Controlled collapse of a Bose-Einstein condensate

The point of instability of a Bose-Einstein condensate (BEC) due to attractive interactions was studied. Stable 85Rb BECs were created and then caused to collapse by slowly changing the atom-atom interaction from repulsive to attractive using a Feshbach resonance. At a critical value, an abrupt transition was observed in which atoms were ejected from the condensate. By measuring the onset of this transition as a function of number and attractive interaction strength, we determined the stability condition to be N(absolute value of a) / a(ho) = 0.459+/-0.012+/-0.054, slightly lower than the predicted value of 0.574.     

玻色-爱因斯坦凝聚态的结构理论研究系列之一:玻色-爱因斯坦凝聚态的复合表示理论

针对近零温度下玻色-爱因斯坦凝聚态的特点,建立了复合原子表示模型.提出了复合原子的概念,研究发现复合原子具有一定的零点半径与结合能,得出了相应的表达式.认为该物态作为一个宏观量子态的说法具有相对性,应该据研究角度而定.同时,给出了二个分离的玻色-爱因斯坦凝聚态之间产生1/R型作用势的几种情况,为同类实验提供了理论依据.     

玻色-爱因斯坦凝聚态的临界温度移动

从平均场理论出发,针对有限温度下的玻色-爱因斯坦凝聚态的特点,从正则系综的自由能函数变化的角度,推演并得出了临界温度移动的表达式。为同类实验提供了理论依据.     

Alternative method for the measurement of the temperature of a Bose-Einstein condensate

Usually the temperature in a Bose-Einstein condensate is experimentally deduced resorting to the comparison between the Maxwell-Boltzmann velocity distribution function and the density profile in momentum space. Though a successful method it is merely an approximation, since it also implies the use of classical statistical mechanics at temperatures close to the condensation temperature where quantum effects play a relevant role and cannot be neglected. The present work puts forward an alternative method in which we use an ultra-intense light pulse and a nonlinear optical material as detectors for differences in times-of-flight, and in this way the temperature is deduced.     

Semiclassical theory of superradiant scattering from a Bose-Einstein condensate

A semiclassical theory of superradiant scattering of light from a Bose-Einstein condensate of a dilute atomic gas is proposed. Choosing atomic states with definite values of the momenta as a basis, we derive a nonlinear Schrödinger equation. Its solutions describe the evolution of the scattered light intensity and of the populations of the coherent atomic states with different recoil momenta.     

On the theory of superradiant Rayleigh Scattering from a Bose-Einstein condensate

A theory of the superradiant Rayleigh scattering by a Bose-Einstein condensate is suggested. The theory is based on a semiclassical approach. Atomic states with definite momenta are used as basis functions. The Maxwell-Bloch equations are derived and solved to describe the intensity of scattered radiation and the evolution of the population of coherent atomic states.     

Structural factor for Bose-Einstein double-component condensate

The structural factors for the single-and double-component Bose-Einstein condensates of neutral atoms in a spherically symmetric harmonic trap are derived. The approach adopted in this paper is applicable in the low excitation energy limit, where the well-known Stringari dispersion relation is valid. Our expression is compared with the quasi-classical one, which is shown to overestimate the structural factor. It is argued that the second component first splits the frequency of collective condensate oscillations, which corresponds to the energy transferred by scattering, into two branches and then multiplies the single-component structural factor by some coefficient that can be greater than unity. The analysis is used to estimate the properties of an experimentally implemented double-component condensate of ⁸⁷Rb atoms.     

Controlling chaos in the Bose-Einstein condensate

The spatial structure of the Bose-Einstein condensate (BEC) is investigated and spatially chaotic distributions of the condensates are revealed. By means of changing the s-wave scattering length with a Feshbach resonance, the chaotic behavior can be well controlled to enter into periodicity. Numerical simulation shows that there are different periodic orbits according to different s-wave scattering lengths only if the Lyapunov exponent of the system is negative.     

Translationally noninvariant path integral for Bose-Einstein condensate

The Bogolyubov [Hartree-Fock-Bogolyubov (HFB)] method performs the one-particle (mean-field) approximation in the theory of Bose-Einstein condensation (BEC). Various generalizations of this method are possible. Apart from a nonlinear theory, taking the correlation effects into consideration, the HFB approximation for translationally noninvariant systems describes an instructive phenomenon. This paper is devoted to the treatment of two cases: superfluid ⁴He in porous media and atomic BEC in traps subjected to the gravitational field. Both these systems show the dependence of a critical BEC temperature T _c on their nonuniform properties in space.     

