Interacting Bose-Einstein Condensate Research Articles

Quantum fluctuation in the segregation phase of two-component weakly interacting Bose–Einstein condensates

Our investigation focuses on the condensate density, excited and condensed fractions of bosons in a mixture of two-component segregated weakly interacting Bose–Einstein condensates (BECs) at zero temperature based on the Gross–Pitaevskii (GP) and Cornwall-Jackiw-Tomboulis (CJT) theories. We find that the repulsive interaction among interspecies of bosons is the origin of the local behavior and the decline of quantum fluctuation. These features are disparate to the single BEC or miscible BECs. The numerical evaluation of the condensed fraction is provided to support the experimental observation.

Entanglement and correlations in an exactly-solvable model of a Bose–Einstein condensate in a cavity

An exactly solvable model of a trapped interacting Bose–Einstein condensate (BEC) coupled in the dipole approximation to a quantized light mode in a cavity is presented. The model can be seen as a generalization of the harmonic-interaction model for a trapped BEC coupled to a bosonic bath. After obtaining the ground-state energy and wavefunction in closed form, we focus on computing the correlations in the system. The reduced one-particle density matrices of the bosons and the cavity are constructed and diagonalized analytically, and the von Neumann entanglement entropy of the BEC and the cavity is also expressed explicitly as a function of the number and mass of the bosons, frequencies of the trap and cavity, and the cavity-boson coupling strength. The results allow one to study the impact of the cavity on the bosons and vice versa on an equal footing. As an application we investigate a specific case of basic interest for itself, namely, non-interacting bosons in a cavity. We find that both the bosons and the cavity develop correlations in a complementary manner while increasing the coupling between them. Whereas the cavity wavepacket broadens in Fock space, the BEC density saturates in real space. On the other hand, while the cavity depletion saturates, and hence does the BEC-cavity entanglement entropy, the BEC becomes strongly correlated and eventually increasingly fragmented. The latter phenomenon implies single-trap fragmentation of otherwise ideal bosons, where their induced long-range interaction is mediated by the cavity. Finally, as a complimentary investigation, the mean-field equations for the BEC-cavity system are solved analytically as well, and the breakdown of mean-field theory for the cavity and the bosons with increasing coupling is discussed. Further applications are envisaged.

Optomechanically induced gain using a trapped interacting Bose-Einstein condensate

We investigate the realization of the phenomenon of optomechanically induced gain in a hybrid optomechanical system consisting of an interacting Bose-Einstein condensate trapped inside the optical lattice of a cavity which is generated by an external coupling laser tuned to the red sideband of the cavity. It is shown that the system behaves as an optical transistor while the cavity is exposed to a weak input optical signal which can be amplified considerably in the cavity output if the system is in the unresolved sideband regime. Interestingly, the system has the capability to switch from the resolved to unresolved sideband regime by controlling the s-wave scattering frequency of atomic collisions. We show that the system gain can be enhanced considerably by controlling the s-wave scattering frequency as well as the coupling laser intensity while the system remains in the stable regime. Based on our obtained results, the input signal can be amplified more than 100 million percent in the system output which is much larger than those already reported in the previously proposed similar schemes.

Noncanonical vortex states and dynamics in Bose-Einstein condensates

Vortex excitations triggered by nonlinear interactions in Bose-Einstein condensates have attracted interest in the study of ultracold atoms. However, most studies focus on canonical vortex states with integer topological charges. In this paper, we study the dynamic properties of noncanonical vortex condensates with three phase distributions: power-exponent, new type power-exponent and oscillation type. The results show that the noncanonical vortices are dynamic unstable and their density distributions obviously depend on the phase parameters of the initial optical phase masks. Different noncanonical vortices decay into canonical clusters with diverse configurations showing rich topological excitation patterns. In particular, a new power exponential noncanonical vortex state decays into a stable canonical polygonal vortex cluster structure. Because the phase structures of the noncanonical optical vortices destroy the rotational symmetry of the condensate, the angular momentum of the condensate is no longer quantized, and its value changes with the power of the azimuthal angle of the optical field or the oscillation frequency, which is obviously different from the evolution of the corresponding noncanonical vortex optical field itself. In the dynamical process, the center-of-mass trajectory of noncanonical vortex condensates with the new type of power exponent phase is always a point, while for the noncanonical vortex condensates with power exponent and oscillating phase, the center-of-mass trajectories are ellipses centering at the origin of coordinates.

