Controlled tunneling of cold atoms: from full suppression to strong enhancement

Two recent experiments have demonstrated the phenomenon of dynamical tunneling of cold atoms interacting with standing electromagnetic waves. We show by quantitative calculations that one can achieve a control of the tunneling period over an orders of magnitude range by changing the frequency difference of the waves by about 10% only. In this narrow parameter region, the mechanism of the tunneling oscillations evolves from the two-state to the three-state one. Our calculations demonstrate that the change in the underlying mechanism leads to the dramatic enhancement of the dynamical tunneling. Moreover, a complete suppression of the dynamical tunneling can also be achieved in the cold atom setup.     

Fermion tunneling from a non-static black hole with the internal global monopole

Kerner and Mann’s recent research shows that the Hawking temperature and tunneling rate can be obtained by the fermion tunneling method from the Rindler space-time and a general non-rotating black hole. In this paper, considering the tunneling particles with spin 1/2 and taking into account the particle’s self-gravitation in the dynamical background space-time, we further improve Kerner and Man’s fermion tunneling method to investigate Hawking radiation via tunneling from a non-static black hole with the internal global monopole. The result shows that the tunneling rate of the non-static black hole is related to the integral of the changing horizon besides the change of Bekenstein–Hawking entropy, which is different from the stationary cases. It also essentially implies that the unitary is violated for the reason that the black hole is non-stationary and cannot be treated as an isolated system.     

Intrinsic asymmetries in spin-valves: a tight binding model study

We have calculated the I–V curves, dynamical conductance, and tunneling magnetoresistance (TMR) of 1D magnetic tunneling junction through singleband tight binding model calculations based on the non-equilibrium Green's function approach. The difference in density of state of two ferromagnetic leads and the bias dependence of the propagator cause intrinsic asymmetries in TMR and dynamical conductance at finite bias. Besides, we have displayed that large TMR can be obtained even at high bias for half metallic leads.     

Fluctuations and decoherence in chaos-assisted tunneling

We study quantum dynamical tunneling between two symmetry-related islands of stability in the phase space of a classically chaotic system. The setting for these experiments is the motion of carefully prepared samples of cesium atoms in an amplitude-modulated standing wave of light. We examine the dependence of the tunneling dynamics on the system parameters and indicate how the observed features provide evidence for chaos-assisted (three-state) tunneling. We also observe the influence of a noisy perturbation of the standing-wave intensity, which destroys the tunneling oscillations, and we show that the tunneling is more sensitive to the noise for a smaller value of the effective Planck constant.     

Coherent state approach to the cross-collisional effects in the population dynamics of a two-mode Bose–Einstein condensate

We reanalyze the non-linear population dynamics of a Bose–Einstein condensate (BEC) in a double well trap considering a semiclassical approach based on a time dependent variational principle applied to coherent states associated to SU(2) group. Employing a two-mode local approximation and hard sphere type interaction, we show in the Schwinger’s pseudo-spin language the occurrence of a fixed point bifurcation that originates a separatrix of motion on a sphere. This separatrix corresponds to the borderline between two dynamical regimes of Josephson oscillations and mesoscopic self-trapping. We also consider the effects of interaction between particles in different wells, known as cross-collisions. Such terms are usually neglected for traps sufficiently far apart, but recently it has been shown that they contribute to the effective tunneling constant with a factor growing linearly with the particle number. This effect changes considerably the effective tunneling of the system for sufficiently large number of trapped atoms, in perfect accord with experimental data. Finally, we identify analytically the transition parameter associated to the bifurcation in the generalized phase space of the model with cross-collision terms, and show how the dynamical regime depends on the initial conditions of the system and the collisional parameters values.     

Continuous-time monitoring of Landau-Zener interference in a cooper-pair box

Landau-Zener (LZ) tunneling can occur with a certain probability when crossing energy levels of a quantum two-level system are swept across the minimum energy separation. Here we present experimental evidence of quantum interference effects in solid-state LZ tunneling. We used a Cooper-pair box qubit where the LZ tunneling occurs at the charge degeneracy. By employing a weak nondemolition monitoring, we observe interference between consecutive LZ-tunneling events; we find that the average level occupancies depend on the dynamical phase. The system's unusually strong linear response is explained by interband relaxation. Our interferometer can be used as a high-resolution Mach-Zehnder-type detector for phase and charge.     

