## Colloquium 17.10.2013, 2:00 pm st, Room D326

### Prof. Dr. Tilman Esslinger

ETH Zürich

Institute for Quantum Electronics

Zürich, Schweitz

Synthetic Quantum Many-Body Systems

Fermionic quantum gases in optical lattices make it possible to physically construct and study key models of condensed matter physics. The riddle of high temperature superconductivity, or the beauty of graphene, are becoming accessible to experiments, in which the Hamiltonian is a direct result of the optical lattice potential created by interfering laser fields and short-ranged collisional interaction between ultracold atoms. Going beyond this approach, we have created cold-atom analogues of mesoscopic conductors and superconductors. A narrow channel made of light connects two macroscopic reservoirs of fermionic atoms. I will introduce the above concepts and report on our most recent results on quantum magnetism and conduction.

## Colloquium 24.10.2013, 2:00 pm st, Room D 326

### Prof. Dr. Päivi Törmä

Department of Applied Physics

Aalto University School of Science

Aalto, Finland

Making a spin-difference with ultracold gases

We present a brief introduction to the field of ultracold gases where many-body quantum physics can be studied with unprecedented accuracy and controllability of the system parameters. One fascinating possibility is to distort the symmetry of the two spins in the usual BCS-type superconductivity scenario. We present experimental advances on this topic as well as several examples of our related work. One example is the the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state where spin-density imbalance and superconductivity can coexist due to spatial oscillations of the superconduting order parameter. We consider ultracold Fermi gases with two pseudospin components and present three different topics where the ability to control and manipulate the pseudospins separately plays a key role. 1) We show that the FFLO state is stabilized in lattice geometries [1] and present a full finite-temperature phase diagram for the one-dimensional (1D) to three-dimensional (3D) crossover of the FFLO state in an attractive Hubbard model of 3D-coupled chains in a harmonic trap, calculated with dynamical mean field theory [2]. 2) We propose a novel way of distorting the two spin species that are forming Cooper pairs: namely, a mixed-geometry system of fermionic species selectively confined in lattices of different geometry [4]. A rich phase diagram of interband pairing with gapped and gapless excitations is found at zero temperature. We also show that the Fermi surface topology further divides the gapless phase into subclasses between which the system undergoes density-driven Lifshitz transitions. 3) We simulate the quantum dynamics of Fermi gases in one dimension: the expansion dynamics of a band-insulator state is shown to be well described by a two-site model [4], and the FFLO state is directly identified from the expansion velocities [5].

[1] T.K. Koponen, T. Paananen, J.-P. Martikainen, and P. Törmä, *Phys. Rev. Lett.* **99**, 120403 (2007)

[2] M.O.J. Heikkinen, D-H. Kim, and P. Törmä, *Phys. Rev. B* **87**, 224513 (2013)

[3] D-H. Kim, J.S.J. Lehikoinen, and P. Törmä, *Phys. Rev. Lett. ***110**, 055301 (2013)

[4] J. Kajala, F. Massel, and P. Törmä, *Phys. Rev. Lett. ***106**, 206401 (2011)

[5] J. Kajala, F. Massel, P. Törmä, *Phys. Rev. A* **84**, 041601(R) (2011)

## Colloquium 31.10.2013, 2:00 pm st, D 326

### Prof. Dr. Kjeld Eikema

Vrije Universiteit

LaserLaB, Faculty of Science (FEW)

Amsterdam, The Netherlands

Exciting frequency combs: why less can be more

Since the first demonstration of frequency comb lasers some 13 years ago, these devices have had a great impact in many fields of physics, in particular that of precision spectroscopy and fundamental tests. In these fields frequency combs are typically used to reference a spectroscopy laser, which is doing the actual excitation of an atomic or molecular transition, to a precise atomic clock. However, direct excitation with pulses from a comb laser is also showing great promise for precision measurements. In the talk I will review the possibilities in that direction, with a focus on the latest developments from our lab: full repetition rate comb excitation with coherent control, and precision Ramsey-comb spectroscopy with just two, highly amplified, comb laser pulses.

## Colloquium 07.11.2012, 2:00 pm st, Room D 326

### Dr. Ulrich Schneider

Ludwig-Maximilians-Universität

Fakultät für Physik

München, Germany

PDF document

Negative absolute Temperatures and the emergence of coherence for bosons in optical lattices

Absolute temperature, that is the fundamental temperature scale in thermodynamics, is usually bound to be positive. Under special conditions, however, negative temperatures - where high-energy states are more occupied than low-energy states - are also possible.