Quench dynamics of a spin-orbital coupled Bose-Einstein condensate with nonlinear interactions

In this work, we study the non-equilibrium quench dynamics from the superfluid stripe phase to the supersolid phase of a two dimensional spin-orbital coupled interacting Bose-Einstein condensate in the presence of a one dimensional optical lattice. The quench protocol here is constructed through varying the lattice depth linearly with the evolution time. By using the time-dependent Gutzwiller method, various physical quantities, such as the vortex number and the overlap of wave-function, have been investigated with respect to the quench time. Through analyzing the dynamical behavior of the above physical quantities, we find out the transition time of the quench procedure, which captures the freeze out time indicating the moment that the system catches the quench speed beginning to evolve quickly. Before the transition time, the dynamics is frozen and the state of the system cannot follow the changes in the Hamiltonian. While passing the transition time, we find that there are significant alterations to both the vortex number and the wave-function. At the transition time, on one hand the vortex number abruptly increases from zero; on the other hand the overlap of wave-function departures from 1 shortly. These signatures indicate that the system evolves rapidly when passing the transition time. Furthermore, we also find that due to the presence of spin-orbital coupling, the spin texture represents a periodic magnetic structure accompanying with the emergence of the supersolid dynamically. It is shown that during the quench procedure, the density distribution of the system are always accompanied with the spatial structure of spin texture, i.e., the central position of topological spin skyrmion (antiskyrmion) corresponding to the minimum position of the density distribution. The topological charge of the above spin structures also shows interesting dynamical properties. We find that the quantized topological charge appears with the emergence of the supersolid dynamically.

The Casimir effect of dual weakly interacting Bose gases at zero-temperature

The Casimir effect in a mixture of two components weakly interacting Bose–Einstein condensates (BECs) is investigated within two-loop approximation of the Cornwall–Jackiw–Tomboulis (CJT) effective action approach at zero temperature. The formulae of the Casimir energy and the resulting Casimir force, including boundary condition (BC) and mixture effects, are determined in immiscible and miscible approaching phases. The origin of BC’s influence on the Casimir effect and the Casimir force’s behavior transition are theoretically predicted.

The Casimir-Type Effect in Two-Component Weakly Interacting Bose-Einstein Condensates Gases Confined between Two Parallel Slabs

The Casimir-Type Effect in Two-Component Weakly Interacting Bose-Einstein Condensates Gases Confined between Two Parallel Slabs

Particle scattering by rotating trapped quantum gases at finite temperature

We have analytically explored the quantum phenomena of particle scattering by rotating trapped quantum gases of electrically neutral bosons and fermions for the short-ranged Fermi-Huang interactions between the incident particle and the scatterers. We have predicted differential scattering cross-sections and their temperature and angular velocity dependencies in this regard, in particular, for an ideal Bose gas in a rotating harmonic trap, an ideal Fermi gas in a rotating harmonic trap, and a weakly interacting Bose gas in a slow rotating harmonic trap. We have theoretically probed the lattice-pattern of the vortices in a rapidly rotating strongly interacting Bose–Einstein condensate by the particle scattering method. We also have obtained de Haas-van Alphen-like oscillations in the differential scattering cross-section for an ideal ultracold Fermi gas in a rotating harmonic trap. Our predictions on the differential scattering cross-sections can be tested within the present-day experimental setups.

The Repulsive Casimir-Type Forces of a Weakly Interacting Bose–Einstein Condensate Gas

The Repulsive Casimir-Type Forces of a Weakly Interacting Bose–Einstein Condensate Gas

The Influence of s-Wave Interactions on Focussing of Atoms

The focusing of a rubidium Bose–Einstein condensate via an optical lattice potential is numerically investigated. The results are compared with a classical trajectory model which underestimates the width of the focused beam. Via the inclusion of the effects of interactions into the classical trajectories model, we show that it is possible to obtain reliable estimates for the width of the focused beam when compared to numerical integration of the Gross–Pitaevskii equation. Finally, we investigate the optimal regimes for focusing and find that for a strongly interacting Bose–Einstein condensate focusing of order 20 nm may be possible.

