## Special Colloquium on February 10, 2016, 4:00 pm (st) Institute for Theoretical Physics Theory Building Appelstr. 2 , Seminar Room 267

### Prof. Dr. Nir Bar-Gill

Hebrew University of Jerusalem

NV centers in diamond - quantum coherence, noise and nanoscale MRI

Nitrogen-Vacancy (NV) color centers in diamond provide a unique nanoscale quantum spin system embedded in a solid-state structure. As such they are well suited for studies in a wide variety of fields, with emerging applications ranging from quantum information processing to magnetic field sensing and nano-MRI (Magnetic Resonance Imaging). Importantly, NVs possess unique optical transitions which allow for optical initialization and readout of their quantum spin state.

## Colloquium 14th January 2016, 1:30 pm st, Room D326

### Prof. Dr. Kai Bongs

School of Physics and Astronomy, University of Birmingham

Birmingham, UK

Developments in the UK National Quantum Technology Hub in Sensors and Metrology

Die UK National Quantum Technology Initiative zielt auf die Beschleunigung des Innovationaprozesses von fundamentaler Forschung bis hin zu kommerziellen Anwendungen. Dieser Vortrag stellt das Konzept und aktualle Entwicklungen des UK National Quantum Technology Hub in Sensors and Metrology vor, der auf die Entwicklung von Sensoren basierend auf atomaren Testteilchen fokussiert ist, d.h. Sensoren basierend aud thermischen und kalten Atomen, sowie Ionen. Dies beinhaltet zugrundeliegende Technologie-Entwicklungen im Bereich Laser und Vakuum, Demonstratoren für Messungen von Gravitation, Magnetfeldern, Rotation, Zeit und Licht und marknahe Forschung in Bereich potentialler Anwendungen.

## Colloquium 07th January 2016, 1:30 pm st, Room D326

### Prof. Dr. Stephan Schiller

Institut für Experimentalphysik, Heinrich Heine Universität Düsseldorf

Düsseldorf, Germany

07th january 2016, The Space Optical Clocks mission on the ISS - Hannover 2016 - Stephan Schiller

The Space Optical Clocks mission on the ISS

The ESA candidate mission “Space Optical Clocks” aims at operating an optical lattice clock on the ISS in approximately 2022. The mission is the natural follow-on of the ACES mission, which will fly in 2017. The scientific goals of the mission are to perform tests of fundamental physics (Einstein’s gravitational time dilation), to enable space-assisted relativistic geodesy at < 1 cm uncertainty level, and to intercompare optical clocks on the ground at the < 1 × 10-18 level. Such comparison of ground clocks via the ISS will be performed using microwave links and frequency-comb-based optical links.

The performance goal of the space clock is less than 1 × 10-17 inaccuracy and instability. Technology development funded by ESA will begin in early 2016.Within an EU-FP7-funded project, a European consortium (16 partners) is developing a strontium optical lattice clock demonstrator [1]. Goal performances are an instability below 1×10-15 τ-1/2 and a fractional inaccuracy at 5×10-17 level. For the realization of the clock, techniques and approaches suitable for later space application are used, such as modular layout, diode lasers, low power consumption, and compact dimensions.A compact, light, and energy-efficient atomics system was developed. A robust frequency stabilization system enables stabilizing the 461 nm and 689 nm cooling lasers (the latter to sub-kHz linewidth), the 813 nm lattice laser and the 679 and 707 nm repumpers. The fully transportable clock laser is based on a 10-cm long cavity. Its current fractional instability is 3×10-15.

The Sr clock apparatus is operational at the point where ≈ 106 Sr atoms are reliably trapped in the optical lattice and the clock transition in 88Sr can be observed. The atomics part was transported by van from Birmingham to PTB (Braunschweig) for the integration of the clock laser and upcoming metrological characterization. After the transportation, the 1-stage MOT was operational 2 days after arrival, demonstrating the robustness and reliability of the system.

[1] K. Bongs et al., C. R. Physique 16, 553 (2015);

dx.doi.org/10.1016/j.crhy.2015.03.009

## Colloquium 17th December 2015, 1:30 pm st, Room D326

### Dr. Gerhard Zürn

Physikalisches Institut, Ultracold Quantum Gases, University Heidelberg

Heidelberg, Germany

17th december 2015, Quantum magnetism with few cold atoms - Gerhard Zuern

Quantum magnetism with few cold atoms

Models describing quantum magnetism are of large interest as they can potentially shed light on exotic properties of matter such as on unconventional superconductivity. To study such models we aim for a bottom-up approach in which we assemble a many-body state from its basic building block using few cold atoms.