Dynamical tunneling in macroscopic systems

We investigate macroscopic dynamical quantum tunneling (MDQT) in the driven Duffing oscillator, characteristic for Josephson junction physics and nanomechanics. Under resonant conditions between stable coexisting states of such systems we calculate the tunneling rate. In macroscopic systems coupled to a heat bath, MDQT can be masked by driving-induced activation. We compare both processes, identify conditions under which tunneling can be detected with present day experimental means and suggest a protocol for its observation.     

A Dynamical Phase Transition of Binary Species BECs Mixtures in a Double Well Potential

We investigate the critical behavior in tunneling dynamics of a binary mixture of Bose-Einstein condensates (BECs) trapped in a symmetric double well potential. By gradually increasing the interspecies interaction, we characterize a continuous dynamical phase transition behavior which involves power law scaling. This dynamical phase transition is a consequence of separatrix crossing as we revealed by poincaré section analysis.     

Effect of frequency dependent electron-electron interaction on resonant tunneling

In electron resonant tunneling through a double barrier structure, we show that dynamical electron-electron interactions in the resonant well can give rise to additional tunneling satellites due to collective electronic excitations. We present a first principle treatment for frequency-dependent electron-electron interactions in the resonant tunneling problem. The result confirms the previously proposed plasmon assisted resonant tunneling mechanism. We also find that the particle-hole excitation has very little effect on resonant tunneling. Our result can be applied to study the effects of various electronic excitations on the resonant tunneling of electrons.     

Hawking Radiation of Charged Particles via Tunneling from a Cylindrically Symmetric Black Hole in Anti-de Sitter Space-Time

Applying Parikh-Wilzcek＇s semi-classical quantum tunneling model, we study the Hawking radiation of charged particles as tunneling from the event horizon of a cylindrically symmetric black hole in anti-de Sitter space-time. The derived result shows that the tunneling rate of charged particles is related to the change of Bekenstein-Hawking entropy and that the radiation spectrum is not strictly pure thermal after taking the black hole background dynamical and self-gravitation interaction into account, but is consistent with the underlying unitary theory.     

Dynamical tunneling in mushroom billiards

We study the fundamental question of dynamical tunneling in generic two-dimensional Hamiltonian systems by considering regular-to-chaotic tunneling rates. Experimentally, we use microwave spectra to investigate a mushroom billiard with adjustable foot height. Numerically, we obtain tunneling rates from high precision eigenvalues using the improved method of particular solutions. Analytically, a prediction is given by extending an approach using a fictitious integrable system to billiards. In contrast to previous approaches for billiards, we find agreement with experimental and numerical data without any free parameter.     

一维双组分玻色-爱因斯坦凝聚体系的量子隧穿特性

利用双模近似方法研究了一维双组分玻色-爱因斯坦凝聚体(Bose-Einstein condensates,BECs)的量子隧穿特性.从描述三维双组分BECs系统的Gross-Pitaevskii方程(GPE)出发,得到了描述一维体系的GP方程.把体系波函数写成原子数和相位指数的乘积,得到描述体系隧穿特性的费曼方程.数值求解费曼方程,研究了原子之间相互作用(双组分BECs体系原子之间的相互作用包括组分内部原子之间的相互作用和不同组分原子之间的相互作用)对隧穿特性的影响.结果显示,当原子之间的相互作用较弱时,体系发生量子隧穿现象,表现为原子数在平衡位置附近作周期振荡;随着原子之间相互作用增强,体系经历一个临界状态,进入自俘获状态,即由于原子之间相互作用的存在,在对称双势阱中演化的BECs可以呈现出原子数高度的不对称分布,好像绝大数原子被其中一个势阱俘获.从隧穿到自俘获原子之间的相互作用存在一个临界值,从而体系的能量也对应一个临界值,根据体系的哈密顿函数,就能求出相互作用临界值的表达式.     

Dynamical control of matter-wave tunneling in periodic potentials

We report on measurements of dynamical suppression of interwell tunneling of a Bose-Einstein condensate (BEC) in a strongly driven optical lattice. The strong driving is a sinusoidal shaking of the lattice corresponding to a time-varying linear potential, and the tunneling is measured by letting the BEC freely expand in the lattice. The measured tunneling rate is reduced and, for certain values of the shaking parameter, completely suppressed. Our results are in excellent agreement with theoretical predictions. Furthermore, we have verified that, in general, the strong shaking does not destroy the phase coherence of the BEC, opening up the possibility of realizing quantum phase transitions by using the shaking strength as the control parameter.     