Nonlinear Bragg interferometer with a trapped Bose-Einstein condensate

We propose a scheme for trapped-atom interferometry using an interacting Bose-Einstein condensate. The condensate is controlled and spatially split into two confined external momentum modes through a series of Bragg pulses. The proposed scheme (i) allows the generation of large entanglement in a trapped-interferometer configuration via one-axis twisting dynamics induced by interatomic interaction and (ii) avoids the suppression of interactions during the interferometer sequence by a careful manipulation of the state before and after phase encoding. The interferometer can be used for the measurement of gravity with a sensitivity beyond the standard quantum limit.

Localization and Damping of Macroscopic Oscillation in Coupled Gross–Pitaevskii Equations without Self-interaction

In Bose–Einstein condensates, quantum phenomena appear in a macroscopic scale. The Gross–Pitaevskii (GP) equation describes the dynamics of weakly interacting Bose–Einstein condensates. The GP equa...

Spatial entanglement in interacting Bose-Einstein condensates

The entanglement between spatial regions in an interacting Bose-Einstein condensate is investigated using a quantum field theoretic formalism. Regions that are small compared to the healing length are governed by a non-relativistic quantum field theory in the vacuum limit, and we show that the latter has vanishing entanglement. In the opposite limit of a region that is large compared to the healing length, the entanglement entropy is like in the vacuum of a relativistic theory where the velocity of light is replaced with the velocity of sound and where the inverse healing length provides a natural ultraviolet regularization scale. Besides the von Neumann entanglement entropy, we also calculate R\'enyi entanglement entropies for a one-dimensional quasi-condensate.

Inversion of coherent backscattering with interacting Bose-Einstein condensates in two-dimensional disorder: A truncated Wigner approach

We theoretically study the propagation of an interacting Bose-Einstein condensate in a two-dimensional disorder potential, following the principle of an atom laser. The constructive interference between time-reversed scattering paths gives rise to coherent backscattering, which may be observed under the form of a sharp cone in the disorder-averaged angular backscattered current. As is found by the numerical integration of the Gross-Pitaevskii equation, this coherent backscattering cone is inversed when a non-vanishing interaction strength is present, indicating a crossover from constructive to destructive interferences. Numerical simulations based on the Truncated Wigner method allow one to go beyond the mean-field approach and show that dephasing renders this signature of antilocalisation hidden behind a structureless and dominant incoherent contribution as the interaction strength is increased and the injected density decreased, in a regime of parameters far away from the mean-field limit. However, despite a partial dephasing, we observe that this weak antilocalisation scenario prevails for finite interaction strengths, opening the way for an experimental observation with $^{87}$Rb atoms.

Zero-energy modes of two-component Bose–Bose droplets

Bose–Bose droplets are self-bound objects emerging from a mixture of two interacting Bose–Einstein condensates when their interactions are appropriately tuned. During droplet formation three continuous symmetries of the system’s Hamiltonian are broken: translational symmetry and two U(1) symmetries, allowing for arbitrary choice of phases of the mean-field wavefunctions describing the two components. Breaking of these symmetries must be accompanied by appearance of zero-energy excitations in the energy spectrum of the system recovering the broken symmetries. Normal modes corresponding to these excitations are the zero-energy modes. Here we find analytic expressions for these modes and introduce Hamiltonians generating their time evolution—dynamics of the droplet’s centers of mass as well as dynamics of the phases of the two droplet’s wavefunctions. When internal types of excitations (quasiparticles) are neglected then the very complex system of a quantum droplet is described using only a few ‘global’ degrees of freedom—the position of the center of mass of the droplet and two phases of two wave-functions, all these being quantum operators. We believe that our work might be useful in describing in a relatively easy way the low energy collisions of quantum droplets in situations where coherent flow of atoms between the droplets takes place.

