## Colloquium on July 23 2015, 1:30 pm st, Room D326

### Dr. Aidan Arnold

Department of Physics - University of Strathclyde

Glasgow, UK

- not available -

Cool things you can do with atoms

Laser cooling and quantum degenerate gases are now routinely used as tools to investigate new and surprising aspects of physics. I will introduce laser cooling in general, before discussing a new technique using micro-fabricated gratings which we hope will dramatically simplify laser cooling to a level where portable high-precision devices become feasible [1,2]. I will also discuss an even cooler experiment, with Bose-Einstein condensates (BECs), where we perform a Young’s slits type experiment which takes CCD pictures of two expanding BECs to observe their de Broglie matter waves [3,4]. We have made the first measurements demonstrating that the Talbot effect is very important in the BEC imaging system.

[1] C.C. Nshii, M. Vangeleyn, J.P. Cotter, P.F. Griffin, E.A. Hinds, C.N. Ironside, P. See, A.G. Sinclair, E. Riis and A.S. Arnold, Nature Nanotech. **8**, 321 (2013).

[2] J.P. McGilligan, P.F. Griffin, E. Riis and A.S. Arnold, Optics Express **23**, 8948 (2015).

[2] M.E. Zawadzki, P.F. Griffin, E. Riis, and A.S. Arnold, Phys. Rev. A **81**, 043608 (2010).

[4] C. H. Carson, Y. Zhai, P.F. Griffin, E. Riis and A. S. Arnold, in preparation (2015).

## Colloquium on July 16 2015, 1:30 pm st, Room D326

### Dr. Lisa Wörner

Vienna Center of Quantum Science and Technology

Faculty of Physics - University of Vienna

Vienna, AUSTRIA

- not available -

Matter wave interferometry

Laser Matter wave interferometry is a fascinating tool to explore the limits of quantum mechanics. Even though the wave nature of matter is a well accepted concept it is not observed in our daily life. Several theories exist that predict the colllaps of the wave function at different masses and only experimental results will be capable of determining the actual limit to quantum mechanics - if there is any. To do so the interference patterns of ever more massive moleculesare recorded. Currently, the most massive particle ever diffracted in a controlled experiment is a complex molecule that consists of roughly 800 atoms and has a mass of 10123 amu. In my talk I will show the different experimental setups currently operated in Vienna and explain possible ways to observe diffraction of more massive particles.

## Colloquium on July 09 2015, 1:30 pm st, Room D326

### Prof. Dr. Alice Sinatra

Laboratoire Kastler Brossel - Paris,

Paris, FRANCE

09th July 2015 Coherence time of a quantum gas

Coherence time of a quantum gas

It is generally assumed that a condensate of bosonic atoms or paired fermions at equilibrium is characterized by a macroscopic wavefunction with a well-defined, immutable phase. In reality, all systems have a finite size and are prepared at non-zero temperature; the condensate has then a finite coherence time, even when the system is isolated in its evolution and the particle number *N* is fixed.The loss of phase memory is due to interactions of the condensate with the excited modes that constitute a dephasing environment. This fundamental effect, crucial for applications using the condensate of pairs' macroscopic coherence, discloses a very rich physics involving the interactions among quasi particles and fascinating concepts as the ``eigenstate thermalization hypothesis" that allows to extend to a quantum system the classical notion of ergodicity.We link the coherence time of the gas to the condensate phase dynamics, and we calculate it with a microscopic theory including the relevant elementary excitations. We propose a method to measure the coherence time with ultracold atoms, and give some numbers, both for the weakly interacting Bose gas and for the unitary Fermi gas.

## Colloquium 02nd July 2015, 1:30 pm st, Room D326

### Dr. Leonardo Fallani

LENS - University Florenz,

Sesto Fiorentini/ Firenze, ITALY

Ultracold fermions in an extra dimension: synthetic gauge fields and chiral edge states

I will report on very recent experiments performed at LENS with ultracold multicomponent 173Yb Fermi gases. We have engineered Raman transitions between different 173Yb nuclear spin states to synthesize an effective lattice dynamics in a finite-sized “extra dimension”, which is encoded in the internal degree of freedom of the atoms [1]. By using this innovative approach, we have realized synthetic magnetic fields for effectively-charged fermions in ladder geometries with a variable number of legs. Direct imaging of the individual legs allowed us to demonstrate the emergence of chiral edge currents and to observe edge-cyclotron orbits propagating along the edges of the system [2], thus providing a direct evidence of a fundamental feature of quantum Hall physics in condensed-matter systems.

