Excitation spectrum and structure factor of a two-component Bose-Einstein condensate in different hyperfine states

The elementary excitation spectrum of a two-component Bose-Einstein condensate in different hyperfine states is obtained by Green＇s function method. It is found to have two branches. In the long wave-length limit, the two branches of the excitation spectrum are reduced to one phonon excitation and one single-particle excitation. The single-particle one has an energy gap. When the energy gap exists, we study the Landau critical velocity and the depletion of the condensate. With the obtained Green＇s functions, we calculate the structure factor of a two-component condensate. It is found that the static structure factor comprises only the branch of the phonon excitation and the single-particle excitation makes no contribution to the structure factor.     

Excitation spectroscopy of vortex lattices in a rotating Bose-Einstein condensate

Excitation spectroscopy of vortex lattices in rotating Bose-Einstein condensates is described. We numerically obtain the Bogoliubov-de Gennes quasiparticle excitations for a broad range of energies and analyze them in the context of the complex dynamics of the system. Our work is carried out in a regime in which standard hydrodynamic assumptions do not hold, and includes features not readily contained within existing treatments.     

Quantum Light Excitation of Atoms in Ideal Bose-Einstein condensate

The spectrum shape of atomic radiation in the case of ideal Bose-Einstein condensate (BEC)excited by quantized light is derived through the Wigner-Weisskopf approximation. Besides the split and gap in the excitation spectrum, a doublet structure appears in the emission spectrum with different widths for the non-resonant case and the same widths for the resonant case. The survival probability is found to decrease exponentially for the non-resonan t case and with oscillation for the resonant case. A suppressed radiation also appears in the nonresonant case. All these results can be used to detect the existence of BE condensation.     

Anomalous frequency shift in the photoassociation spectrum of a Bose-Einstein condensate

We theoretically investigate the effect of anomalous quantum correlations on the light-induced frequency shift in the photoassociation spectrum of a Bose-Einstein condensate. Anomalous quantum correlations arise because, although formed from a pair of zero-momentum condensate atoms, a condensate molecule need not dissociate back to the atomic condensate, but may just as well form a noncondensate atom pair with equal and opposite momentum, i.e., due to rogue photodissociation. The uncorrelated frequency shift of the photoassociation spectrum is to the red and linearly dependent on the laser intensity I. In contrast, anomalous correlations due to rogue dissociation lead to a blueshifted photoassociation spectrum. For sufficiently low light intensities, the rogue blueshift is dominant and proportional to sqrt[I].     

Chaos in a Bose-Einstein condensate

It is demonstrated that Smale-horseshoe chaos exists in the time evolution of the one-dimensional Bose-Einstein condensate driven by time-periodic harmonic or inverted-harmonic potential.A formally exact solution of the time-dependent Gross-Pitaevskii equation is constructed,which describes the matter shock waves with chaotic or periodic amplitudes and phases.     

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.     

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.     

Fast rotation of a Bose-Einstein condensate

We study the rotation of a 87Rb Bose-Einstein condensate confined in a quadratic plus quartic potential. This trap configuration allows one to increase the rotation frequency of the gas above the trap frequency. In such a fast rotation regime we observe a dramatic change in the appearance of the quantum gas. The vortices which were easily detectable for a slower rotation become much less visible, and their surface density is well below the value expected for this rotation frequency domain. We discuss some possible tracks to account for this effect.     

Vortex-antivortex pair in a Bose-Einstein condensate

Presented is a type-II quantum algorithm for superfluid dynamics, used to numerically predict solutions of the GP equation for a complex scalar field (spinless bosons) in φ⁴ theory. The GP equation is a long wavelength effective field theory of a microscopic quantum lattice gas with nonlinear state reduction. The quantum lattice gas algorithm for modeling the dynamics of the one-body BEC state in 3+1 dimensions is presented. To demonstrate the method's strength as a computational physics tool, a difficult situation of filamentary singularities is simulated, the dynamics of solitary vortex-antivortex pairs, which are a basic building block of morphologies of quantum turbulence.     

Knots in a spinor Bose-Einstein condensate

We show that knots of spin textures can be created in the polar phase of a spin-1 Bose-Einstein condensate, and discuss experimental schemes for their generation and probe, together with their lifetime.     

Bose-Einstein condensate in a random potential

An optical speckle potential is used to investigate the static and dynamic properties of a Bose-Einstein condensate in the presence of disorder. With small levels of disorder, stripes are observed in the expanded density profile and strong damping of dipole and quadrupole oscillations is seen. Uncorrelated frequency shifts of the two modes are measured and are explained using a sum-rules approach and by the numerical solution of the Gross-Pitaevskii equation.     

