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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 24. April 2014, 1:30 pm st, Room D326

Dr. Pierre Cladé
Laboratoire Kastler Brossel
Physique quantique et applications
Paris, France

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Bloch oscillations in atom interferometry : determination of the fine structure constant

We are using an atom interferometer to precisely measure the recoil velocity of an atom that absorbs a photon. In order to reach a high sensitivity, many recoils are transferred to atoms using the Bloch oscillations technique. In this talk, I will present in details this technique and its application to high precision measurement. I will describe in details how this method allows us to perform an atom recoil measurement at the level of 1.3 part per billion. I will also present how we plan to use Bloch oscillations to improve the beamsplitter of the atom interferometer.

Colloquium 08. May 2014, 1:30 pm st, Room D326

Prof. Dr. Stefan Kuhr
University of Strathclyde
Department of Physics
Glasgow, United Kingdom

Towards single-site-resolved detection of fermions in an optical lattice

Ultracold atoms in optical lattices have become a tool to simulate and test fundamental concepts of condensed matter physics, in particular to simulate electrons in solid crystals. Recent experiments with single-site resolution of single atoms at individual lattice sites have resulted in the direct observation of quantum phase transitions, such as the superfluid to Mott insulator transition for bosonic particles [1], and, e.g. single-site addressing [2] and the quantum dynamics of spin-impurities [3].

However, an experimental proof of single-site-resolved detection of correlated phases of ultracold fermions in a lattice is still missing. I will report on our current progress to realise single-site resolved, in-situ imaging and manipulation of strongly correlated fermionic 40K in an optical lattice. Such a system would be an ideal environment to simulate the Fermi-Hubbard Hamiltonian, allowing for the direct observation and characterisation of, e.g., temperature, spin-structure, or entropy distribution of quantum phases such as fermionic Mott insulators, Band insulators or Néel antiferromagnets.

[1] J. F. Sherson, C. Weitenberg, M. Endres, M. Cheneau, I. Bloch, S. Kuhr, Single-atom-resolved fluorescence imaging of an atomic Mott insulator, Nature 467, 68 (2010).

[2] C. Weitenberg, M. Endres, J. F. Sherson, M. Cheneau, P. Schauß, T. Fukuhara, I.Bloch, S. Kuhr, Single-spin addressing in an atomic Mott insulator, Nature 471, 319 (2011).

[3] T. Fukuhara, A. Kantian, M. Endres, M. Cheneau, P. Schauß, S. Hild, D. Bellem, U. Schollwöck, T. Giamarchi, C. Gross, I. Bloch, S. Kuhr, Quantum dynamics of a single, mobile spin impurity, Nature Physics 9

Colloquium 22. May 2014, 1:30 pm st, Room D326

Prof. Dr. Jörg Schmiedmayer
Vienna Center for Quantum Science and Technology (VCQ), Atominstitut, TU-Wien

 Does an isolated many body quantum system relax?

Understanding non-equilibrium dynamics of many-body quantum systems is crucial for many fundamental and applied physics problems ranging from de-coherence and equilibration to the development of future quantum technologies such as quantum computers, which are inherently non-equilibrium quantum systems.

One of the biggest challenges in probing non-equilibrium dynamics of many-body quantum systems is that there is no general approach to characterize the resulting quantum states. Using the full distribution functions of a quantum observable [1,2], and the full phase correlation functions allows us to study the relaxation dynamics in one-dimensional quantum systems and to characterize the underlying many body states.

Interfering two isolated one-dimensional quantum gases we study how the coherence created between the two many body systems by the splitting process slowly dies by coupling to the many internal degrees of freedom available. Two distinct regimes are clearly visible: for short length scales the system is characterized by spin diffusion, for long length scales by spin decay [3]. The system approaches a pre-thermalized state [4], which is characterized by thermal like distribution functions but exhibits an effective temperature over five times lower than the kinetic temperature of the initial system.  A detailed study of the correlation functions reveals that these thermal-like properties emerge locally in their final form and propagate through the system in a light-cone-like evolution [5]. Furthermore we demonstrate that the pre-thermalized state is connected to a Generalized Gibbs Ensemble and that its higher order correlation functions factorize. Finally we show two distinct ways for subsequent evolution away from the pre-thermalized state. One proceeds by further de-phasing, the other by higher order phonon scattering processes.  In both cases the final state is indistinguishable from a thermally relaxed state.  We conjecture that our experiments points to a universal way through which relaxation in isolated many body quantum systems proceeds if the low energy dynamics is dominated by long lived excitations.

Supported by the Wittgenstein Prize, the Austrian Science Foundation (FWF) SFB FoQuS: F40-P10 and the EU through the ERC-AdG QuantumRelax

[1] A. Polkovnikov, et al. PNAS 103, 6125 (2006); V. Gritsev, et al., Nature Phys. 2, 705 (2006);

[2] S. Hofferberth et al. Nature Physics 4, 489 (2008);

[3] M. Kuhnert et al. Phys. Rev. Lett 110, 090405 (2013).

[4] M. Gring et al., Science 337, 1318 (2012); D. Adu Smith et al. NJP 15, 075011(2013).

[5] T. Langen et al. Nature Physics 9, 640–643 (2013).