[1] A. Celi et al., Synthetic gauge fields in synthetic dimensions, Phys. Rev. Lett. 112, 043001 (2014).

[2] M. Mancini et al., Observation of chiral edge states with neutral fermions in a synthetic Hall ribbon, preprint arXiv:1502.02495 (2015).

## Colloquium 25th June 2015, 1:30 pm st, Room Bielefeldhörsaal B 305

### Prof. Dr. Herwig Ott

Department of Physics, University of Kaiserslautern

Kaiserslautern, GERMANY

25th June 2015 „Driven-dissipative Bose-Einstein condensates”.pdf

Driven - dissipative Bose-Einsein condessates

Ultracold quantum gases are usually well isolated from the environment. This allows to study the ground state properties and the unitary dynamics of a many-body quantum system under almost ideal conditions. Introducing a controlled coupling to the environment “opens” the quantum system and non-unitary dynamics can be investigated. Such an approach provides new opportunities to study fundamental quantum effects in open systems and to engineer robust many-body quantum states.

In this talk I will present an experimental platform [1,2] that allows for the controlled engineering of dissipation in ultracold quantum gases by means of localized particle losses. This technique is exploited to study quantum Zeno dynamics [3] and non-equilibrium dynamics [4]. We were also able to realize non-equilibrium steady-states in a driven-dissipative Bose-Einstein condensate.

References:

[1] T. Gericke et al., Nature Physics **4**, 949 (2008).

[2] P. Würtz et al., Phys. Rev. Lett. **103**, 080404 (2009).

[3] G. Barontini et al., Phys. Rev. Lett. **110**, 035302 (2013).

[4] R. Labouvie et al., accepted for publication in Phys Rev. Lett. (2015)

## Colloquium 18th June 2015, 1:30 pm st, Room D326

### Prof. Dr. Jan W. Thomsen

University of Copenhagen, Nils Bohr Institute

Copenhagen, DENMARK

- not available -

Nonlinear Dispersion with Narrow Linewidth Atoms in an Optical Cavity

As an alternative to state-of-the-art laser frequency stabilization using ultra-stable cavities, we proposed to exploit the non-linear effects from coupling of atoms with a narrow transition to a single mode of an optical cavity. We have constructed such a system and observed non-linear phase shifts of the 3P1 -1S0 narrow optical line strontium-88 atoms .Our sample temperature of a few mK provides a domain where the Doppler energy scale is several orders of magnitude larger than the narrow linewidth of the optical transition. This makes the system sensitive to velocity dependent multi-photon scattering events (Dopplerons) that changes the cavity field transmission and phase. This demonstration in a relatively simple system opens new possibilities for alternative routes to laser stabilization at the sub 100 mHz level and super-radiant laser sources involving narrow line atoms. The understanding of relevant motional effects obtained here has direct implications for other atomic clocks when used in relation with ultra-narrow clock transitions.

## Colloquium 11th June 2015, 1:30 pm st, Room D326

### Dr. Carlo Sias

LENS - University Florenz,

Sesto Fiorentini/ Firenze, ITALY

11th June 2015 - Quantum simulation with ultracold Ytterbium atoms.pdf

Quantum simulation with ultracold Ytterbium atoms: one-dimensional physics and beyond

In quantum simulation of condensed matter models with ultracold atoms, the choice of the atomic element strongly depends on the model that has to be realized. Alkaline-earth-like atoms have several peculiarities which make them extremely attractive for quantum simulation, including the possibility of simulating multi-component fermionic systems possessing SU(N) symmetry and having an orbital degree of freedom. Here we present two experiments realized with the Florence Ytterbium machine in which Ytterbium properties are used to realize different quantum simulations. In a first experiment, we exploit the internal states of a gas of 173Yb to realize one-dimensional liquids of ultracold fermions interacting repulsively within SU(N) symmetry. We observe that static and dynamic properties of the system deviate from those of ideal fermions, and that in the large-N limit, the system exhibits properties of a bosonic spinless liquid.In a second experiment, we exploit a clock transition to study the evolution of excited states of two atoms. We witness spin-exchange coherent oscillations between different long-lived electronic orbitals. This observation allows us to retrieve important information on the inter-orbital collisional properties of 173Yb atoms, paving the way to novel quantum simulations of paradigmatic models of two-orbital quantum magnetism.

