our analysis is based on the continuation of the symmetric ground state and anti-symmetric first excited state of the non-interacting (linear) limit into their nonlinear counterparts. The collisional inhomogeneity produces a saddle–node bifurcation scenario between two additional solution branches; as the inhomogeneity becomes stronger, the turning point of the saddle–node tends to infinity and eventually only the two original branches remain, which is completely different from the standard double-well phenomenology. Finally, one of these branches changes its monotonicity as a function of the chemical potential, a feature especially prominent, when the sign of the nonlinearity changes between the two wells. Our theoretical predictions, are in excellent agreement with the numerical results.

Spatial Pair Correlations of Atoms in Molecular Dissociation

We perform first-principles quantum simulations of dissociation of trapped, spatially inhomogeneous Bose-Einstein condensates of molecular dimers. Specifically, we study spatial pair correlations of atoms produced in dissociation after time of flight. We find that the observable correlations may significantly degrade in systems with spatial inhomogeneity compared to the predictions of idealized uniform models. We show how binning of the signal can enhance the detectable correlations and lead to the violation of the classical Cauchy-Schwartz inequality and relative number squeezing.

Modulated amplitude waves in collisionally inhomogeneous Bose–Einstein condensates

We investigate the dynamics of an effectively one-dimensional Bose–Einstein condensate (BEC) with scattering length a subjected to a spatially periodic modulation, a = a ( x ) = a ( x + L ) . This “collisionally inhomogeneous” BEC is described by a Gross–Pitaevskii (GP) equation whose nonlinearity coefficient is a periodic function of x . We transform this equation into a GP equation with a constant coefficient and an additional effective potential and study a class of extended wave solutions of the transformed equation. For weak underlying inhomogeneity, the effective potential takes a form resembling a superlattice, and the amplitude dynamics of the solutions of the constant-coefficient GP equation obey a nonlinear generalization of the Ince equation. In the small-amplitude limit, we use averaging to construct analytical solutions for modulated amplitude waves (MAWs), whose stability we subsequently examine using both numerical simulations of the original GP equation and fixed-point computations with the MAWs as numerically exact solutions. We show that “on-site” solutions, whose maxima correspond to maxima of a ( x ) , are more robust and likely to be observed than their “off-site” counterparts.

Collisional phase shifts of ring dark solitons in inhomogeneous Bose–Einstein condensates

Abstract The head-on collisions of two ring dark solitons in inhomogeneous Bose–Einstein condensates (BECs) with thin disk-shaped potential are studied by the extended Poincare–Lighthill–Kuo (PLK) perturbation method. The result shows that the system admits a solution with two concentric ring solitons, one moving inwards and the other moving outwards, which in small-amplitude limit, are described by two modified cylindrical KdV equations in the respective reference frames. In particular, the analytical phase shifts induced by the head-on collisions between two ring dark solitary waves are derived, and the result shows that the phase shifts change with the radial coordinate r according to the ( 1 + σ r 2 ) r − 1 / 3 law (where σ ∼ ω r 2 / ω z 2 ), which are quite different with the homogeneous case.

Velocity of vortices in inhomogeneous Bose-Einstein condensates.

We derive, from the Gross-Pitaevskii equation, an exact expression for the velocity of any vortex in a Bose-Einstein condensate, in equilibrium or not, in terms of the condensate wave function at the center of the vortex. In general, the vortex velocity is a sum of the local superfluid velocity, plus a correction related to the density gradient near the vortex. A consequence is that in rapidly rotating, harmonically trapped Bose-Einstein condensates, unlike in the usual situation in slowly rotating condensates and in hydrodynamics, vortices do not move with the local fluid velocity. We indicate how Kelvin's conservation of circulation theorem is compatible with the velocity of the vortex center being different from the local fluid velocity. Finally, we derive an exact wave function for a single vortex near the rotation axis in a weakly interacting system, from which we derive the vortex precession rate.

The dynamics of ring dark soliton in inhomogeneous Bose-Einstein condensates

The dynamics of the ring dark soliton in an inhomogeneous Bose-Einstein condensates (BEC) with thin disk-shaped potential trapping is investigated analytically and numerically. Analytical result shows that the ring dark soliton is governed by a variable coefficients Korteweg-de Vries (KdV) equation. The effect of the ring curvature (nonplanar geometry) and the inhomogeneous of the background on soliton amplitude and the emitted radiation profiles are obtained analytically. The theoretical results are confirmed by the direct numerical results.

