Superfluid and dissipative dynamics of a Bose-Einstein condensate in a periodic optical potential

We create Bose-Einstein condensates of 87Rb in a static magnetic trap with a superimposed blue-detuned 1D optical lattice. By displacing the magnetic trap center we are able to control the condensate evolution. We observe a change in the frequency of the center-of-mass oscillation in the harmonic trapping potential, in analogy with an increase in effective mass. For fluid velocities greater than a local speed of sound, we observe the onset of dissipative processes up to full removal of the superfluid component. A parallel simulation study visualizes the dynamics of the Bose-Einstein condensate and accounts for the main features of the observed behavior.     

Cyclotron dynamics of a Bose–Einstein condensate in a quadruple-well potential with synthetic gauge fields

We investigate the cyclotron dynamics of Bose–Einstein condensate (BEC) in a quadruple-well potential with synthetic gauge fields. We use laser-assisted tunneling to generate large tunable effective magnetic fields for BEC. The mean position of BEC follows an orbit that simulated the cyclotron orbits of charged particles in a magnetic field. In the absence of atomic interaction, atom dynamics may exhibit periodic or quasi-periodic cyclotron orbits. In the presence of atomic interaction, the system may exhibit self-trapping, which depends on synthetic gauge fields and atomic interaction strength. In particular, the competition between synthetic gauge fields and atomic interaction leads to the generation of several discontinuous parameter windows for the transition to self-trapping, which is obviously different from that without synthetic gauge fields.     

有效质量法调控原子玻色-爱因斯坦凝聚体的双阱动力学

操控原子玻色-爱因斯坦凝聚体在双势阱中的动力学通常是通过改变势阱深度来实现,本文提出了一种基于调节原子有效质量的控制方案,可以在不改变双阱势的前提下操控凝聚体的双阱动力学.利用双模近似,本文解析地导出了超冷原子在双阱势中的隧穿强度和相互作用强度对有效质量的依赖关系,并基于平均场近似数值模拟了在有效质量调节下的凝聚体动力学演化,展示了隧穿振荡和自束缚等典型的双阱动力学行为.此外,本文的研究还发现,借助负有效质量效应,这一方案甚至可以等效地实现对负散射长度原子凝聚体双阱动力学行为的操控.     

Dynamical superfluid-insulator transition in a chain of weakly coupled bose-Einstein condensates

We predict a dynamical classical superfluid-insulator transition in a Bose-Einstein condensate trapped in an optical and a magnetic potential. In the tight-binding limit, this system realizes an array of weakly coupled condensates driven by an external harmonic field. For small displacements of the parabolic trap about the equilibrium position, the condensates coherently oscillate in the array. For large displacements, the condensates remain localized on the side of the harmonic trap with a randomization of the relative phases. The superfluid-insulator transition is due to a discrete modulational instability, occurring when the condensate center of mass velocity is larger than a critical value.     

Effects of periodic optical lattice potential on dark solitons in a Bose Einstein condensate

This paper investigates the dynamics of dark solitons in a Bose--Einstein condensate with a magnetic trap and an optical lattice (OL) trap, and analyses the effects of the periodic OL potential on the dynamics by applying the variational approach based on the renormalized integrals of motion. The results show that the dark soliton becomes only a standing-wave and free propagation of the dark soliton is not possible when the periodic length of the OL potential is approximately equal to the effective width of the dark soliton. When the periodic length is very small or very large, the effects of the OL potential on the dark soliton will be sharply reduced. Finally, the numerical results confirm these theoretical findings.     

基于磁悬浮大体积交叉光学偶极阱的Dimple光阱装载研究

三维拉曼边带冷却后的铯原子样品装载于一个磁悬浮的大体积交叉光学偶极阱中, 继续加载一个小体积的光学偶极阱后, 实现了Dimple光学偶极阱对铯原子的高效装载. 对不同磁场下磁悬浮大体积光阱的有效装载势能进行理论分析与实验测量, 得出最优化的梯度磁场和均匀偏置磁场, 获得了基于磁悬浮大体积光阱的Dimple光学偶极阱的装载势能曲线, 实现了Dimple光学偶极阱对经拉曼边带冷却后俘获在磁悬浮的大体积光阱中的铯原子样品的有效装载. 比较了Dimple光学偶极阱分别从拉曼边带冷却、大体积的交叉光阱和消除反俘获势后的磁悬浮大体积光阱装载的结果, 将俘获在磁悬浮大体积光阱中的铯原子样品装载到Dimple光学偶极阱, 铯原子样品的密度提高了约15倍.     

