Stable Bose-Einstein Condensate Research Articles

Domain-wall dynamics in Bose-Einstein condensates with synthetic gauge fields.

Interactions in many-body physical systems, from condensed matter to high-energy physics, lead to the emergence of exotic particles. Examples are mesons in quantum chromodynamics and composite fermions in fractional quantum Hall systems, which arise from the dynamical coupling between matter and gauge fields1,2. The challenge of understanding the complexity of matter-gauge interaction can be aided by quantumsimulations, for which ultracold atoms offer a versatile platform via the creation of artificial gauge fields. An important step towards simulating the physics of exotic emergent particles is the synthesis of artificial gauge fields whose state depends dynamically on the presence of matter. Here we demonstrate deterministic formation of domain walls in a stable Bose-Einstein condensate with a gauge field that is determined by the atomic density. The density-dependent gauge field is created by simultaneous modulations of an optical lattice potential and interatomic interactions, and results in domains of atoms condensed into two different momenta. Modelling the domain walls as elementary excitations, we find that the domain walls respond to synthetic electric field with a charge-to-mass ratio larger than and opposite to that of the bare atoms. Our work offers promising prospects to simulate the dynamics and interactions of previously undescribed excitations in quantum systems with dynamical gauge fields.

Few Excited State Energies and Chemical Potential Energy of 87Rb Condensate by Many-Body Approach

We consider experimental 87Rb condensate which is trapped by harmonic plus quartic trap [V(r) = ½mω2r2 + λr4]. Keeping similarity with experiments, the anharmonic parameter (λ) is considered as a controllable parameter. The excited state energies of stable Bose-Einstein condensate are strongly influenced by the presence of an anharmonic term, even when the interatomic interaction is repulsive. The necessary dependencies of excited state energies of the trapped condensate on λ are discussed in detail. In addition, the variation of chemical potential energy as a function of λ is also investigated to explore the role of interaction. The many-particle Schrödinger equation is solved by the potential harmonic expansion method, where all possible two-body correlations are considered by utilizing the correlated two-body basis function. Specifically, we present a clear physical explanation of excited state energies and chemical potential energy of the experimental repulsive condensate confined by the anharmonic trap.

A new possibility for light-quark dark matter

Despite many decades of study the physical origin of ‘dark matter’ (DM) in the Universe remains elusive. In this letter we calculate the properties of a completely new DM candidate—Bose–Einstein condensates formed from a recently discovered bosonic particle in the light-quark sector, the hexaquark. In this first study, we show stable Bose–Einstein condensates could form in the primordial early universe, with a production rate sufficiently large that they are a plausible new candidate for DM. Some possible astronomical signatures of such DM are also presented.

Observation of fermion-mediated interactions between bosonic atoms.

In high-energy and condensed-matter physics, particle exchange has an essential role in the understanding of long-range interactions and correlations. For example, the exchange of massive bosons leads to the Yukawa potential1,2, and phonon exchange between electrons gives rise to Cooper pairing in superconductors3. Here we show that, when a Bose-Einstein condensate of caesium atoms is embedded in a degenerate Fermi gas of lithium atoms, interspecies interactions can give rise to an effective trapping potential, damping, and attractive boson-boson interactions mediated by fermions. The latter, which is related to the Ruderman-Kittel-Kasuya-Yosida mechanism4, results from a coherent three-body scattering process. Such mediated interactions are expected to form new magnetic phases5 and supersolids6. We show that under suitable conditions, the mediated interactions can convert a stable Bose-Einstein condensate into a train of 'Bose-Fermi solitons'7,8. The predicted long-range nature of the mediated interactions opens up the possibility of correlating distant atoms and preparing new quantum phases.

Parafermionic behavior of Bose-Einstein condensates

Bright solitons of 7Li atoms in a quasi one-dimensional optical trap, formed in a stable Bose–Einstein condensate in which the interactions have been magnetically tuned from repulsive to attractive, have been seen to exhibit repulsive interactions among them when set in motion by offsetting the optical potential. Solving first the Gross–Pitaevskii equation for the special conditions corresponding to the experiment, we show then that this system can be described in terms of generalized parafermionic oscillators, the order of the parafermions being related to the strength of the interaction among the atoms and being a measure of the bosonic behavior vs. the fermionic behavior of the system.

Bosonic topological excitations from the instability of a quadratic band crossing

We investigate the interaction-driven instability of a quadratic band crossing arising for ultracold bosonic atoms loaded into a two-dimensional optical lattice. We consider the case when the degenerate point becomes a local minimum of both crossing energy bands such that it can support a stable Bose-Einstein condensate. Repulsive contact interaction among the condensed bosons induces a spontaneously time-reversal symmetry broken superfluid phase and a topological gap is opened in the excitation spectrum. We propose two concrete realizations of the desired quadratic band crossing in lattices with either fourfold or sixfold rotational symmetries via suitable tuning of the unit cell leading to reduced Brillouin zones and correspondingly folded bands. In either case, topologically protected edge excitations are found for a finite system.