We present how we prepare such a building block, a singlet state of two spin-1/2 fermions in a double well potential. By tuning the system into a regime where the superexchange energy is the dominant energy scale we realize the smallest possible Heisenberg Hamiltonian. Combining several of such singlet states should allow us to study quantum magnetism in larger systems.

As a first step towards this we have performed an experiment with few strongly interacting fermions in a 1D environment. The strong repulsion leads to a Wigner-crystal-like state which can therefore be described by a spin-chain Hamiltonian. Using novel methods to probe the system we map out the spin correlations of the prepared antiferromagnetic state for up to four particles and can directly observe quantum magnetism beyond two-particle correlations.

## Colloquium 10th December 2015, 1:30 pm st, Room D326

### Prof. Dr. David Guery-Odelin

Laboratoire Collisions Agrégats Réactivité, University of Toulouse

Toulouse, France

Spatial gaps and transport

In the first part of the talk, I will discuss a few experiments of atom optics that we have performed in guided environment using a finite size optical lattice. I will present the use of the optical lattice as a distributed Bragg reflector for matter waves. The envelope of the optical lattice projects in position space the gaps of the lattice. As a result Landau Zener transitions become effective tunnel barriers in real space, commonly referred to as spatial gaps. Using a symmetric envelope, we have realized a matter wave cavity with virtual walls provided by spatial gaps and having an energy dependent reflectivity. In this system, we can observe directly a single tunneling event and even evanescent matter waves. We will discuss the possible applications of this technique including the realization of a mode locked atom laser or the design of arrays of coupled cavities. We will explain how atoms can be coherently manipulated and transported between such cavities.

In the second part, I propose to explain briefly some key ideas of shortcuts to adiabaticity techniques applied to the transport of wave packets by a moving external potential.

## Colloquium 03th December 2015, 1:30 pm st, Room D326

### Prof. Dr. Achim Peters

Humboldt-University Berlin, Department of Physics

Berlin, Gernany

A High Precision Mobile Atom Interferometer

Inertial sensors based on interferometry with ultra cold matter waves are a valuable tool for many experiments. The spectrum of applications covers a broad area from metrology through gravimetry and geodesy up to addressing fundamental questions in physics, such as testing the validity of the Einstein equivalence principle (EEP) in the quantum domain.

This talk will discuss the mobile Quantum Gravimeter GAIN (targeted accuracy of a few parts in 10^10 for measurements of local gravity) as an example of a high performance instrument developed for terrestrial applications.

## Colloquium 26th November 2015, 1:30 pm st, Room D326

### Dr. Michael Tarbutt

Centre for Cold Matter, Blackett Laboratory, Department of Physics

Imperial College London

London, UK

Laser-cooled molecules and their applications

Ultracold molecules can be used for several applications in fundamental physics. An array of ultracold molecules in an optical lattice can be used as a simulator for studying many-body quantum systems with long range interactions. Cold molecules are already being used to measure the electron’s electric dipole moment and to search for changes in the fundamental constants, both of which test modern theories of particle physics. We aim to produce the ultracold molecules needed for these applications by direct laser cooling. I will explain how to apply laser cooling to molecules and present results on the laser cooling of CaF. I will discuss our progress towards a magneto-optical trap of these molecules, and outline a plan for making a fountain of ultracold YbF molecules for measuring the electron’s electric dipole moment.

## Colloquium 19th November 2015, 1:30 pm st, Room D326

### Dr. Christian Groß

Max-Planck-Institut für Quantenoptik München

Garching, Germany

Exploring long-range interacting quantum many-body systems with Rydberg atoms

Ultracold atoms laser coupled to Rydberg states provide a unique system to explore long-range interacting many-body systems. Here we summarize recent experiments on synthetic quantum magnets realized with Rydberg atoms, which we studied locally with single spin resolution using a quantum gas microscope. We discuss entanglement in collective "superatoms" as well as the dynamical formation of small Rydberg crystals. Finally, we report on recent progress towards Rydberg dressing, a promising scheme to realize dipolar quantum gases in the future.