Beyond-mean-field results for atomic Bose-Einstein condensates at interaction strengths near Feshbach resonances: A many-body dimensional perturbation-theory calculation

We present semianalytical many-body results for energies and excitation frequencies for an inhomogeneous Bose-Einstein condensate over a wide range of atom numbers $N$ for both small $s$-wave scattering lengths, typical of most laboratory experiments, and large scattering lengths, achieved by tuning through a Feshbach resonance. Our dimensional perturbation treatment includes two-body correlations at all orders and yields analytical results through first order by taking advantage of the high degree of symmetry of the condensate at the zeroth-order limit. Because $N$ remains a parameter in our analytical results, the challenge of calculating energies and excitation frequencies does not rise with the number of condensate atoms. In this proof-of-concept paper the atoms are confined in a spherical trap and are treated as hard spheres. Our many-body calculations compare well to Gross-Pitaevskii results in the weakly interacting regime and depart from the mean-field approximation as the density approaches the strongly interacting regime. The excitation frequencies provide a particularly sensitive test of beyond-mean-field corrections. For example, for $N=2000$ atoms and an experimentally realized large scattering length of $a=0.433{a}_{ho}\phantom{\rule{0.3em}{0ex}}({a}_{ho}=\sqrt{\ensuremath{\hbar}∕m{\ensuremath{\omega}}_{ho}})$ we predict a $75%$ shift from the mean-field breathing mode frequency.

N identical particles under quantum confinement: a many-body dimensional perturbation theory approach

Systems that involve N identical interacting particles under quantum confinement appear throughout many areas of physics, including chemical, condensed matter, and atomic physics. In this paper, we present the methods of dimensional perturbation theory, a powerful set of tools that uses symmetry to yield simple results for studying such many-body systems. We present a detailed discussion of the dimensional continuation of the N-particle Schrödinger equation, the spatial dimension D→∞ equilibrium ( D 0) structure, and the normal-mode ( D −1) structure. We use the FG matrix method to derive general, analytical expressions for the many-body normal-mode vibrational frequencies, and we give specific analytical results for three confined N-body quantum systems: the N-electron atom, N-electron quantum dot, and N-atom inhomogeneous Bose–Einstein condensate with a repulsive hard-core potential.

Deformation of dark solitons in inhomogeneous Bose–Einstein condensates

A dark soliton becomes unstable when it is incident on a background densitygradient, and the induced instability results in the emission of sound. Detailedquantitative studies of sound emission are performed for various potentials,such as steps, linear ramps and Gaussian traps. The amount of soundemission is found to be a significant fraction of the soliton energy for typicalpotentials. Continuous emission of sound is found to lead to an apparentdeformation of the soliton profile. The power emitted by the soliton isshown to be parametrized by the square of the displacement of the centreof mass of the soliton from its density minimum, thus highlighting thesignificance of the inhomogeneity-induced soliton deformation.

Collective excitations of trapped Bose-Einstein condensates in the energy and time domains

A time-dependent method for calculating the collective excitation frequencies and densities of a trapped, inhomogeneous Bose-Einstein condensate with circulation is presented. The results are compared with time-independent solutions of the Bogoliubov--de Gennes equations. The method is based on time-dependent linear-response theory combined with spectral analysis of moments of the excitation modes of interest. The technique is straightforward to apply, extremely efficient in our implementation with parallel fast Fourier transform methods, and produces highly accurate results. For high dimensionality or low symmetry the time-dependent approach is a more practical computational scheme and produces accurate and reliable data. The method is suitable for general trap geometries, condensate flows and condensates permeated with defects and vortex structures.

Vortex line and ring dynamics in trapped Bose-Einstein condensates

Vortex dynamics in inhomogeneous Bose-Einstein condensates are studied numerically in two and three dimensions. We simulate the precession of a single vortex around the center of a trapped condensate, and use the Magnus force to estimate the precession frequency. Vortex ring dynamics in a spherical trap are also simulated, and we discover that a ring undergoes oscillatory motion around a circle of maximum energy. The position of this locus is calculated as a function of the number of condensed atoms. In the presence of dissipation, the amplitude of the oscillation will increase, eventually resulting in self-annihilation of the ring.

Density functional theory of the inhomogeneous Bose-Einstein condensate

I present a new bosonic density functional especially conceived for the alkali atom Bose gas, together with the corresponding Kohn-Sham equation. The new formalism provides the simplest theory of the observed inhomogeneous Bose-Einstein condensate, beyond the level of mean-field theory.