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.

By moving the pivot of a pendulum rapidly up and down one can create a stable position with the pendulum's bob above the pivot rather than below it [1]. This surprising and counterintuitive phenomenon is a widespread feature of driven systems and carries over into the quantum world. Even when the static properties of a quantum system are known, its response to an explicitly timedependent variation of its parameters may be highly nontrivial, and qualitatively new states can appear that were absent in the original system. In quantum mechanics the archetype for this kind of behaviour is an atom in a radiation field, which exhibits a number of fundamental phenomena such as the modification of its g-factor in a radiofrequency field [2] and the dipole force acting on an atom moving in a spatially varying light field [3]. These effects can be successfully described in the so-called dressed atom picture [4]. Here we show that the concept of dressing can also be applied to macroscopic matter waves [5], and that the quantum states of "dressed matter waves" can be coherently controlled. In our experiments we use BoseEinstein condensates in driven optical lattices and demonstrate that the many-body state of this system can be adiabatically and reversibly changed between a superfluid and a Mott insulating state [6, 7, 8] by varying the amplitude of the driving. Our setup represents a versatile testing ground for driven quantum systems, and our results indicate the direction towards new quantum control schemes for matter waves.An atom in a radiation field can be described in the dressed atom picture [4] (or in equivalent approaches using, e.g., Floquet quasienergy states) in which the modified properties of the driven system arise from "dressing" the atom's electronic states with the photons of the radiation field. This concept can also be applied to macroscopic matter waves in driven periodic potentials [5], where the "dressing" is provided by the oscillatory motion of the lattice potential. In analogy to the dressed atom picture, such "dressed matter waves" can exhibit new properties absent in the original system and thus allow enhanced control of its quantum states. Here we demonstrate that matter waves can be adiabatically transferred into a well-defined Floquet quasienergy state of a driven periodic potential while preserving their quantum coherence.Cold atoms in optical lattices [7] can be described in the Bose-Hubbard model by the parameter U/J, where J is the hopping term relating to tunneling between adjacent sites, and U is the on-site interaction energy (see Fig. 1a). When U/J is small, tunneling dominates and the atoms are delocalized over the lattice, whereas a large value means that the inter- action term is large compared to J and phase coherence is lost through the formation of number-squeezed states with increased quantum phase fluctuations. At a critical value of U/J the system undergoes a quantum phase transition to a Mott insulator state. Using optical lattices one can tune U/J by changing the lattice dept...

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.

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