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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 05th February 2015, 1:30 pm st, Room D326

Prof. Dr. Augusto Smerzi
QSTAR, INO-CNR and Lens,
Firenze, Italy

29th January  - 05th February  2015 Quantum Interferometry, PDF

Distinguishability of Quantum States:from Interferometry to Bell Inequalities and Quantum Zeno Dynamics

A most striking physical consequence of entanglement is non-locality. We show that entanglement is also deeply connected to the concept of distinguishability of quantum states. This because, under the same Hamiltonian, entangled states can evolve faster in the probability space than classical states: Entanglement is high distinguishability speed. This has important implications in technological applications like interferometry and in foundational problems like non-locality and the quantum Zeno paradox. 

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

Prof. Dr. Marianna Safronova
University of Delaware and Joint Quantum Institute
University of Maryland and NIST,  Newark, USA

January 22, 2015 Atomic Clocks and the Search for Variation of Fundamental Constants PDF

Atomic Clocks and the Search for Variation of Fundamental Constants

Recent advances in atomic and optical physics have led to unprecedented improvements in the optical frequency metrology leading to more sensitive quantum-based standards for various applications including search for variation of fundamental constants.I will describe the recent advances in theoretical calculations relevant to atomic clock research and review the present status of the blackbody radiation shifts for all frequency standards that are currently being developed. In the second part of my talk, I will review the present status of the laboratory and astrophysical searches for the variation of the fine-structure constant, and discuss future proposals based on highly-charged ions. 

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

Dr. Marco Fattori
CNR-INO LENS, University of Firenze
Firenze, Italy

 - not available -

Atom interferometry with trapped BEC with tunable interactions

We report on the operation of an atom interferometer with trapped Bose Einstein condensates of potassium 39 where interactions can be tuned using broad magnetic Feshbach resonances. An innovative double well trapping potential allows to split and recombine  coherently the atomic matter wave in two distinct spatial modes. Canceling the homo-nuclear scattering length it is possible to achieve long coherence times and demonstrate the operation of a sensor with high sensitivity and high spatial resolution in the measurement of forces. In addition we report on preliminary studies performed with condensates with repulsive and attractive interactions. A tunable positive non linearity allows to study the Rabi-Josephson transition in a bosonic Josephson Junction while a negative one allows to observe a parity breaking quantum phase transition. We will conclude describing the advantages offered by our system in the production of non-classical states for quantum enhanced metrology.

Colloquium 08th January 2015, 1:30 pm st, Room D326

Prof. Dr. Axel Görlitz
Institut für Experimentalphysik, Heinrich-Heine-Universität Düsseldorf
Düsseldorf, Germany

15th January 2015, Photoassociation spectroscopy of RbYb - On the way towards paramagnetic dipolar molecules - Görlitz

Photoassociation spectroscopy of RbYb - On the way towards paramagnetic dipolar molecules

The creation of ultracold heteronuclear molecules with anisotropic electric dipole interaction is one of the prominent goals in ultracold atom physics. While the widely used bialkalis possess no magnetic moment in the electronic ground state, diatomic molecules with an unpaired electron are paramagnetic and thus have an additional degree of freedom. One example for such a molecule is RbYb which is at the focus of the experimental investigations in our group.

Our most recent step towards the creation of ultracold RbYb ground state molecules is photoassociation spectroscopy in a conservative trap. In a newly designed trap consisting of a magnetic trap for Rb and an optical trap at 556 nm for Yb we perform one-photon photoassociation spectrosocopy on weakly-bound states of excited Rb*Yb molecules. This combines our previous studies on photoassociation spectroscopy of RbYb in a magnetooptical trap and simultaneous conservative trapping of the two species.

 

 

 

Colloquium 11. December 2014, 1:00 pm st, Room D326

Prof. Dr. Tilman Pfau
5. Physikalisches Institut, Universität Stuttgart
Stuttgart, Germany

 - not available - 

A single charge in a Bose-Einstein condensate: from two to few to many-body physics

Electrons attract polarizable atoms via a 1/r^4 potential. For slow electrons the scattering from that potential is purely s-wave and can be described by a Fermi pseudopotential. To study this interaction Rydberg electrons are well suited as they are slow and trapped by the charged nucleus. In the environment of a high pressure discharge Amaldi and Segre, already in 1934 observed a lineshift proportional to the scattering length [1].

At ultracold temperatures and Rydberg states with medium size principle quantum numbers n, one or two ground state atoms can be trapped in the meanfield potential created by the Rydberg electron, leading to so called ultra-long range Rydberg molecules [2].

