玻色-费米混合气体的非线性Landau-Zener隧穿

在有相互作用的非线性两模系统中, 通过对相互作用参数的调节, 研究了该系统中玻色-费米混合气体的Landau-Zener隧穿现象. 研究发现, 其中某一组分的自相互作用会影响另一组分粒子的隧穿, 使得隧穿出现临界现象. 关键词： 玻色-费米混合气体 Landau-Zener隧穿 量子绝热隧穿     

双势阱中玻色-费米混合气体的周期调制效应

在周期调制场下, 通过对双势阱中费米子数目及相互作用参数的调节, 研究了该系统中玻色子的自俘获现象. 研究发现, 系统中费米子数目及相互作用参数都会影响玻色子的自俘获现象, 并且随着 相互作用及粒子数目的变化, 玻色子的自俘获发生临界现象. 关键词： 玻色-费米混合气体 周期调制 自俘获     

Spontaneous symmetry breaking of a Bose-Fermi mixture in a two-dimensional double-well potential

We study the spontaneous symmetry breaking of a superfluid Bose-Fermi mixture in a two-dimensional double- well potential. The mixture is described by a set of coupled Gross-Pitaevskii equations. The symmetry breaking phenomenon is demonstrated in the two-dimensional double-well potential in the mixture. The results are summarized in the phase diagrams of the mixture particle numbers, which are divided into symmetric and asymmetric regions by the asymmetry ratios. The dynamical pictures of the spontaneous symmetry breaking induced by a gradual transformation of the single-well potential into a double-well one are also illustrated. The properties of the quantum degenerate mixture are explored using the realistic parameters for a 40 K- 87 Rb system.     

Nonlinear Landau-Zener tunneling under biharmonic driving

We address the response of a two-mode Bose-Einstein condensate to a biharmonic high-frequency driving field that directly couples the two mode. By use of a high-frequency approximation we find that asymmetric biharmonic high-frequency driving yields new effects on Nonlinear Landau-Zener tunneling as compared to a symmetric off-diagonal modulation. In particular, we detect an unclosed loop structure and the disappearance of the energy level below the loop structure.     

Degenerate Bose-Fermi mixture of metastable atoms

We report the observation of simultaneous quantum degeneracy in a dilute gaseous Bose-Fermi mixture of metastable atoms. Sympathetic cooling of helium-3 (fermion) by helium-4 (boson), both in the lowest triplet state, allows us to produce ensembles containing more than 10(6) atoms of each isotope at temperatures below 1 microK, and achieve a fermionic degeneracy parameter of T/TF = 0.45. Because of their high internal energy, the detection of individual metastable atoms with subnanosecond time resolution is possible, permitting the study of bosonic and fermionic quantum gases with unprecedented precision. This may lead to metastable helium becoming the mainstay of quantum atom optics.     

二维周期光子晶格中的非线性Landau-Zener隧穿

在二维周期光子晶格中研究了Kerr型非线性Landau-Zener隧穿. 首先在六方晶系光子晶格中推导出非线性三能级Landau-Zener隧穿模型, 在特殊初值条件下将其转化为非线性二能级模型. 对于三能级隧穿模型, 选择特殊初值, 研究了隧穿率随参数的变化规律. 关键词： 光子晶格 Kerr型非线性 Landau-Zener隧穿     

Atomic Landau-Zener tunneling in Fourier-synthesized optical lattices

We report on an experimental study of quantum transport of atoms in variable periodic optical potentials. The band structure of both ratchet-type asymmetric and symmetric lattice potentials is explored. The variable atom potential is realized by superimposing a conventional standing wave potential of lambda/2 spatial periodicity with a fourth-order multiphoton potential of lambda/4 periodicity. We find that the Landau-Zener tunneling rate between the first and the second excited Bloch band depends critically on the relative phase between the two spatial lattice harmonics.     

Asymmetric Landau-Zener tunneling in a periodic potential

Using a simple model for nonlinear Landau-Zener tunneling between two energy bands of a Bose-Einstein condensate in a periodic potential, we find that the tunneling rates for the two directions of tunneling are not the same. Tunneling from the ground state to the excited state is enhanced by the nonlinearity, whereas in the opposite direction it is suppressed. These findings are confirmed by numerical simulations of the condensate dynamics. Measuring the tunneling rates for a condensate of rubidium atoms in an optical lattice, we have found experimental evidence for this asymmetry.     

Single-particle tunneling in strongly driven double-well potentials

We report on the first direct observation of coherent control of single-particle tunneling in a strongly driven double-well potential. In our setup atoms propagate in a periodic arrangement of double wells allowing the full control of the driving parameters such as frequency, amplitude, and even the space-time symmetry. Our experimental findings are in quantitative agreement with the predictions of the corresponding Floquet theory and are also compared to the predictions of a simple two mode model. Our experiments reveal directly the critical dependence of coherent destruction of tunneling on the generalized parity symmetry.     

Localization or tunneling in asymmetric double-well potentials

An asymmetric double-well potential is considered, assuming that the wells are parabolic around the minima. The WKB wave function of a given energy is constructed inside the barrier between the wells. By matching the WKB function to the exact wave functions of the parabolic wells on both sides of the barrier, for two almost degenerate states, we find a quantization condition for the energy levels which reproduces the known energy splitting formula between the two states. For the other low-lying non-degenerate states, we show that the eigenfunction should be primarily localized in one of the wells with negligible magnitude in the other. Using Dekker’s method (Dekker, 1987), the present analysis generalizes earlier results for weakly biased double-well potentials to systems with arbitrary asymmetry.