Sandro Wimberger

Time-resolved measurement of Landau-Zener tunneling in periodic potentials.

We report time-resolved measurements of Landau-Zener tunneling of Bose-Einstein condensates in accelerated optical lattices, clearly resolving the steplike time dependence of the band populations. Using different experimental protocols we were able to measure the tunneling probability both in the adiabatic and in the diabatic bases of the system. We also experimentally determine the contribution of the momentum width of the Bose condensates to the temporal width of the tunneling steps and discuss the implications for measuring the jump time in the Landau-Zener problem.

Dissipation induced coherence of a two-mode Bose-Einstein condensate.

We discuss the dynamics of a Bose-Einstein condensate in a double-well trap subject to phase noise and particle loss. The phase coherence of a weakly interacting condensate as well as the response to an external driving show a pronounced stochastic resonance effect: Both quantities become maximal for a finite value of the dissipation rate matching the intrinsic time scales of the system. Even stronger effects are observed when dissipation acts in concurrence with strong interparticle interactions, restoring the purity of the condensate almost completely and increasing the phase coherence significantly.

Quantum resonances and decoherence for δ-kicked atoms

The quantum resonances occurring with δ-kicked atoms when the kicking period is an integer multiple of the half-Talbot time are analysed in detail. Exact results about the momentum distribution at exact resonance are established, both in the case of totally coherent dynamics and in the case when decoherence is induced by spontaneous emission. A description of the dynamics when the kicking period is close to, but not exactly at resonance, is derived by means of a quasi-classical approximation where the detuning from exact resonance plays the role of the Planck constant. In this way scaling laws describing the shape of the resonant peaks are obtained. Such analytical results are supported by extensive numerical simulations, and explain some recent surprising experimental observations.

Negative Differential Conductivity in an Interacting Quantum Gas.

We report on the observation of negative differential conductivity (NDC) in a quantum transport device for neutral atoms employing a multimode tunneling junction. The system is realized with a Bose-Einstein condensate loaded in a one-dimensional optical lattice with high site occupancy. We induce an initial difference in chemical potential at one site by local atom removal. The ensuing transport dynamics are governed by the interplay between the tunneling coupling, the interaction energy, and intrinsic collisions, which turn the coherent coupling into a hopping process. The resulting current-voltage characteristics exhibit NDC, for which we identify atom number-dependent tunneling as a new microscopic mechanism. Our study opens new ways for the future implementation and control of complex neutral atom quantum circuits.

Resonant tunneling of Bose–Einstein condensates in optical lattices

In this paper, we present the theoretical as well as experimental results on resonantly enhanced tunneling of Bose–Einstein condensates in optical lattices both in the linear case and for small nonlinearities. Our results demonstrate the usefulness of condensates in optical lattices for simulating Hamiltonians originally used for describing solid-state phenomena.

Induced delocalization by correlation and interaction in the one-dimensional Anderson model

We consider long-range correlated disorder and mutual interacting particles according to a dipole-dipole coupling as modifications to the one-dimensional Anderson model. Technically we rely on the (numerical) exact diagonalization of the systems Hamilitonian. From the perspective of different localization measures we confirm and extend the picture of the emergence of delocalized states with increasing correlations. Beside these studies a definition for multi-particle localization is proposed. In the case of two interacting bosons we observe a sensitivity of localization with respect to the range of the particle-particle interaction and insensitivity to the couplings sign, which should stimulate new theoretical approaches and experimental investigations with e.g. dipolar cold quantum gases. This revised manuscript is much more explicit compared to the initial version of the paper. Major extensions have been applied to Sects. II and III where we updated and added figures and we more extensively compared our results to the literature. Furthermore, Sect. III additionally contains a phenomenological line of reasoning that bridges from delocalization by correlation to delocalization by interaction on the basis of the multi-particle Hamilton matrix.

Resonant nonlinear quantum transport for a periodically kicked bose condensate

Our realistic numerical results show that the fundamental and higher-order quantum resonances of the delta-kicked rotor are observable in state-of-the-art experiments with a Bose condensate in a shallow harmonic trap, kicked by a spatially periodic optical lattice. For stronger confinement, interaction-induced destruction of the resonant motion of the kicked harmonic oscillator is predicted.

Bosonic transport through a chain of quantum dots

The particle transport through a chain of quantum dots coupled to two bosonic reservoirs is studied. For the case of reservoirs of non-interacting bosonic particles, we derive an exact set of stochastic differential equations, whose memory kernels and driving noise are characterised entirely by the properties of the reservoirs. Going to the Markovian limit an analytically solvable case is presented. The effect of interparticle interactions on the transient behaviour of the system, when both reservoirs are instantaneously coupled to an empty chain of quantum dots, is approximated by a semiclassical method, known as the Truncated Wigner approximation. The steady-state particle flow through the chain and the mean particle occupations are explained via the spectral properties of the interacting system.

A Pseudoclassical Method for the Atom-Optics Kicked Rotor: from Theory to Experiment and Back

Abstract We review the concept and applications of a semiclassical ( ϵ -classical or pseudoclassical) approximation to the resonant dynamics of an atom “kicked” by a pulsed, periodic potential. This powerful method allows us to derive analytical results in the deep quantum limit of the kicked rotor. Additionally, classical phase space portraits may be used to represent the dynamics even though the system is fundamentally quantum mechanical. The technique has been successfully adapted for systems including noise and decoherence, as well as systems for which the initial state is a trivial quantum superposition (leading to directed transport at quantum resonance). For almost a decade, theoretical investigations and experimental investigations have been proceeding hand-in-hand in this field, which has been stimulated regularly by experimental progress in controlling driven dynamical systems. Here, we review both theoretical and experimental advances, which in turn may inspire future applications of the presented pseudoclassical method.

Collapse and revival in inter-band oscillations of a two-band Bose?Hubbard model

We study the effect of a many-body interaction on inter-band oscillations in a two-band Bose?Hubbard model with an external Stark force. Weak and strong inter-band oscillations are observed, where the latter arise from a resonant coupling of the bands. These oscillations collapse and revive due to a weak two-body interaction between the atoms. Effective models for oscillations in and out of resonance are introduced that provide predictions for the systems behaviour, particularly for the time scales for the collapse and revival of the resonant inter-band oscillations.