Long-lived nonlinear oscillatory states in interacting relativistic Bose-Einstein condensates

In this paper we study a mean field model for the dynamics of an interacting Bose-Einstein condensate in two dimensional pseudo-relativistic materials. This model is relatively simple, but contains stable solutions called oscillons which are absent in non-relativistic condensates. We report on a variety of scenarios including interactions between pairs of oscillons and oscillons propagating across an inhomogeneous material boundary. Hitherto relativistic oscillons have been studied only in high energy physics and cosmology and their relevance has not been highlighted so far in condensed matter physics.

Nonlinear Rosen-Zener-Stückelberg interferometry of a Bose-Einstein condensate

We adopt a nonlinear two-mode model to investigate the Rosen-Zener-St\uckelberg (RZS) interferometry of an interacting Bose-Einstein condensate trapped in double-well potential with a periodically modulating barrier. For symmetric potential, we analyze the properties of interference patterns and derive the interference conditions. For asymmetric potential, we explore the resonance structures caused by destructive interference at the weakly coupling limit and obtain the resonance conditions. In particular, we find that the chaotic motions in the asymmetric case with low-frequency modulation, can destroy the formation of interference fringes. In the symmetric case with high-frequency modulation, we see Fano-line-like resonance structures and find that the reso- nance positions exactly correspond to the boundary of macroscopic quantum self-trapping. We show that the nonlinearity can significantly alter both the shape and position of interference fringes and resonance structures. We discuss the experimental feasibility by comparing the solution of the Gross-Pitaevskii equation with both the solution of the two-mode model and the truncated Wigner simulation as well. The results suggest important applications of nonlinear RZS interferometry both in precisely controlling the dynamics and in accurately measuring the parameters of interacting many-body systems.

Shortcuts to adiabaticity for an interacting Bose-Einstein condensate via exact solutions of the generalized Ermakov equation.

Shortcuts to adiabatic expansion of the effectively one-dimensional Bose-Einstein condensate (BEC) loaded in the harmonic-oscillator (HO) trap are investigated by combining techniques of variational approximation and inverse engineering. Piecewise-constant (discontinuous) intermediate trap frequencies, similar to the known bang-bang forms in the optimal-control theory, are derived from an exact solution of a generalized Ermakov equation. Control schemes considered in the paper include imaginary trap frequencies at short time scales, i.e., the HO potential replaced by the quadratic repulsive one. Taking into regard the BEC's intrinsic nonlinearity, results are reported for the minimal transfer time, excitation energy (which measures deviation from the effective adiabaticity), and stability for the shortcut-to-adiabaticity protocols. These results are not only useful for the realization of fast frictionless cooling, but also help us to address fundamental problems of the quantum speed limit and thermodynamics.

Casimir and Surface Tension Forces on a Single Interacting Bose–Einstein Condensate in Canonical Ensemble

The forces on a single Bose–Einstein condensate confined between two parallel plates consist of two components, namely, surface tension force and Casimir force. In canonical ensemble, these forces are quite different from the one in grand canonical ensemble. In small region with distance $$\ell $$ between two parallel plates, using double parabola approximation, we find that surface tension force decreases as $${{\ell }^{{ - 3}}}$$, whereas the Casimir force, in one-loop approximation of the quantum field, is proportional to $${{\ell }^{{ - 13/2}}}$$. The total force is also considered and its veer is found.

Nonclassical properties and entanglement generation in an optical cavity containing two atomic Bose–Einstein condensates

We consider the model of a weakly driven optical cavity containing two clouds of atomic Bose–Einstein condensates (BECs). Nonclassical photon correlations and correlations between the two atomic BECs are investigated under different cavity conditions including strong atom-field coupling and bad cavity regime. We show that the nonlinear interatom collisional interactions in BEC leads to a significant loss of cavity light coherence. Various types of nonclassical properties are investigated such as sub-Poissonian statistics, antibunching and entanglement. We show that the entanglement can be generated between BECs and the cavity field. The time evolution of entanglement is also numerically studied.