Magic tilt angle for stabilizing two-dimensional solitons by dipole-dipole interactions

In the framework of the Gross-Pitaevskii equation, we study the formation and stability of effectively two-dimensional solitons in dipolar Bose-Einstein condensates (BECs), with dipole moments polarized at an arbitrary angle $\theta$ relative to the direction normal to the system's plane. Using numerical methods and the variational approximation, we demonstrate that unstable Townes solitons, created by the contact attractive interaction, may be completely stabilized (with an anisotropic shape) by the dipole-dipole interaction (DDI), in interval $\theta ^{\text{cr}}<\theta \leq \pi /2$. The stability boundary, $\theta ^{\text{cr}}$, weakly depends on the relative strength of DDI, remaining close to the "magic angle", $\theta_{m}=\arccos \left( 1/\sqrt{3}\right) $. The results suggest that DDIs provide a generic mechanism for the creation of stable BEC\ solitons in higher dimensions.

Bosons star at finite temperature

By using a simple thermodynamical method, we confirm the finding of Chavanis and Harko that stable Bose-Einstein condensate stars can form. However, by using a thermodynamically consistent boson equation of state, we obtain a less massive Bose-Einstein condensate star compared to the one predicted by Chavanis and Harko. We also obtain that the maximum mass of a boson star is insensitive to the change of matter temperature. However, the mass of a boson star with relatively large radius depends significantly on the temperature of the boson matter.

Generation of ultracold paraexcitons in cuprous oxide: A path toward a stable Bose-Einstein condensate

Cooling of a gas of trapped paraexcitons in Cu${}_{2}$O to 100 mK, close to the lattice temperature, is reported. We show that the activation of a transverse acoustic scattering mechanism by the applied stress is responsible for the production of ultracold excitons. This low temperature is indispensable to achieving a stable and pure Bose-Einstein condensate of three-dimensional excitons in a strongly inelastic environment.

Vortical Solitons of Three-Dimensional Bose—Einstein Condensates under Both a Bichromatic Optical Lattice and Anharmonic Potentials

We study Bose—Einstein condensate vortical solitons under both a bichromatic optical lattice and anharmonic potential. The vortical solitons are built in the form of a layer-chain structure made up of two fundamental vortices along the bichromatic optical lattice direction, which have not been reported before in the three-dimensional Bose—Einstein condensate. A variation approach is applied to find the optimum initial solutions of vortical solitons. The stabilities of the vortical solitons are confirmed by the numerical simulation of the time-dependent Gross—Pitaevskii equation. In particular, stable Bose—Einstein condensate vortical solitons with fundamental vortices of different atomic numbers in the external potential within a range of experimentally achievable timescales are found. We further manipulate the vortical solitons to an arbitrary position by steadily moving the bichromatic optical lattice, and find a stable region for the successful manipulation of vortical solitons without collapse. These results provide insight into controlling and manipulating the Bose—Einstein condensate vortical solitons for macroscopic quantum applications.

Effects of three-body scattering processes on BCS-BEC crossover

We investigate the BCS-BEC crossover taking into account an additional three-body interaction, which is essentially the scattering between the Cooper pairs and the newly formed bosons. We show that if the two-body interaction is attractive, the presence of this additional three-body term makes the crossover process a nonreversible one. Starting from a stable Bose-Einstein condensate (BEC) state, crossover to BCS can be achieved; but if the BCS state is the starting point, instead of a stable BEC region, what the system crosses over to is a metastable condensed state.

R 85 b tunable-interaction Bose–Einstein condensate machine

We describe our experimental setup for creating stable Bose–Einstein condensates (BECs) of R85b with tunable interparticle interactions. We use sympathetic cooling with R87b in two stages, initially in a tight Ioffe–Pritchard magnetic trap and subsequently in a weak, large-volume, crossed optical dipole trap, using the 155 G Feshbach resonance to manipulate the elastic and inelastic scattering properties of the R85b atoms. Typical R85b condensates contain 4×104 atoms with a scattering length of a=+200a0. Many aspects of the design presented here could be adapted to other dual-species BEC machines, including those involving degenerate Fermi–Bose mixtures. Our minimalist apparatus is well suited to experiments on dual-species and spinor Rb condensates, and has several simplifications over the R85b BEC machine at JILA, which we discuss at the end of this article.

Design of Infrared Laser Pulses for the Vibrational De-excitation of Translationally Cold Li2 Molecules

In connection with attempts to form molecular Bose-Einstein condensates, there have been reports in the literature of the preparation of samples of translationally cold alkali metal dimers. The molecules in these samples are generally in excited vibrational levels. To form a stable Bose-Einstein condensate, the molecules must be de-excited to their lowest vibrational state. In this paper, we demonstrate that through the use of optimal control theory, it is possible to design a sequence of infrared laser pulses that will de-excite a sample of (7)Li2 molecules from the v = 10 vibrational level of their (1)Sigma(g)+ ground electronic state to their lowest v = 0 vibrational level with an overall efficiency of 91.1%.

