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Quantum simulation of many-body quantum systems using Rydberg-atom platforms has become of extreme interest in the last years. The possibility to realize spin Hamiltonians and the accurate control at the single atom level paved the way for the study of quantum phases of matter and dynamics. Here, we propose a quantum optimal control protocol to engineer current states: quantum states characterized by Rydberg excitations propagating in a given spatially closed tweezer networks. Indeed, current states with different winding numbers can be generated on demand. Besides those ones with single winding number, superposition of quantum current states characterized by more winding numbers can be obtained. The single current states are eigenstates of the current operator that therefore can define an observable that remains persistent at any time. In particular, the features of the excitations dynamics reflects the nature of current states, a fact that in principle can be used to characterize the nature of the flow experimentally without the need of accessing high order correlators., Comment: 16 pages, 10 figures

We demonstrate a simple technique for adding controlled dissipation to Rydberg atom experiments. In our experiments we excite cold rubidium atoms in a magneto-optical trap to $70$-S Rydberg states, whilst simultaneously inducing forced dissipation by resonantly coupling the Rydberg state to a hyperfine level of the short-lived $6$-P state. The resulting effective dissipation can be varied in strength and switched on and off during a single experimental cycle., Comment: 8 pages, 7 figures

Networks of Rydberg atoms provide a powerful basis for quantum simulators and quantum technologies. Inspired by matter-wave atomtronics, here we engineer switches, diodes and universal logic gates. Our schemes control the Rydberg excitation dynamics via the anti-blockade or facilitation mechanism, allowing for much faster devices compared to cold atom systems. Our approach is robust to noise and can be applied to individually trapped atoms and extensive three-dimensional gases. In analogy to electronics, Rydberg atomtronic devices promise to enhance quantum information processors and quantum simulators.

The coherent quantum transport of matter wave through a ring-shaped circuit attached to leads defines an iconic system in mesoscopic physics that has allowed both to explore fundamental questions in quantum science and to draw important avenues for conceiving devices of practical use. Here we study the source-to-drain transport of excitations going through a ring-network, without propagation of matter waves. We model the circuit in terms of a spin system with specific long-range interactions that are relevant for quantum technology, such as Rydberg atoms trapped in optical tweezers or ion traps. Inspired by the logic of rf- and dc-SQUIDs, we consider rings with one and two local energy offsets, or detunings. As a combination of specific phase shifts in going though the localized detunings and as a result of coherent tunneling, we demonstrate how the transport of excitations can be controlled, with a distinctive dependence on the range of interactions., Comment: Main text: 9 pages, 4 figures. Appendix: 16 pages, 10 figures

We propose to use a quantum spin chain as a device to store and release energy coherently (namely, a quantum battery) and we investigate the interplay between its internal correlations and outside decoherence. We employ the quantum Ising chain in a transverse field, and our charging protocol consists of a sudden global quantum quench in the external field to take the system out of equilibrium. Interactions with the environment and decoherence phenomena can dissipate part of the work that the chain can supply after being charged, measured by the ergotropy. We find that the system shows overall remarkably better performances, in terms of resilience, charging time, and energy storage, when topological frustration is introduced by setting AFM interactions with an odd number of sites and periodic boundary conditions. Moreover, we show that in a simple discharging protocol to an external spin, only the frustrated chain can transfer work and not just heat.

Highly excited Rydberg atoms are a powerful platform for quantum simulation and information processing. Here, we propose atomic ring networks to study chiral currents of Rydberg excitations. The currents are controlled by a phase pattern imprinted via a Raman scheme and can persist even in the presence of dephasing. Depending on the interplay between the Rabi coupling of Rydberg states and the dipole-dipole atom interaction, the current shows markedly different features. The excitations propagate with a velocity displaying a characteristic peak in time, reflecting the chiral nature of the current. We find that the time-averaged current in a quench behaves similarly to the ground-state current. This analysis paves the way for the development of new methods to transport information in atomic networks., Comment: 6 pages main, 6 figures + appendices

