Alessandro Zenesini

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.

Observation of photon-assisted tunneling in optical lattices.

We have observed tunneling suppression and photon-assisted tunneling of Bose-Einstein condensates in an optical lattice subjected to a constant force plus a sinusoidal shaking. For a sufficiently large constant force, the ground energy levels of the lattice are shifted out of resonance and tunneling is suppressed; when the shaking is switched on, the levels are coupled by low-frequency photons and tunneling resumes. Our results agree well with theoretical predictions and demonstrate the usefulness of optical lattices for studying solid-state phenomena.

Coherent control of dressed matter waves.

We demonstrate experimentally that matter waves can be coherently and adiabatically loaded and controlled in one-, two-, and three-dimensional strongly driven optical lattices. This coherent control is then used in order to reversibly induce the superfluid-Mott insulator phase transition by changing the strength of the driving. Our findings pave the way for studies of driven quantum systems and new methods for controlling matter waves.

Universality of the three-body parameter for Efimov states in ultracold cesium.

We report on the observation of triatomic Efimov resonances in an ultracold gas of cesium atoms. Exploiting the wide tunability of interactions resulting from three broad Feshbach resonances in the same spin channel, we measure magnetic-field dependent three-body recombination loss. The positions of the loss resonances yield corresponding values for the three-body parameter, which in universal few-body physics is required to describe three-body phenomena and, in particular, to fix the spectrum of Efimov states. Our observations show a robust universal behavior with a three-body parameter that stays essentially constant.

Exploring dynamic localization with a Bose-Einstein condensate

We report on the experimental observation of dynamic localization of a Bose-Einstein condensate in a shaken optical lattice, both for sinusoidal and square-wave forcing. The formulation of this effect in terms of a quasienergy band collapse, backed by the excellent agreement of the observed collapse points with the theoretical predictions, suggests the feasibility of systematic quasienergy band engineering.

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.

Efimov Resonances in Ultracold Quantum Gases

Ultracold atomic gases have developed into prime systems for experimental studies of Efimov three-body physics and related few-body phenomena, which occur in the universal regime of resonant interactions. In the last few years, many important breakthroughs have been achieved, confirming basic predictions of universal few-body theory and deepening our understanding of such systems. We review the basic ideas along with the fast experimental developments of the field, focussing on ultracold cesium gases as a well-investigated model system. Triatomic Efimov resonances, atom-dimer Efimov resonances, and related four-body resonances are discussed as central observables. We also present some new observations of such resonances, supporting and complementing the set of available data.

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.

Feshbach resonances, weakly bound molecular states, and coupled-channel potentials for cesium at high magnetic fields.

We explore the scattering properties of ultracold ground-state Cs atoms at magnetic fields between 450 G (45 mT) and 1000 G. We identify 17 previously unreported Feshbach resonances, including two very broad ones near 549 and 787 G. We measure the binding energies of several different dimer states by magnetic field modulation spectroscopy. We use least-squares fitting to these experimental results, together with previous measurements at lower field, to determine a six-parameter model of the long-range interaction potential, designated M2012. Coupled-channels calculations using M2012 provide an accurate mapping between the s-wave scattering length and the magnetic field over the entire range of fields considered. This mapping is crucial for experiments that rely on precise tuning of the scattering length, such as those on Efimov physics.

Resonant atom-dimer collisions in cesium: Testing universality at positive scattering lengths

Efimov’s solution to the problem of three resonantly interacting particles [1] is widely considered to be the most prominent example of a universal few-body system, where the knowledge of the two-body scattering length a and an additional three-body parameter is sufficient to define the who le energy spectrum and to locate all the bound states. The details of the interparticle potential become irrelevant and diffe rent systems very far apart in energy and length scales can be described in the same way. The famous discrete scaling of the Efimov spectrum (scaling factor of 22 .7) and the precise ratios that link its different parts have attracted large interest in the scientific community. Universal behavior arises from the presence of resonant interactions leading to collisions on a length scale exceedin g the typical size of the interparticle potential. In trimer s ystems, the contributions of the short-range details are commonly included in the “three-body parameter”. This parameter fixes the starting point of the spectrum and can be expressed in terms of the scattering length a− at which the most deeply bound Efimov state crosses the zero-energy threshold [2]. Within the ideal Efimov scenario, the positions of all th e other features of the spectrum are uniquely determined, both at positive and negative values of a. The validation of Efimov’s scenario had remained elusive for decades until experiments on ultracold atoms provided evidence for its existence [3‐13]. The appearance of trimer bound states has been shown by measuring inelastic collisional rates in atomic samples or atom-dimer mixtures by tuning the scattering length via magnetically controlled Fesh bach resonances [14]. The presence of trimer bound states leads to enhancement and suppression of losses [15‐17]. In particular the loss resonances represent a “smoking gun” for Efimov’s spectrum and occur where the trimer energy state crosses the atomic threshold (at a−, in the region of negative a) or merges into the state of a dimer plus a free atom (at a∗, in the region of positive a).