After a general introduction into experiments with ultracold atoms in optical lattices, I will present a negative temperature state for motional degrees of freedom: By tailoring the Bose-Hubbard Hamiltonian we experimentally created an attractively interacting ensemble of ultracold bosons, which is stable against collapse for arbitrary atom numbers despite a negative pressure. In this negative temperature state, the quasi-momentum distribution develops sharp peaks at the upper band edge, revealing thermal equilibrium and bosonic coherence over several lattice sites.

I will also discuss the connection to classical thermodynamics and present counterintuitive effects, such as above-unity Carnot efficiencies, which can occur when negative temperature and positive temperature systems are combined.

## Colloquium 14.11.2013, 2:00 pm st, Room D326

### Dr. Avinash Kumar

European Laboratory of Non-linear Spectroscopy

Florence, Italy

Transport of a 1D Bose gas in disorder

The study of combined effect of disorder and interaction on the transport properties of a system is essential to characterize the fluid-insulator transition. In this talk I will report on the experiments on a 1D Bose gas with tunable interaction in a quasiperiodic lattice, which are aimed to understand the effect of disorder on the transport properties of a system. Primarily we observe a weakly dissipative transport at low momenta followed by a sudden instability after a critical value of momentum. We study the reduction of this critical momentum under different strengths of disorder and interaction and identify the set of values at which the critical momentum vanishes, which we relate to the predicted zero temperature superfluid-Bose glass crossover.

## Colloquium: 21.11.2013, 2:00 pm st, Room D326

### Prof. Dr. Martin Weitz

Universität Bonn

Institut für Angewandte Physik

Veselago lensing with ultracold atoms in an optical lattice

Veselago lensing is a concept based on negative refraction, which allows for an effective time reversal and has prospects for sub-diffraction limit optical imaging. I will here describe a recent experiment of our group demonstrating Veselago lensing and negative refraction for matter waves. The experiment uses ultracold rubidium atoms in a variable optical lattice that allows for a tailoring of the dispersion relation. In particular, the atomic dispersion can be made relativistic, and resemble that of light both in positive and negative index materials respectively. Using a Raman pi-pulse technique to transfer the atomic de Broglie waves between different branches of the dispersion relation, we demonstrate both a one-dimensional Veselago lens and provide a ray-tracing simulation of a two-dimensional Veselago lens. In my talk, I will also mention recent work of our group on cold photon gases, in which we recently realised Bose-Einstein condensation in the grand-canonical ensemble regime

## Colloquium: 12.12.2013, 2:00 pm st, Room D326

### Prof. Dr. Thomas Udem

Max-Planck Institut für Quantenoptik

Garching

Precision Spectroscopy of Atomic Hydrogen

Precise determination of transition frequencies of simple atomic systems are required for a number of fundamental applications such as tests of quantum electrodynamics (QED), the determination of fundamental constants and nuclear charge radii. The sharpest transition in atomic hydrogen occurs between the metastable 2S state and the 1S ground state.

Its transition frequency has now been measured with almost 15 digits accuracy using an optical frequency comb and a cesium atomic clock as a reference. A recent measurement of the Lamb shift in muonic hydrogen is in significant contradiction to the hydrogen data if QED calculations are assumed to be correct. We hope to contribute to the resolution of this so called “proton size puzzle” by providing additional experimental input from the hydrogen side.

## Colloquium: 09.01.2014, 2:00 pm st, Room D326

### Dr. Garrett Cole

Faculty of Physics

University of Vienna and Vienna Center for Quantum Science and Technology

Cavity optomechanics: minimizing thermal noise in precision measurement

Cavity optomechanics has recently emerged as one of the most dynamic fields in modern optics. The ultimate objective of this interdisciplinary endeavor is to gain access to a completely new parameter regime, in terms of size and complexity, for experimental quantum physics. The fundamental process at the heart of this effort is the enhancement of radiation pressure within a high-finesse optical cavity. Exploiting this weak interaction, i.e. the momentum transfer of photons onto the cavity boundaries, requires the development of mechanical resonators simultaneously exhibiting high reflectivity and low mechanical dissipation. Interestingly, similar requirements—as a means of minimizing the deleterious effects of thermal noise—are found in a broad spectrum of applications, ranging from interferometric gravitational wave detectors to cavity-stabilized lasers for optical atomic clocks. This overlap leads to an intimate link between advances in the disparate areas of optical precision measurement and micro- and nanoscale optomechanical systems. In this presentation I will outline the fascinating perspectives of cavity optomechanics and introduce a related spin-off technology focusing on the development of ultra-stable optical reference cavities.