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Research Training Group 1729/Leibniz Universität Hannover
Logo Leibniz Universität Hannover
Research Training Group 1729/Leibniz Universität Hannover
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Colloquium 14th July 2016, 1:30 pm st, Room D326

Prof. Dr. Tommaso Calarco
Institut für Komplexe Quantensysteme
Universität Ulm

Bringing quantum technologies to reality

Quantum mechanics is at the basis of all present-day information and communication technologies: to name just two examples, transistors and lasers would be impossible to build without understanding the quantum behaviour of matter and light.

But the control of individual quantum systems has long been considered only a “thought experiment”, something only possible in theory. However, today this is routinely achieved in labs around the world, and it is the basis of quantum technologies.

The exquisite level of control needed for quantum technologies to outperform present-day technologies can be achieved by quantum optimal control theory (QOCT). 

After reviewing the main applications of quantum technologies, ranging from secure communications to ultra-high precision sensing and metrology, and from extremely powerful computers to the simulation of complex materials, I will explain how QOCT can be applied to a few of these fields and present the experimental results obtained with the CRAB (Chopped Random Basis) optimisation algorithm that I proposed and developed in recent years.

I will conclude by outlining the upcoming steps to turn the vision of quantum technologies into reality at European level.

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

Prof. Dr. Jörg Helge Müller
Quantop - Niels Bohr Institute
University of Copenhagen
Copenhagen, DENMARK

Experiments with Cesium atoms trapped around a tapered nanofiber

Over the past 15 years optical fibers thinned to subwavelength diameters have developed into useful tools for a plethora of applications ranging from coupling light to nanoscale optics devices, sensitive detection of biologically relevant molecules, to trapping of atoms for spectroscopy and quantum optics experiments.

After the pioneering work of the Mainz/Vienna group, several research teams around the world are now pursuing experiments to trap and interrogate atoms in the evanescent light field above the surface of such nanofibers. The key motivation for these projects is the high coupling strength achievable due to the strong focusing of light over macroscopic distances.

After an introduction to the possibilities and peculiarities encountered with evanescent light field traps I will discuss several experiments performed in Copenhagen with Cs atoms. We study methods to determine trapped atom number, dispersive interfacing of trapped ensembles with the guided light modes, microwave spectroscopy, and transient Bragg reflection from structured ensembles.

Colloquium 05th July 2016, 2:00 pm st, Room D326

Prof. Dr. Rejish Nath
Indian Institute of Science Education and Research (IISER)
University of Pune

Pancake-like Dipolar condensates with tilted dipoles

With the recent experimental realization of Erbium and Dysprosium condensates, there has been a revival of studies in dipolar condensates, especially on the quest of new states of matter. Here, we address the properties of pancake-type or quasi-2D condensates with tilted dipoles.
In particular,  the possibility of generating two-dimensional anisotropic solitons using an adiabatic approach, which has so far been a difficult task experimentally. In addition, on the dynamical preparation of stable structured states, may be an en route to supersolid phase in the dipolar condensates.

Colloquium 30th June 2016, 1:30 pm st, Room D326

Prof. Dr. Helmut Ritsch
Institut für Theoretische Physik
Universität Innsbruck
Innsbruck, AUSTRIA

A free space selforderd atom-photon crystal

The dispersive interaction of a cold gaseous medium with electromagnetic waves involves an exchange of energy and momentum leads to a wealth of intriguing nonlinear dynamical phenomena. A prominent example is self-ordering, whereby the particles spontaneously form stationary periodic structures, simultaneously maximising Bragg scattering and minimizing their potential energy. This has been experimentally observed with thermal gases and more recently also with Bose-Einstein condensates (BECs) inside optical resonators.The latter selects a discrete set of electromagnetic modes so that self-ordering leads to discrete symmetry breaking.