Experimental realization of strong effective magnetic fields in optical superlattice potentials

We present the experimental generation of large effective magnetic fields for ultracold atoms using photon-assisted tunneling in an optical superlattice. The underlying method does not rely on the internal structure of the atoms and, therefore, constitutes a general approach to realize widely tunable artificial gauge fields without the drawbacks of near-resonant optical potentials. When hopping in the lattice, the accumulated phase shift by an atom is equivalent to the Aharonov–Bohm phase of a charged particle exposed to a staggered magnetic field of large magnitude, on the order of one flux quantum per plaquette. We study the ground state of this system and observe that the frustration induced by the magnetic field can lead to a degenerate ground state for non-interacting particles. We provide a local measurement of the phase acquired by single particles due to photon-assisted tunneling. Furthermore, the quantum cyclotron orbit of single atoms in the lattice exposed to the effective magnetic field is directly revealed.     

一维光晶格中玻色凝聚气体的干涉

对捕限在三维轴对称谐振势阱叠加一维光晶格的组合势中的玻色凝聚气体，文章基于平均场G-P方程并运用传播子方法，求解玻色凝聚气体基态波函数及其随时间的演化，给出了物质波干涉图样的空间分布与光晶格势周期结构之间的关系.研究表明，运用这一方法得到的光晶格势中物质波干涉条纹与光学中的多光束干涉相类似，并且与Andrews 和Peil等人的实验结果一致.物质波干涉图样随时间的演化也与Sadhan K. Adhikari通过直接数值求解G-P方程所得结果基本相同. 关键词： 玻色凝聚气体 子凝聚原子云 光晶格势 干涉     

Evolution of Matter Wave Interference of Bose-Condensed Gas in a 2D Optical Lattice

We investigate the average particle-number distribution of the atoms in the combined potential of 2D optical lattices and 3D harmonic magnetic trap based on the Gross-Pitaevskii equation. After the combined potential is switched off, and only the optical lattice is switched off, we give the analytical results of the wavefunction of the Bosecondensed gas at any time t by using a propagator method. For both disk-shaped and cigar-shaped Bose-condensed gas,we discuss the evolution process of the central and side peaks of the interference pattern.     

Evolution of Matter Wave Interference of Bose-Condensed Gas in a 2D Optical Lattice

We investigate the average particle-number distribution of the atoms in the combined potential of 2D optical lattices and 31) harmonic magnetic trap based on the Gross-Pitaevskii equation. After the combined potential is switched of[, and only the optical lattice is switched off, we give the analytical results of the wavefunction of the Bosecondensed gas at any time t by using a propagator method. For both disk-shaped and cigar-shaped Bose-condensed gas,we discuss the evolution process of the central and side peaks of the interference pattern.     

Dynamics and Manipulation for Gap Solitons in Bose-Einstein Condensatesunder Optical Lattice and Harmonic Potentials

By means of an extended variational approach, we study dynamics for gap solitons in a repulsive interaction Bose-Einstein condensate under both a harmonic and an optical lattice confinement. The simplified analytic theory gives the critical strength ratio of harmonic to optical lattice necessary to support multiple stable lattice sites for the condensate. Moreover, we use numerical experiments to guide and manipulate the gap solitons to an arbitrary position via a time-dependent potential. All predictions of the extended variational approach are reasonably close to results of the simulations. In particular,
the variational model helps capture the composition relationship between the variations of chirp and amplitude.     

Effect of the weak interaction on the in situ radii of condensate boson atoms in one or two-dimensional deep optical lattices

In this letter we suggest a theoretical model to investigate the weak interaction dependence of the in situ radii of condensate boson atoms in a combined harmonic with one or two-dimensional deep optical lattice. The semiclassical approximation is employed to calculate the above mentioned parameters. The calculated results showed that the in situ radii depends crucially on the interatomic interaction. Our results can be extended to investigate the moment of inertia of condensate boson atoms in combined harmonic-optical potential as well as superfluidity nature of synthetically charged boson atoms in combined potential.     

Creation of solitons and vortices by bragg reflection of Bose-Einstein condensates in an optical lattice

We study the dynamics of Bose-Einstein condensates in an optical lattice and harmonic trap. The condensates are set in motion by displacing the trap and initially follow simple semiclassical paths, shaped by the lowest energy band. Above a critical displacement, the condensate undergoes Bragg reflection. For high atom densities, the first Bragg reflection generates a train of solitons and vortices, which destabilize the condensate and trigger explosive expansion. At lower densities, soliton and vortex formation requires multiple Bragg reflections, and damps the center-of-mass motion.     

简谐势阱中具有吸引相互作用原子体系的玻色-爱因斯坦凝聚

给出了简谐势阱中具有吸引相互作用原子体系的非线性定态薛定谔方程的基态解,得到了凝 聚原子数随能量本征值变化的双稳态曲线.并由此得到与实验报道相符的吸引型玻色-爱因斯 坦凝聚体所能包含的最大原子数. 关键词： 玻色-爱因斯坦凝聚 双稳态     

Simulating cyclotron-Bloch dynamics of a charged particle in a 2D lattice by means of cold atoms in driven quasi-1D optical lattices

Quantum dynamics of a charged particle in a two-dimensional (2D) lattice subject to magnetic and electric fields is a rather complicated interplay between cyclotron oscillations (the case of vanishing electric field) and Bloch oscillations (zero magnetic field), details of which has not yet been completely understood. In the present work we suggest to study this problem by using cold atoms in optical lattices. We introduce a one-dimensional (1D) model which can be easily realized in laboratory experiments with quasi-1D optical lattices and show that this model captures many features of the cyclotron-Bloch dynamics of the quantum particle in 2D square lattices.     