Ground-state properties of a Bose-Einstein condensate tuned by a far-off-resonant optical field

In this paper, we explore a scheme to tackle a challenging problem of stable Bose-Einstein condensates (BECs) with attractive atom interactions. In this scheme, the $s$-wave scattering length is tuned in space, rather than in time as previously studied, by a far-off-resonant Gaussian optical field, from negative to positive in the center region of the potential well. We find that this tuning leads to coexisting repulsive and attractive interactions within a single atomic gas and consequently a stable BEC in the repulsive region. We investigate the ground-state properties of the tuned BECs and show them to exhibit a strikingly different spatial density distribution from a conventional one with a positive $s$-wave scattering length. The tuned BEC is formed only when the condensed number is less than a critical number. We derive a formula for the critical number.

Cold collisions of ground-state calcium atoms in a laser field: A theoretical study

State-of-the-art ab initio techniques have been applied to compute the potential-energy curves for the ground $X{}^{1}{\ensuremath{\Sigma}}_{g}^{+}$ and excited ${}^{1}{\ensuremath{\Pi}}_{g}(4s3d)$ states of the calcium dimer in the Born-Oppenheimer approximation. The weakly bound ground state was calculated by symmetry-adapted perturbation theory, while the strongly bound excited state was computed using a combination of the linear-response theory within the coupled-cluster singles and doubles framework for the core-valence electronic correlation and of the full configuration interaction for the valence-valence correlation. The ground-state potential has been corrected by considering the relativistic terms resulting from the first-order many-electron Breit theory, and the retardation corrections. The magnetic electronic transition dipole moment governing the ${}^{1}{\ensuremath{\Pi}}_{g}{\ensuremath{\leftarrow}}^{1}{\ensuremath{\Sigma}}_{g}^{+}$ transitions has been obtained as the first residue of the polarization propagator computed with the coupled-cluster method restricted to single and double excitations. The computed energies and transition moments have been analytically fitted and used in the dynamical calculations of the rovibrational energy levels, ground-state scattering length, photoassociation intensities at ultralow temperatures, and spontaneous emission coefficients from the ${}^{1}{\ensuremath{\Pi}}_{g}(4s3d)$ to the $X{}^{1}{\ensuremath{\Sigma}}_{g}^{+}$ state. The spectroscopic constants of the theoretical ground-state potential are in a good agreement with the experimental values derived from the Fourier-transform spectra [O. Allard et al., Eur. Phys. J. D (to be published)]. The theoretical s-wave scattering length for the ground state is $a=44\mathrm{bohrs},$ suggesting that it should be possible to obtain a stable Bose-Einstein condensate of calcium atoms. Finally, the computed photoassociation intensities and spontaneous emission coefficients suggest that it should be possible to obtain cold calcium molecules by photoassociation of cold atoms to the first ${}^{1}{\ensuremath{\Pi}}_{g}$ state followed by a spontaneous emission to the ground state.

Formation and propagation of matter-wave soliton trains.

Attraction between the atoms of a Bose-Einstein condensate renders it unstable to collapse, although a condensate with a limited number of atoms can be stabilized by confinement in an atom trap. However, beyond this number the condensate collapses. Condensates constrained to one-dimensional motion with attractive interactions are predicted to form stable solitons, in which the attractive forces exactly compensate for wave-packet dispersion. Here we report the formation of bright solitons of (7)Li atoms in a quasi-one-dimensional optical trap, by magnetically tuning the interactions in a stable Bose-Einstein condensate from repulsive to attractive. The solitons are set in motion by offsetting the optical potential, and are observed to propagate in the potential for many oscillatory cycles without spreading. We observe a soliton train, containing many solitons; repulsive interactions between neighbouring solitons are inferred from their motion.

Positronium-positronium scattering using the stochastic variational method

The stochastic variational method is used in conjunction with stabilization ideas to compute the low-energy s-wave phase shifts and scattering lengths for Ps-Ps scattering. The scattering lengths obtained were 8.44a0 for the singlet-singlet and 3.00a0 for the triplet-triplet states (the spin state refers to the coupling of the two electrons and the coupling of the two positrons in the system). The positive scattering length in the triplet-triplet channel means a stable Bose-Einstein condensate of triplet Ps atoms is physically possible. The large value of the 3Ps3Ps→1Ps1Ps cross section, 7.41πa02 suggests the use of spin-polarized positrons as advanced by Platzmann and Mills [Phys. Rev. B 49 454 (1994)].

Elastic positronium-atom scattering using the stochastic variational method.

The stochastic variational method is used in conjunction with stabilization ideas to compute the low energy phase shifts and scattering lengths for positronium-atom scattering. Results are obtained for the Ps-H, Ps- Li+, Ps-He, and Ps-Ps systems. The Ps-H scattering lengths are probably accurate to better than 5% and are the most accurate so far computed. The results for Ps- Li+ and Ps-Ps scattering represent the first published scattering lengths for these systems. The positive scattering length for completely spin-aligned 3Ps-3Ps scattering, namely 2.95a(0), is particularly significant since it demonstrates the feasibility of forming a stable Bose-Einstein condensate of 3Ps atoms.