Overcritical rotation of a trapped Bose-Einstein condensate

The rotational motion of an interacting Bose-Einstein condensate confined by a harmonic trap is investigated by solving the hydrodynamic equations of superfluids, with the irrotationality constraint for the velocity field. We point out the occurrence of an overcritical branch where the system can rotate with angular velocity larger than the oscillator frequencies. We show that in the case of isotropic trapping the system exhibits a bifurcation from an axisymmetric to a triaxial configuration, as a consequence of the interatomic forces. The dynamical stability of the rotational motion with respect to the dipole and quadrupole oscillations is explicitly discussed.     

Dynamical properties of a rotating Bose-Einstein condensate

Within a variational approach to solving the Gross-Pitaevskii equation we investigate dynamical properties of a rotating Bose-Einstein condensate confined in an anharmonic trap. In particular, we calculate the eigenfrequencies of low-energy excitations out of the equilibrium state and the aspect ratio of the condensate widths during the free expansion.     

Photoassociation of sodium in a Bose-Einstein condensate

We form ultracold Na2 molecules by single-photon photoassociation of a Bose-Einstein condensate, measuring the photoassociation rate, linewidth, and light shift of the J = 1, v = 135 vibrational level of the A1 Sigma (+)(u) molecular state. The photoassociation rate constant increases linearly with intensity, even where it is predicted that many-body effects might limit the rate. Our observations are in good agreement with a two-body theory having no free parameters.     

周期驱动玻色-爱因斯坦凝聚系统的棘齿效应

研究了周期脉冲驱动下的玻色-爱因斯坦凝聚体系（BEC）的动力学演化.其中着重考虑了BEC原子间的非线性相互作用对量子棘齿效应的影响.数值计算结果表明,较弱的非线性相互作用可以减弱定向动量流的强度.而较强的非线性相互作用则会使量子棘齿效应消失甚至发生反转,即系统会出现反向的定向动量流,而且随着时间的演化,动量流会表现出微弱的饱和趋势.计算还发现,高阶量子共振下系统的棘齿效应变得很不明显,而且外部驱动势的周期噪声很容易破坏体系的棘齿效应. 关键词： 玻色-爱因斯坦凝聚 量子混沌 量子共振 棘齿效应     

Atom-molecule dark states in a Bose-Einstein condensate

We have created a dark quantum superposition state of a Rb Bose-Einstein condensate and a degenerate gas of Rb2 ground-state molecules in a specific rovibrational state using two-color photo-association. As a signature for the decoupling of this coherent atom-molecule gas from the light field, we observe a striking suppression of photo-association loss. In our experiment the maximal molecule population in the dark state is limited to about 100 Rb2 molecules due to laser induced decay. The experimental findings can be well described by a simple three mode model.     

Texture control in a pseudospin Bose-Einstein condensate

We describe a wave function engineering approach to the formation of textures in nonrotated multicomponent Bose-Einstein condensates. With numerical simulations of a viable two-component condensate experiment, we demonstrate the formation of a ballistically?expanding regular lattice?texture, composed of half-quantum vortices and spin-2 textures. The formation is described by a linear interference process in which the geometry and phase of three initially separated wave packets provide deterministic control over the resulting lattice?texture.     

偶极玻色-爱因斯坦凝聚体中的各向异性耗散

针对偶极相互作用的玻色-爱因斯坦凝聚体,解析计算了点状杂质沿平行极化轴和垂直极化轴运动的能量耗散率,证明了在超流临界速度更大的方向上耗散率也更高.该结论为最近在162Dy原子气体中观测到的实验现象提供了理论支持.对于一般的运动方向,给出了耗散率在高速极限下以及临界速度附近的渐近形式.结合数值计算的结果,论证了耗散率随方向角的变化总是表现出与临界速度一致的各向异性.     

Dark solitons in a two-component Bose-Einstein condensate

The creation and interaction of dark solitons in a two-component Bose-Einstein condensate is investigated. For a miscible case, the interaction of dark solitons in different components is studied. Various possible scenarios are presented, including the formation of a soliton-soliton bound pair. We also analyze the soliton propagation in the presence of domains, and show that a dark soliton can be transferred from one component to the other at the domain wall when it exceeds a critical velocity. For lower velocities multiple reflections within the domain are observed, where the soliton is evaporated and accelerated after each reflection until it finally escapes from the domain.     

Tunable miscibility in a dual-species Bose-Einstein condensate

We report on the observation of controllable phase separation in a dual-species Bose-Einstein condensate with 85Rb and 87Rb. Interatomic interactions between the different components determine the miscibility of the two quantum fluids. In our experiments, we can clearly observe immiscible behavior via a dramatic spatial separation of the two species. Furthermore, a magnetic-field Feshbach resonance is used to change them between miscible and immiscible by tuning the 85Rb scattering length. The spatial density pattern of the immiscible quantum fluids exhibits complex alternating-domain structures that are uncharacteristic of its stationary ground state.