Collective excitations of a trapped Bose-Einstein condensate in the presence of a 1D optical lattice

We study low-lying collective modes of an elongated 87Rb condensate produced in a 3D magnetic harmonic trap with the addition of a 1D periodic potential which is provided by a laser standing wave along the axial direction. While the transverse breathing mode remains unperturbed, quadrupole and dipole oscillations along the optical lattice are strongly modified. Precise measurements of the collective mode frequencies at different heights of the optical barriers provide a stringent test of the theoretical model recently introduced [M. Kr?mer, Phys. Rev. Lett. 88, 180404 (2002)]].     

量子遂穿在双阱异核分子形成过程中的作用(英文）

对于束缚在双阱中的两组分玻色－爱因斯坦凝聚体,本文研究了基于光缔合机制形成分子的过程中,量子隧穿所起的作用．该双阱外势可以在实验上利用磁场生成的谐振子势和由蓝失谐光或射频源生成的中心势垒来实现．在合适的光场中,我们分析了系统在不同初始状态下的短时极限量子动力学,同时对系统做了在平均场近似下的数值计算分析．我们发现无论是隧穿强度还是初始状态的制备都对系统的动力学有很大的影响．我们还计算了同核分子在这些情况下的结果,并和异核分子计算的结果作了比较．量子化的光缔合光场在分子形成过程的影响也在文章中给予了讨论．     

A wire trap for neutral atoms

We present new ways of trapping a neutral atom with static electric and magnetic fields. We discuss the interaction of a neutral atom with the magnetic field of a current carrying wire and the electric field of a charged wire. Atoms can be trapped by the 1/r magnetic field of a current-carrying wire in a two-dimensional trap. The atoms move in Kepler-like orbits around the wire and angular momentum prevents them from being absorbed at the wire. Trapping was demonstrated in an experiment by guiding atoms along a 1 m long current-carrying wire. Stable traps using the interaction of a polarizable atom with the electric field of a charged wire alone are not possible because of the 1/r ² form of the interaction potential. Nevertheless, we show that one can build a microscopic trap with a combination of a magnetic field generated by a current in a straight wire and the static electric field generated by a concentric charged ring which provides the longitudinal confinement. In all of these traps, the neutral atoms are trapped in a region of maximal field, in theirhigh-field seeking state.Dedicated to H. Walther on the occasion of his 60th birthday     

Dynamical stability of dipolar condensate in a parametrically modulated one-dimensional optical lattice

We study the stabilization properties of dipolar Bose–Einstein condensate in a deep one-dimensional optical lattice with an additional external parametrically modulated harmonic trap potential. Through both analytical and numerical methods, we solve a dimensionless nonlocal nonlinear discrete Gross–Pitaevskii equation with both the short-range contact interaction and the long-range dipole–dipole interaction. It is shown that, the stability of dipolar condensate in modulated deep optical lattice can be controled by coupled effects of the contact interaction, the dipolar interaction and the external modulation. The system can be stabilized when the dipolar interaction, the contact interaction, the average strength of potential and the ratio of amplitude to frequency of the modulation satisfy a critical condition. In addition, the breather state, the diffused state and the attractive-interaction-induced-trapped state are predicted. The dipolar interaction and the external modulation of the lattice play important roles in stabilizing the condensate.     

Mean-field model for Josephson oscillation in a Bose-Einstein condensate on an one-dimensional optical trap

Using the axially-symmetric time-dependent Gross-Pitaevskii equation we study the phase coherence in a repulsive Bose-Einstein condensate (BEC) trapped by a harmonic and an one-dimensional optical lattice potential to describe the experiment by Cataliotti et al. on atomic Josephson oscillation [Science 293, 843 (2001)]. The phase coherence is maintained after the BEC is set into oscillation by a small displacement of the magnetic trap along the optical lattice. The phase coherence in the presence of oscillating neutral current across an array of Josephson junctions manifests in an interference pattern formed upon free expansion of the BEC. The numerical response of the system to a large displacement of the magnetic trap is a classical transition from a coherent superfluid to an insulator regime and a subsequent destruction of the interference pattern in agreement with the more recent experiment by Cataliotti et al. [New J. Phys. 5, 71 (2003)].Received: 20 March 2003, Published online: 30 July 2003PACS: 03.75.-b Matter waves - 03.75.Lm Tunneling, Josephson effect, Bose-Einstein condensates in periodic potentials, solitons, vortices and topological excitations - 03.75.Kk Dynamic properties of condensates; collective and hydrodynamic excitations, superfluid flow