Colloquim 28. May 2014, 11:00 am st, Room D326

Prof. Dr. Gediminas Juzeliūnas
Institute of Theoretical Physics and Astronomy
Vilnius University

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Artificial electromagnetism and spin-orbit coupling for ultra cold atoms

In the initial part of the talk there will be some background material on the artificial magnetic field and spin-orbit coupling for ul-tracold atoms. Subsequently we shall talk about possibilities to simulate the spin-orbit coupling (SOC) of the Rashba-Dresselhaus type for ultra cold atoms using several laser beams or a sequence of properly chosen magnetic pulses, and discuss manifestations of such a SOC. We shall al-so talk about a recent work on the synthetic gauge fields in synthetic dimensions, as well as on the multicomponent slow light and its experi-mental implementation.

Colloquium 05. June 2014, 1:30 pm st, Room D326

Prof. Dr. Christoph Westbrook
Institut d´Optique / Graduate School
Laboratoire Charles Fabry
Paris, France

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Observing correlated atoms in three dimensions 

I will discuss several cold atom experiments which draw inspiration from the field of quantum optics. Our principal tools are a Bose-Einstein condensate of metastable helium atoms and a microchannel plate detector with which we can reconstruct three dimensional momentum distributions at the single atom level. The detector can play a role similar to similar to a photon counter optics while atomic interactions in a Bose-Einstein condensate can act as a non-linear medium for the generation of non-classical states of the matter wave field. With these two ingredients we can explore the generation of two mode squeezed states via various non-linear processes and attempt to observe their quantum correlations. Our principal current effort is to perform tomography on such states. I will also discuss prospects for generating states with very small particle numbers which could violate Bell inequalities.  

Colloquium, 16. June 2014, 2:00 pm, Room D326

Étienne Wodey
École polytechnique fédérale de Lausanne

Numerical modelling of the coupling of excitonic quantum dots and photonic crystal nanocavities

Colloquium 19. June 2014, 1:30 pm st, Room D326

Prof. Dr. Philippe Bouyer
Laboratoire Photonique, Numérique et Nanosciences
Bordeaux, France

Matter wave optics and interferometry: from concepts to applications

Cooled close to absolute zero, atoms move at velocities of or below a few centimetres per Second and no longer behave as particles, but as de Broglie waves whose propagation can lead to interference phenomena. This presentation will describe how to observe matter-wave interferences, to reproduce, for example, phenomena found in the propagation of electrons in semiconductors. It will also introduce how to use the interferences to build highly accurate measuring devices and use them for guidance and navigation, or perform accurate test of fundamental physics.

Colloquium 26. June 2014, 1:30 pm st, Room D326

Prof. Dr. Fabrice Gerbier
Laboratoire Kastler Brossel
Physique quantique et applications
Paris, France

Antiferromagnetism and Fragmentation in Spin 1 Bose-Einstein Condensates

In this talk, I will present a set of experiments done at Laboratoire Kastler Brossel (ENS Paris) on the magnetic properties of ultracold bosons. After a brief introduction to ultracold gases, I will present an experimental study of the phase diagram of spin-1 bosons with antiferromagnetic interactions. Antiferromagnetic interactions results in this system in an unusual kind of magnetic ordering called spin-nematic ordering, where the order parameter has the symmetry of an ellipsoid as in nematic liquid crystals. I will show how this order can be detected directly by driving coherent Rabi oscillations and looking at the magnetization statistics.

I will finally discuss in details the behavior of the system for small magnetic
fields and magnetizations, where anomalously large fluctuations are observed. We show they can be explained by collective spin fluctuations (fluctuations in the direction of the spin-nematic order parameter), that would vanish in the thermodynamic limit but are important due to the small size (atom number~few thousands) of the samples we study. This illustrates on a particular example how collective fluctuations in small systems are effective to restore a broken symmetry (here spin rotational symmetry).

Colloquium 16. July 2014, 1:30 pm st, Room D326

Sebastian Schmid
Laboratoire de Photonique Numérique et Nanosciences
Institut d'Optique Graduate School IOA
Talence, France

The Matter-Wave Laser Interferometer Gravitation Antenna (MIGA):
Control of a Large Scale Cavity for Matter-Wave Interferometry

We are building a new, hybrid detector that couples laser and matter-wave interferometry to study sub Hertz variations of the strain tensor of space-time and gravitation. Using a novel approach exploiting a set of atomic interferometers simultaneously manipulated by the resonant optical field of a 200m cavity, this instrument will allow at the same time a better understanding of the evolution of the gravitational field and a new tool for gravitational waves (GW) detection. This new infrastructure will be embedded into the LSBB underground laboratory, ideally located away from major anthropogenic disturbances and benefitting from very low background noise.

Each atomic ensemble of the antenna will be manipulated by cavity enhanced Bragg pulses to create an at-om interferometer that will simultaneously read out motion of the cavity, GW and inertial effects. Using the spatial resolution offered by a set of AIs placed along the cavity axis we will separate these contributions. This will bring to unprecedented sensitivities to gravity gradients fluctuations and open new perspectives for sub Hertz GW detection.

Indeed, sensitivity of state-of-the-art GW detectors based on giant optical interferometers is limited under a few tens of Hertz by several sources of cavity length noise that mimic the effects of GW (Newtonian noise, seismic noise, radiation pressure noise...). The MIGA interrogation scheme will allow to read GW signals free of cavity length noise which opens new perspective for ground based GW detection.
In addition, MIGA will also provide measurements of gravity gradients fluctuations limited only by detection noise of single atom interferometers which will allow sensitivities of about 10-13 s-2/Sqrt(Hz) @ 2Hz. This instrument will then be capable to resolve 1 cubic-meter of water a distances of about 100 m bringing new applications in geosciences.

Besides an overview of the whole experiment the talk will give an insight into the technical challenges de-signing and controlling large scale cavities dedicated for atom interrogation in the Bragg-Regime.