Rate limit for photoassociation of a Bose-Einstein condensate

We simulate numerically the photodissociation of molecules into noncondensate atom pairs that accompanies photoassociation of an atomic Bose-Einstein condensate into a molecular condensate. Such rogue photodissociation sets a limit on the achievable rate of photoassociation. Given the atom density rho and mass m, the limit is approximately 6(planck)rho(2/3)/m.     

玻色-爱因斯坦凝聚中的混沌反控制

提出一种实现玻色-爱因斯坦凝聚中的混沌反控制方法——周期信号驱动法.数值模拟结果表明,用小的周期信号控制系统,采用恰当的调制相位和强度,只要满足系统的最大Lyapunov指数大于零即可实现不同的混沌轨道重构.调制相位在混沌轨道重构中起了很重要的作用. 关键词： 玻色-爱因斯坦凝聚 混沌反控制 小周期信号 Lyapunov指数     

Strongly dipolar Bose-Einstein condensate of dysprosium

We report the Bose-Einstein condensation (BEC) of the most magnetic element, dysprosium. The Dy BEC is the first for an open f-shell lanthanide (rare-earth) element and is produced via forced evaporation in a crossed optical dipole trap loaded by an unusual, blue-detuned and spin-polarized narrowline magneto-optical trap. Nearly pure condensates of 1.5 × 10(4) (164)Dy atoms form below T = 30 nK. We observe that stable BEC formation depends on the relative angle of a small polarizing magnetic field to the axis of the oblate trap, a property of trapped condensates only expected in the strongly dipolar regime. This regime was heretofore only attainable in Cr BECs via a Feshbach resonance accessed at a high-magnetic field.     

Explosion of a collapsing Bose-Einstein condensate

We show that elastic collisions between atoms in a Bose-Einstein condensate with attractive interactions can lead to an explosion that ejects a large fraction of the collapsing condensate. We study variationally the dynamics of this explosion and find excellent agreement with recent experiments on magnetically trapped 85Rb. We also determine the energy and angular distribution of the ejected atoms during the collapse.     

Excitation spectrum of a Bose-Einstein condensate

We report a measurement of the excitation spectrum omega(k) and the static structure factor S(k) of a Bose-Einstein condensate. The excitation spectrum displays a linear phonon regime, as well as a parabolic single-particle regime. The linear regime provides an upper limit for the superfluid critical velocity, by the Landau criterion. The excitation spectrum agrees well with the Bogoliubov spectrum in the local density approximation, even close to the long-wavelength limit of the region of applicability. Feynman's relation between omega(k) and S(k) is verified, within an overall constant.     

Levinson-like theorem for scattering from a Bose-Einstein condensate

A relation between the number of bound elementary excitations of an atomic Bose-Einstein condensate and the phase shift of elastically scattered atoms is derived. Within the Bogoliubov model of a weakly interacting Bose gas this relation is exact and generalizes Levinson's theorem. Specific features of the Bogoliubov model such as complex energy and continuum bound states are discussed and a numerical example is given.     

两分量玻色-爱因斯坦凝聚系统的自旋压缩

在周期性脉冲撞击的两分量玻色－爱因斯坦凝聚系统中研究了自旋压缩动力学性质,结果表明：原子自旋压缩动力学能够揭示相空间的混沌和规则结构,即当初始相干态处在混沌区域时,自旋压缩在很短时间后消失,而当初始相干态处在规则区域时,自旋压缩则存在很长时间.特别是随着时间的演化,系统的平均自旋方向的分布和摆动也与初态所处的空间结构有着密切的联系.最后,研究了相空间的整体自旋压缩动力学,得到了一种较好的量子-经典对应. 关键词： 玻色-爱因斯坦凝聚 混沌 自旋压缩 平均自旋方向     

Dissipative solitons of the Bose-Einstein condensate of excitons

A mirrorless scheme for forming dissipative solitons has been proposed. It consists of a thin layer of a nonlinear medium excited by coherent laser radiation. The existence of dark dissipative solitons of the Bose-Einstein condensate of excitons in such a semiconductor film has been numerically demonstrated. The sizes of an excition soliton and required laser-radiation level are two orders of magnitude smaller than the respective quantities for the case of optical dissipative solitons.