We report experimental measurements of the rates of blackbody radiation-induced transitions between high-lying (n>60) S, P and D Rydberg levels of rubidium atoms in a magneto-optical trap using a hybrid field ionization and state-selective depumping technique. Our results reveal significant deviations of the measured transition rates from theory for well-defined ranges of the principal quantum number. We assume that the most likely cause for those deviations is a modified blackbody spectrum inside the glass cell in which the magneto-optical trap is formed, and we test this assumption by installing electrodes to create an additional microwave cavity around the cell. From the results we conclude that it should be possible to use such external cavities to control and suppress the blackbody radiation-induced transitions., Comment: 9 pages, 5 figures

Understanding and probing phase transitions in non-equilibrium systems is an ongoing challenge in physics. A particular instance are phase transitions that occur between a non-fluctuating absorbing phase, e.g., an extinct population, and one in which the relevant order parameter, such as the population density, assumes a finite value. Here we report the observation of signatures of such a non-equilibrium phase transition in an open driven quantum system. In our experiment rubidium atoms in a quasi one-dimensional cold disordered gas are laser-excited to Rydberg states under so-called facilitation conditions. This conditional excitation process competes with spontaneous decay and leads to a crossover between a stationary state with no excitations and one with a finite number of excitations. We relate the underlying physics to that of an absorbing state phase transition in the presence of a field (i.e. off-resonant excitation processes) which slightly offsets the system from criticality. We observe a characteristic power-law scaling of the Rydberg excitation density as well as increased fluctuations close to the transition point. Furthermore, we argue that the observed transition relies on the presence of atomic motion which introduces annealed disorder into the system and enables the formation of long-ranged correlations. Our study paves the road for future investigations into the largely unexplored physics of non-equilibrium phase transitions in open many-body quantum systems., Comment: 14 pages, 8 figures

We report an experimental investigation of the facilitated excitation dynamics in off-resonantly driven Rydberg gases by separating the initial off-resonant excitation phase from the facilitation phase, in which successive facilitation events lead to excitation avalanches. We achieve this by creating a controlled number of initial seed excitations. Greater insight into the avalanche mechanism is obtained from an analysis of the full counting distributions. We also present simple mathematical models and numerical simulations of the excitation avalanches that agree well with our experimental results., Comment: 13 pages, 6 figures

We study the time evolution of a Bose-Einstein condensate in an accelerated optical lattice. When the condensate has a narrow quasimomentum distribution and the optical lattice is shallow, the survival probability in the ground band exhibits a steplike structure. In this regime we establish a connection between the wave-function renormalization parameter $Z$ and the phenomenon of resonantly enhanced tunneling., Comment: 12 pages, 3 figures. arXiv admin note: substantial text overlap with arXiv:1201.6286

The dipole blockade of Rydberg excitations is a hallmark of the strong interactions between atoms in these high-lying quantum states. One of the consequences of the dipole blockade is the suppression of fluctuations in the counting statistics of Rydberg excitations, of which some evidence has been found in previous experiments. Here we present experimental results on the dynamics and the counting statistics of Rydberg excitations of ultra-cold Rubidium atoms both on and off resonance, which exhibit sub- and super-Poissonian counting statistics, respectively. We compare our results with numerical simulations using a novel theoretical model based on Dicke states of Rydberg atoms including dipole-dipole interactions, finding good agreement between experiment and theory., Comment: accepted for publication in PRL; 10 pages, 4 figures (including Supplemental Material)

We analyze time-of-flight absorption images obtained with dilute Bose-Einstein con-densates released from shaken optical lattices, both theoretically and experimentally. We argue that weakly interacting, ultracold quantum gases in kilohertz-driven optical potentials constitute equilibrium systems characterized by a steady-state distri-bution of Floquet-state occupation numbers. Our experimental results consistently indicate that a driven ultracold Bose gas tends to occupy a single Floquet state, just as it occupies a single energy eigenstate when there is no forcing. When the driving amplitude is sufficiently high, the Floquet state possessing the lowest mean energy does not necessarily coincide with the Floquet state connected to the ground state of the undriven system. We observe strongly driven Bose gases to condense into the former state under such conditions, thus providing nontrivial examples of dressed matter waves., Comment: 36 pages, 3 figures, Advance Atomic Molecular Physics in press