## Colloquium 04th June 2015, 1:30 pm st, Room D326

### Prof. Dr. Christiane Koch

Kassel University

Kassel, GERMANY

04th June 2015 Non-resonant light control of ultracold collisions.pdf

Non-resonant light control of ultracold collisions

Non-resonant light universally couples to any polarizable object,independent of the particu-lar energy level structure, frequency of the light (as long as it is non-resonant), or presence of a permanent dipole moment. For molecules, this coupling shifts rotational and vibrational levels; it induces molecular alignment and may alter the spin character of wavefunctions. Most importantly, non-resonant light also modifies intermolecular interactions and can thus be used to control atomic and molecular collisions.

I will show that non-resonant light control becomes particularly useful for ultracold matter where scattering is dominated by tunneling and resonances. In particular, non-resonant light can be employed to shift the position of shape resonances, induce Feshbach resonances and engineer them in their position and width. Non-resonant light control thus facilitates photo- and magneto-association of molecules that would otherwise be very hard to produce.

## Colloquium 21th May 2015, 1:30 pm st, Room D326

### Prof. Dr. Nigel Cooper

Cambridge University

Cambridge, GREAT BRITAIN

21th May 2015 - Effects of Berry Curvature in Ultracold Atomic Gases.pdf

Effects of Berry Curvature in Ultracold Atomic Gases

Topological energy bands exhibit many fascinating physical phenomena. For instance, topological invariants underlie both the quantum Hall effect and more general topological insulators. There is currently great interest in exploring such physics in ultracold gases. Recent experiments have explored optical lattices with novel geometrical and topological features, and there is much ongoing activity to extend to other situations. Less widely appreciated is the fact that the energy bands of these new forms of optical lattice also have important geometrical properties. In particular, the Berry curvature is a geometrical property of the energy eigenstates, defined locally in the Brillouin zone. When integrated over the Brillouin zone of a two-dimensional band, it gives the Chern number, the topological invariant of the quantum Hall effect. The Berry curvature has many physical consequences in 2D and 3D systems, such as in the anomalous quantum Hall effect. I shall summarize how the Berry curvature can manifest itself in experimental measurements of transport and of collective modes in ultracold atomic gases.

## Colloquium 30th April 2015, 1:30 pm st, Room D326

### Dr. Jérôme Lodewyck

### SYRTE - Observatoire de Paris

Paris, FRANCE

30th April 2015 Optical lattice clocks with strontium atoms.pdf

Optical lattice clocks with strontium atoms

In Optical lattice clocks have recently beome the most stable and the most accurate frequency standards. These record performances are achieved by associating a large quality factor and a large numer of atoms simultaneously interrogated. I will present the general principles of optical lattice clocks, as well as recent developments. The presentation will focus on local and remote clock comparisons, which are a keystone for asserting the performances of clocks, and to reach an international agreement that demonstrates the reproducibility of these clocks.Furthermore such comparisons already yield tests of fundamental physics. Finally, i will introduce new concepts and methods that can bring optical lattice clocks to their ultimate performances. In particular, i will present a non-destructive detection for the transition probability.

## Colloquium 23th April 2015, 1:30 pm st, Room D326

### Dr. Olivier Dulieu

Universite Paris-Sud X

Directeur de Recherche CNRS, Equipe "THEOMAL", Directeur - adjont, Orsay Cedex, FRANCE

23th April 2015 Modeling routes for ultracold molecule formation.pdf

Modeling routes for ultracold molecule formation

In the continuously developing research field of cold and ultracold matter, cold molecules occupy a particular position. They can be considered as an atom with “one atom too much”, as an atom with supplementary degrees of freedom for novel control opportunities, as a pair of weakly interacting atoms for precision measurements, as a system ready for unconventional chemical reactions, as a tool for quantum simulation and quantum information.

Despite spectacular advances, creating dense samples of ultracold molecules in their ground state remains a challenging objective. In this talk, I will present the recent investigations of our group aiming at modeling two routes toward ultracold molecule formation: the optically controlled formation of ground state KRb and KCs polar molecules, and the formation of cold molecular ions in merged traps of laser-cooled atoms and ions.