## Colloquium 05th November 2015, 1:30 pm st, Room D326

### Prof. Dr. Philippe Grangier

Laboratoire Charles Fabry, Institut d`Optique, CNRS - National Center for Scientific Research

Universite´Paris-Sacllay

Palaiseau, France

Quantum Communications with Gaussian and non-Gaussian States of the Light

In recent years various various quantum communication protocols have been implemented, using either Gaussian states of the light (e.g. for continuous-variable quantum cryptography [1,2] ), or non-Gaussian states (e.g. for the generation of optical "Schrödinger's cat states" [3,4]). After reviewing these protocols, we will present recent results towards the possible realization of non-linear optical effects that are large enough to induce photon-photon interactions [5, 6]. Such effects would be a significant step forward for quantum information processing and communications, including the implementation of efficient two-photon phase gates.

[1] F. Grosshans, G. Van Assche, J. Wenger, R. Brouri, N. Cerf, P. Grangier, “Quantum key distribution using Gaussian-modulated coherent states”, Nature 421, 238 (2003).

[2] P. Jouguet, S. Kunz-Jacques, A. Leverrier, P. Grangier, E. Diamanti, “Experimental demonstration of long-distance continuous-variable quantum key distribution”, Nature Photonics 7, 378 (2013).

[3] A. Ourjoumtsev, R. Tualle-Brouri, J. Laurat, and Ph. Grangier, “Generating Optical Schrödinger Kittens for Quantum Information Processing”, Science 312, 83 (2006).

[4] A. Ourjoumtsev, F. Ferreyrol, R. Tualle-Brouri, P. Grangier, “Preparation of non-local superpositions of quasi-classical light states”, Nature Physics 5, 189 (2009).

[5] “Observation of Interaction-Induced Dispersive Optical Nonlinearities in an Ensemble of Cold Rydberg Atoms”, V. Parigi et al, Phys. Rev. Lett. 109, 233602 (2012)

[6] “Homodyne Tomography of a Single Photon Retrieved on Demand from a Cavity-Enhanced Cold Atom Memory”, E. Bimbard et al, Phys. Rev. Lett. 112, 033601 (2014)

## Colloquium 22th October 2015, 1:30 pm st, Room D326

### Dr. Magdalena Zych

ARC - Centre for Engineered Quantum Systems (EQuS)

University of Queensland

Queensland, Australia

Clocks in superpositions of proper time - testing general relativistic effects in quantum mechanics

Phenomena stemming jointly from quantum theory and general relativity are often thought to be relevant only at high energies and in strong gravitational fields. This colloquium considers low-energy quantum systems under weak time dilation. We study a quantum version of the “twin paradox’’: where a single* *quantum system follows in superposition two world lines with different proper times (e.g.: at different heights above earth) and show that time dilation in general leads to entanglement between internal degrees of freedom and the centre of mass of a particle. This results in new effects which could be tested in earth-based interference experiments with atoms, molecules (or photons). We further derive that time dilation provides a universal decoherence mechanism for sufficiently complex systems, substantial already at micro-scales. Thus far largely overlooked in theoretical research, the regime of low-energy quantum systems under weak gravity is particularly promising for experimental exploration and allows intriguing insights into joint foundation of quantum theory and general relativity.

## Colloquium 15th October 2015, 1:30 pm st, Room D326

### Dr. Oliver Morsch

INO-CNR and Department of Physics - University of Pisa,

Pisa, ITALY

15th october 2015 Kinetic constraints in strongly interacting Rydberg gases, Morsch

Kinetic constraints in strongly interacting Rydberg gases

Cold atoms excited to high-lying Rydberg states exhibit a variety of intriguing effects such as the dipole blockade and facilitated off-resonant excitation, which are both due to the strong van der Waals interaction between the atoms. In my talk I will present recent results on the observation of kinetic constraints in such systems. Kinetic constraints were first conceived to model soft matter systems, which exhibit slow and often complex relaxation to equilibrium. Rydberg gases can be used to simulate the many-body dynamics of soft matter systems, and the kinetic constraints leading to it can be controlled through the frequency of the excitation lasers. Our experimental findings agree well with numerical simulations of a simple model based on an Ising chain, in which the internal states of the Rydberg atoms are represented as interacting spins. I will discuss our experiments, which were carried out in a semi-classical (incoherent) excitation regime, and give an outlook on future work in the quantum (coherent) regime.