The ability to accurately control a quantum system is a fundamental requirement in many areas of modern science such as quantum information processing and the coherent manipulation of molecular systems. It is usually necessary to realize these quantum manipulations in the shortest possible time in order to minimize decoherence, and with a large stability against fluctuations of the control parameters. While optimizing a protocol for speed leads to a natural lower bound in the form of the quantum speed limit rooted in the Heisenberg uncertainty principle, stability against parameter variations typically requires adiabatic following of the system. The ultimate goal in quantum control is to prepare a desired state with 100% fidelity. Here we experimentally implement optimal control schemes that achieve nearly perfect fidelity for a two-level quantum system realized with Bose-Einstein condensates in optical lattices. By suitably tailoring the time-dependence of the system's parameters, we transform an initial quantum state into a desired final state through a short-cut protocol reaching the maximum speed compatible with the laws of quantum mechanics. In the opposite limit we implement the recently proposed transitionless superadiabatic protocols, in which the system perfectly follows the instantaneous adiabatic ground state. We demonstrate that superadiabatic protocols are extremely robust against parameter variations, making them useful for practical applications., Comment: 17 pages, 4 figures

The time it takes a quantum system to complete a tunneling event (which in the case of cross-barrier tunneling can be viewed as the time spent in a classically forbidden area) is related to the time required for a state to evolve to an orthogonal state, and an observation, i.e., a quantum mechanical projection on a particular basis, is required to distinguish one state from another. We have performed 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. The use of different protocols enabled us to access the tunneling probability, in two different bases, namely, the adiabatic basis and the diabatic basis. The adiabatic basis corresponds to the eigenstates of the lattice, and the diabatic one to the freeparticle momentum eigenstates. Our findings pave the way towards more quantitative studies of the tunneling time for LZ transitions, which are of current interest in the context of optimal quantum control and the quantum speed limit., Comment: 5 pages, 2 figures. Fifth International Workshop DICE2010, September 13-17 2010

Matter waves can be coherently and adiabatically loaded and controlled in strongly driven optical lattices. This coherent control is used in order to modify the modulus and the sign of the tunneling matrix element in the tunneling Hamiltonian. Our findings pave the way for studies of driven quantum systems and new methods for engineering Hamiltonians that are impossible to realize with static techniques., Comment: 6 pages, 3 figures. 3rd Italian Quantum Information Science Conference IQIS 2010. Preprint of an article submitted for consideration in IJQI 2011 [copyright World Scientific Publishing Company]

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., Comment: 7 pages, 4 figures

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. 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 time-dependent 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 radio-frequency field and the dipole force acting on an atom moving in a spatially varying light field. These effects can be successfully described in the so-called dressed atom picture. Here we show that the concept of dressing can also be applied to macroscopic matter waves, and that the quantum states of "dressed matter waves" can be coherently controlled. In our experiments we use Bose-Einstein 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 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., Comment: 5 pages, 4 figures

In this article, we present 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., Comment: New J. Phys., in press

We present our experimental investigations on the subject of nonlinearity-modified Bloch-oscillations and of nonlinear Landau-Zener tunneling between two energy bands in a rubidium Bose Einstein condensate in an accelerated periodic potential. Nonlinearity introduces an asymmetry in Landau-Zener tunneling. We also present measurements of resonantly enhanced tunneling between the Wannier-Stark energy levels for Bose-Einstein condensates loaded into an optical lattice., Comment: Chapter of "Nonlinearities of Periodic Structures and Metamaterials" (edited by C. Denz, S. Flach, and Yu. Kivshar) to be published by Springer

Quantum tunnelling from a tilted, but otherwise periodic potential is studied. Our theoretical and experimental results show that, by controlling the system's parameters, we can engineer the escape rate of a Bose-Einstein condensate to an exceptional degree. Possible applications of this atom-optics realization of the open Wannier-Stark system are discussed., Comment: 6 pp, proceedings DICE 11-15 September 2006, Castello di Piombino, Tuscany, Italy

We present a very simple model for realizing directed transport with cold atoms in a pair of periodically flashed optical lattices. The origin of this ratchet effect is explained and its robustness demonstrated under imperfections typical of cold atom experiments. We conclude that our model offers a clear-cut way to implement directed transport in an atom optical experiment., Comment: 7 pages, 7 figures

We present detailed numerical results on the dynamics of a Bose-Einstein condensate in a tilted periodic optical lattice over many Bloch periods. We show that an increasing atom-atom interaction systematically affects coherent tunneling, and eventually destroys the resonant tunneling peaks., Comment: to be published in Phys. Rev. A; amended version, 4 p., 4 figs

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., Comment: amended version, new Fig.4