As a new regime we study steady-state self-ordering with a driven BEC in a free-space configuration, where no pre-selected wave vectors are imposed by the mirror boundary conditions. Hence self-ordering amounts to a crystallization breaking a continuous symmetry and showing an intrinsic length scale. In the self-ordered phase both matter and light crystallize: the BEC into a supersolid and the light field into an optical standing wave pattern. As a decisive signature of crystallization the close to infinite range atomic interactions generate a gapped acoustic phonon branch.

Colloquium 23th June 2016, 1:30 pm st, Room D326

Prof. Dr. Ferdinand Schmidt-Kaler
Institut für Physik,
Johannes Gutenberg-Universität Mainz

Trapped ion quantum computing

Quantum computing is fascinating fundamental science but may lead to future applications as well.

For the implementation of qubit operations and quantum algorithms in trapped ions, I discuss the fidelity and speed of operations. This regards the internal qubit state manipulation in single and two-qubit operations. However, for investigating scalable quantum computing, segmented ion traps are used. Here, we need to shuttle ions, separate them or merge single ion into linear crystals, and reorder the ions, according to the needs of a quantum algorithm. I also discuss the speed and quality of such qubit-register-reconfiguration operations. Time scales of these operations on the internal qubit state and the position of ions are several orders of magnitude faster as compared with the measured time scale of coherence. Quantum error correction is the most urgent next level to keep logic qubits alive forever.

Rydberg excitations of trapped ions are a novel, complementary approach for quantum gate operations and I describe the most recent results from the Mainz ion-trapping group.

Colloquium 16th June 2016, 1:30 pm st, Room D326

Prof. Dr. Erling Riis
Experimental Quantum Optics and Photonics
Department of Physics
University of Strathclyde

Interferometry with BECs

Atom interferometers allow the measurement of forces through detection of the differential phase shifts induced in the atomic wavefunction by the interaction. The atomic phase can then be read out against a lab-frame reference, typically the spatial phase of an optical standing wave. This readout is a leading limitation to practical measurement, requiring long temporal stability of the optical phase, without which the resolution of the atomic signal can be lost. An atom interferometer is described, that is inherently insensitive to the phase noise of the readout system. Starting with a Bose-Einstein condensate it is based on tuneable, high-fidelity, symmetric atomic-beamsplitters through a multi-pulse Kapitza-Dirac scheme. We use an atomic homodyne detection that transfers the atomic phase into a temporal atomic beat-note, and show how the entire interferometric signal can be read out in a single shot.

Colloquium 09th June 2016, 1:30 pm st, Room D326

Dr. Sebastiaan van de Meerakker
Institute for Molecules and Materials
Radboud University

Taming Molecular Collisions

The study of molecular collisions with the highest possible detail has been an important research theme in physical chemistry for decades. Over the last years we have developed methods to get improved control over molecules in a molecular beam [1]. With the Stark decelerator, a part of a molecular beam can be selected to produce bunches of molecules with a computer-controlled velocity and with longitudinal temperatures as low as a few mK. The molecular packets that emerge from the decelerator have small spatial and angular spreads, and have almost perfect quantum state purity. These tamed molecular beams allow for crossed beam scattering experiments with unprecedented levels of precision and sensitivity [2,3].

I will discuss our most recent results on the combination of Stark deceleration and velocity map imaging. The narrow velocity spread of Stark-decelerated beams results in scattering images with an unprecedented sharpness and angular resolution. This has facilitated the observation of diffraction oscillations in the state-to-state differential cross sections for collisions of NO with rare gas atoms [4,5], and the observation of scattering resonances at low-energy inelastic NO-He collisions [6].


[1] S.Y.T. van de Meerakker, H.L. Bethlem, G. Meijer, Nature Physics 4, 595 (2008)

[2] J.J. Gilijamse, S. Hoekstra, S.Y.T. van de Meerakker, G.C. Groenenboom, G. Meijer, Science 313, 1617 (2006).

