Albert Werner

Leibniz Universität Hannover

Propagation properties of quantum walks

Quantum walks describe the time discrete evolution of a single particle with an internal degree of freedom on a lattice with an additional locality condition. We study the propagation behaviour of such quantum walks in the translation invariant case as well as for different regimes of decoherence and disorder. We show that the possible propagation behaviour of the particle ranges from ballistic transport over diffusive spreading to dynamical localization. In the translation invariant case it is known that the asymptotic position distribution is governed by the group velocity that is the gradient of the dispersion relation. We prove that this convergence obeys a large deviation estimate.

In the case of a two particle quantum walk with on site interactions we show the existence of a bound state of these two particles that itself undergoes a quantum walk dynamics.

Patrick Windpassinger

Institut für Laserphysik, Universität Hamburg

Simulating classical magnetism and creating artificial gauge fields in optical lattices

Ultracold bosons in optical lattices have during the past years been applied to emulate various solid state systems. The presentation will discuss the experimentally realized simulation of a frustrated classical spin system in a triangular optical lattice and the verification of the corresponding spin-phase diagram. Particular focus will be put on the frustrated regions of the phase diagram and the associated phase transitions.

The second part will be concerned with the recent realization of an artificial vector gauge field in a one dimensional optical lattice and the extensions to two dimensional systems. In the latter case, the gauge field will be used to control the frustration on the classical spin system described in the first part.

As the findings rely on the application of an external periodic force, the main underlying technical method will also be discussed in detail.

Anne Nielsen

Max-Planck Institute of Quantum Optics

Laughlin-like states on finite lattices

Laughlin's wavefunctions have played a key role in explaining the fractional quantum Hall effect, and lattice versions of the states are expected to be important for understanding topological phases of spin models. Guided by conformal field theory, we propose a set of wavefunctions, which can be seen as a slight modification of Laughlin's states on lattices. The modification builds convenient mathematical properties into the states, which allow us to obtain various analytical results, while important physical characteristics of the states remain practically unchanged. In particular, we derive a parent Hamiltonian for the state with Landau level filling factor 1/2 and show that the resulting model unifies the Haldane-Shastry model and the Kalmeyer-Laughlin state into the same model. The Hamiltonian contains long-range two- and three-body interactions, but we find numerically that the ground state has a large overlap with the ground state of a Hamiltonian containing only short-range interactions, which makes the model simpler to simulate experimentally.

Axel Kuhn

University of Oxford

Photonic Qubits, Qutrits and Ququads in Linear Optical Quantum Circuits

The ability of encoding arbitrary information in elementary quantum systems is the key to a novel approach to computing based on quantum mechanics. Particular attention has been paid toward the field of linear optics quantum computing (LOQC) which in principle is a scalable, albeit often restricted by the spontaneous nature of parametric down-conversion sources. Here, I will show that single photons emitted on demand from a single atom into an optical cavity can be used to get past those limits. With a coherence time greater than 500 ns, a subdivision of photons into d time bins of arbitrary amplitudes and phases has been achieved, which we use for encoding arbitrary qu-d-its in one single photon. The fidelity of the quantum state preparation is verified in time-resolved quantum-homodyne measurements, and the photons are used to operate elementary quantum gates in integrated photonic circuits.

Jonathan Simon

Harvard/MIT Center for Ultracold Atoms

Building Synthetic Materials from Ultracold Atoms: Quantum Magnetism in an Optical Lattice

Ultracold atoms in optical lattices form a unique testbed for quantum many-body physics. Using such systems it has recently become possible to engineer strongly-correlated materials from the ground up and probe them with single-atom resolution. I will describe experiments in which we have synthesized the first magnetic material composed of ultracold atoms, and watched it undergo a quantum phase transition from a paramagnet to an antiferromagnet. I will then introduce a new algorithmic cooling scheme that we have demonstrated, which points the way to even more exotic quantum phases that exist at lower temperatures. In addition, I will describe recent experiments studying bilayer quantum gases, with promising applications in generation and detection of even more varied strongly correlated materials.

Antoine Browaeys

CNRS – Institut d'Optique

Experimental investigation of the dipole-dipole interaction between cold atoms

This talk will present our ongoing effort to understand and use the dipole-dipole interaction between ultra-cold atoms. This dipole interaction can result from the interaction of the atoms with near-resonant light. In this case the scattering of light by an ensemble of atoms is modified if the inter-atomic distance is on the order of the wavelength of the light. The atoms then behave collectively, leading to super or sub-radiant phenomena. The dipoles can also be prepared by exciting the atoms to a Rydberg state leading to interaction strength large enough to make the atoms interact even at long distance. This Rydberg interaction is at the basis of the preparation of entangled states.

Yoav Lahini

The Weizmann Institute of Science

Correlated quantum walks and topological phases in photonic lattices

I will describe the study of quantum walks in photonic lattices, including the emergence of Hanbury-Brown Twiss correlations between multiple particles and the effects of interactions, disorder and quasi-periodicity on the dynamics. In addition, I will describe the emergence of non-trivial topological effects in quasi-periodic potentials.

Artur Widera

TU Kaiserslautern

Immersing Single Atomic Impurities into a Quantum Gas

I will present our approach to merge the two worlds of single individually controlled atoms and large-scale many-body systems for realizing a paradigm of experimental quantum physics: a single well controlled impurity immersed in a quantum gas. Such systems are promising candidates for studies of condensed matter problems such as Fröhlich-type polarons, or for non-destructive, local probing of quantum many-body systems.

We have immersed single Cs atoms into an ultracold Rb gas and observed cooling of the single atom down to the temperature of the many-body system through s-wave collisions. Our results not only suggest a method for the preparation of ultracold mesoscopic samples, they also allow to probe non-equilibrium dynamics of quantum systems, where the full energy distribution is accessible at all times. Furthermore, inelastic collisions can be studied event-by-event with single atom sensitivity, allowing to distinguish various channels of molecule formation and to extract a precise number for the three-body recombination coefficient.