A single-electron circuit as a discrete dynamical system

A single-electron circuit can be operated as a discrete dynamical system because it changes its internal state discontinuously because of electron tunneling. To confirm this idea, we designed a sample circuit for discrete dynamical operation and confirmed by computer simulation that the circuit successfully generated a sequence of discrete-time outputs by following a return map. The concept of discrete dynamical systems will be useful in developing new functional systems that consist of quantum devices and nanostructures.     

强激光场原子隧道电离的研究新进展

光与物质的相互作用一直是科学的主旋律之一.随着超强超短激光技术的快速发展,如今人们可以研究单个原子的内部世界,并调控光与电子的相互作用,从而实现了对原子内电子的超快动力学过程的探索.强激光诱导的原子隧道电离是众多强场物理现象的基石,具有重要的研究意义,也是研究前沿的热点之一.综述了强场原子隧道电离的最新研究进展,基于隧...     

Regular-to-chaotic tunneling rates using a fictitious integrable system

We derive a formula predicting dynamical tunneling rates from regular states to the chaotic sea in systems with a mixed phase space. Our approach is based on the introduction of a fictitious integrable system that resembles the regular dynamics within the island. For the standard map and other kicked systems we find agreement with numerical results for all regular states in a regime where resonance-assisted tunneling is not relevant.     

Entropy of Quantum Dynamical Systems and Sufficient Families in Orthomodular Lattices with Bayessian State

The purpose of the present paper is to study the entropy h_s(Φ) of a quantum dynamical systems Φ=(L,s,φ), where s is a bayessian state on an orthomodular lattice L. Having introduced the notion of entropy h_s(φ,A) of partition A of a Boolean algebra B with respect to a state s and a state preserving homomorphism φ, we prove a few results on that, define the entropy of a dynamical system h_s(Φ), and show its invariance. The concept of sufficient families is also given and we establish that h_s(Φ) comes out to be equal to the supremum of h_s(φ,A), where A varies over any sufficient family. The present theory has then been extended to the quantum dynamical system (L,s,φ), which as an effect of the theory of commutators and Bell inequalities can equivalently be replaced by the dynamical system (B, s₀,φ), where B is a Boolean algebra and s₀ is a state on B.     

Effect of quantum fluctuations on the dipolar motion of bose-einstein condensates in optical lattices

We reexamine dipolar motion of condensate atoms in one-dimensional optical lattices and harmonic magnetic traps including quantum fluctuations within the truncated Wigner approximation. In the strong tunneling limit we reproduce the mean field results with a sharp dynamical transition at the critical displacement. When the tunneling is reduced, on the contrary, strong quantum fluctuations lead to finite damping of condensate oscillations even at infinitesimal displacement. We argue that there is a smooth crossover between the chaotic classical transition at finite displacement and the superfluid-to-insulator phase transition at zero displacement. We further analyze the time dependence of the density fluctuations and of the coherence of the condensate and find several nontrivial dynamical effects, which can be observed in the present experimental conditions.     

Nonlinear dynamics of a spinor Bose-Einstein condensate in a double-well potential

We examine the nonlinear dynamical behavior of a spinor Bose-Einstein condensate in a double-well potential. Considering a condensate with large number of atoms, such that it can be described using the mean field theory, we separate the spinor dynamics from the spatial dynamics under the single-mode approximation. We limit ourselves to certain initial conditions under which the spatial mode is frozen so that we can focus on the spinor dynamics only. Identifying collective spin variables of our system, we derive the corresponding nonlinear equations of motion for them. Employing standard stability analysis, we find and characterize fixed points of the system. For a wide range of physical parameters such as tunneling strength and non-linear interactions, as well as for various initial preparations of the system, we identify qualitatively different dynamical regimes possible in the system. In particular, complete and incomplete oscillations of spin variables between quantum wells are found. We also show that by bringing some fixed points close to each other in the phase space of the system, it is possible to induce amplitude modulation to those otherwise regular tunneling oscillations.     

非对称的玻色-爱因斯坦凝聚中的约瑟夫森结的动力学性质

利用半经典量子理论，研究了玻色-爱因斯坦凝聚体处于非对称的约瑟夫森结的动力学行为.结果表明双势阱中不同势阱的基态能量差与其相互作用能量的比率χ=0时，凝聚体表现为约瑟夫森效应；当χ≠0时，凝聚体中既存在量子宏观隧穿效应，又存在量子宏观局域效应. 关键词： 玻色爱因斯坦凝聚 约瑟夫森结 动力学性质