[3] M. Kirste, X. Wang, H.C. Schewe, G. Meijer, K. Liu, A. van der Avoird, L.M.C. Janssen, K.B. Gubbels, G.C. Groenenboom, S.Y.T. van de Meerakker, Science 338, 1060 (2012).

[4] A. von Zastrow, J. Onvlee, S.N. Vogels, G.C. Groenenboom, A. van der Avoird,  S.Y.T. van de Meerakker, Nature Chemistry 6, 216 (2014).

[5] S.N. Vogels, J. Onvlee, A. von Zastrow, G.C. Groenenboom, A. van der Avoird, S.Y.T. van de Meerakker, Phys. Rev. Lett. 113, 263202 (2014).

[6] S.N. Vogels, J. Onvlee, S. Chefdeville, A. van der Avoird, G.C. Groenenboom, S.Y.T.  van de Meerakker, Science 350, 787 (2015). 

Colloquium 02nd June 2016, 1:30 pm st, Room D326

Prof. Dr. Géza Tóth
Department of Theoretical Physics and ERC Fellow,
University of the Basque Country UPV/EHU
Bilbao, SPAIN

Witnessing metrologically useful multipartite entanglement

Entanglement witnesses have been used many times in theory and experiments. However, it is not clear that the entanglement detected by them is useful for some task. In this work, we discuss how to construct entanglement witnesses that detect only entangled states that are also useful for quantum metrology.

First, we will present an approach that can detect metrologically useful entanglement in the Dicke states prepared in experiments [1].Then, we introduce a general method that can detect metrologically useful entanglement based on the measurement of a couple of observables, and can be used in a number of experimental setups [2].Finally, we discuss the differences between multipartite entanglement in general and metrologically useful multipartite entanglement.

[1] I. Apellaniz, B. Lücke, J. Peise, C. Klempt, and G. Tóth, Detecting metrologically useful entanglement in the vicinity of Dicke states, New J. Phys. 17, 083027 (2015).

[2] I. Apellaniz, M. Kleinmann, O. Gühne, and G. Tóth, Optimal detection of metrologically useful entanglement, arXiv:1511.05203.

Colloquium 12th May 2016, 1:30 pm st, Room D326

Dr. Oleg Prudnikov
Institut für Laser Physik, SB RAS, Novosibirsk
Novosibirsk, RUSSIA

12th may 2016 Deep sub-Doppler cooling of Mg in MOT formed by light waves with elliptical polarization, Prudnikov

Deep sub-Doppler cooling of Mg in MOT formed by light waves with elliptical polarization

We study magneto-optical trap of 24 Mg atoms operating on the closed triplet 3P2->3D3 (lam = 383.3 nm) transition formed by the light waves with elliptical polarization (e-theta-e* configuration). Compare to well-known trap formed by light waves with circular polarization ( sigma+ - sigma- configuration) the suggested configuration offer the lower sub-Doppler temperature for trapped Mg atoms, that can’t be reached in conventional MOT.

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

Prof. Dr. Artur Widera
Fachbereich Physik der Technischen Universität Kaiserslautern
Kaiserslautern, GERMANY

Dynamics of Single Atoms in Designer Potentials and in Ultracold Gases

Recent advances in manipulation and detection of individual neutral atoms have paved the way to study single particle phenomena in a highly controlled environment. In my talk, I will discuss some of the methods we have developed to cool and trap individual neutral atoms and to interface them with an ultracold gas. Furthermore I will show how the observation of dynamics of single neutral atoms in different environments can shed light onto transport phenomena in various fields, including non-equilibrium statistical physics, classical kinetic theory of gases, and condensed matter quantum physics. I will present results on nonergodic single atom diffusion in a periodic potential, where single particle trajectories are not representative for an atomic ensemble. Moreover, I will show recent results on diffusion of a tagged particle in a gas, where single collisional events distinguish the dynamical regime describing the diffusion dynamics.