At higher Rydberg states the spatial extent of the Rydberg electron orbit is increasing. For principal quantum numbers n in the range of 100-200 and typical BEC densities, up to several ten thousand ground state atoms are located inside one Rydberg atom, We excite a single Rydberg electron in the BEC, the orbital size of which becomes comparable to the size of the BEC. We study the coupling between the electron and phonons in the BEC [3].

We also observe evidence for ultracold charge transfer processes for a single ion which is shielded by a Rydberg electron. Also reactive processes due to few-body Langevin dynamics involving a single ion can be studied.

As an outlook, the trapping of a full condensate inside a Rydberg atom of high principal quantum number and the imaging of the Rydberg electron's wave function by its impact onto the surrounding ultracold cloud seem to be within reach [4].

[1] E. Amaldi and E. Segre, Nature 133, 141 (1934)

[2] C. H. Greene, et al., PRL 85, 2458 (2000); V. Bendkowsky et al., Nature 458, 1005 (2009)

[3] J . B. Balewski,  et al., Nature 502, 664 (2013)

[4] T. Karpiuk, et al., arXiv:1402.6875

 

 

Colloquium 27. November 2014, 1:30 pm st, Room D326

Sylvain Schwartz
Thales Research and Technology
Palaiseau, France

 - not available -

Transparent atom chips and microwave-stimulated Raman adiabatic passage in a Bose-Einstein condensate

In this talk, I will report the experimental work in progress at Thales Research and Technology France with atom chips.

I will first describe our study of transparent atom chips made with silicon carbide. We have observed a very favorable thermal behavior, and demonstrated the possibility to create a magneto-optical trap with some of the laser beams passing through the chip. I will discuss some potential applications of this technique including detecting the atoms through the chip with high numerical aperture, and combining the advantages of atom chips with more complex systems requiring full optical access to the atoms.

In a second part, I will report our recent results on microwave-stimulated Raman adiabatic passage (STIRAP) in a Bose-Einstein condensate (BEC). Using a combination of two microwave frequencies with the appropriate time sequence, we are able to transfer a 87Rb BEC from the Zeeman sublevel |F=2,mF=1> of the 52S1/2 ground state to the |F=1,mF=-1> sublevel of the same state with 90% efficiency in less than 1ms, with all the benefits of the STIRAP protocol in terms of robustness to the fluctuations of external parameters. I will describe the basic principles of STIRAP and discuss the potential applications in the context of atom chips.

Finally, I will describe the experimental protocol we are developing to build an interferometer involving trapped thermal atoms with reduced mean-field effects, based on microwave dressing with two coplanar waveguides on the atom chip.

 

 

 

Colloquium 20. November 2014, 1:30 pm st, Room D326

Francesca Ferlaino
Institute for Experimental Physics
Innsbruck, Austria

 - not available -

Dipolar physics with ultracold atomic magnets

Given their strong magnetic moment and exotic electronic configuration, rare-earth atoms disclose a plethora of intriguing phenomena in ultracold quantum physics. Here, we report on the first degenerate Fermi gas of erbium atoms, based on direct cooling of identical fermions via dipolar collisions [1]. We study the impact of the anisotropic character of the interaction following the re-thermalization dynamics of a dipolar Fermi gas driven out of equilibrium [2]. At the many-body level, we prove the long-standing prediction of a deformed Fermi surface in dipolar gas [3]. Finally, scattering experiments show a spectacularly high number of Fano-Feshbach resonances. This complexity, arising from the anisotropy of the interactions, escapes to traditional scattering models and requires novel approaches based on statistical analysis. Using the powerful toolset provided by Random-Matrix theory, we elucidate the chaotic nature of the scattering [4].

 

[1] K. Aikawa, A. Frisch, M. Mark, S. Baier, R. Grimm, and F. Ferlaino, 
Phys. Rev. Lett. 112, 010404 (2014).

[2] K. Aikawa, A. Frisch, M. Mark, S. Baier, R. Grimm, J. L. Bohn, D. S. Jin, G. M. Bruun, F. Ferlaino
arXiv:1405.1537 (2014)

[3] K. Aikawa, S. Baier, A. Frisch, M. Mark, C. Ravensbergen, F. Ferlaino
arXiv:1405.2154 (2014)

[4] A. Frisch, M. Mark, K. Aikawa, F. Ferlaino, J. L. Bohn, C. Makrides, A. Petrov, and S. Kotochigova, Nature 507, 475-479 (2014).


Colloquium 13. November 2014, 1:30 pm st, Room D326

Dr. Benjamin Canuel
Institut d'Optique d'Aquitaine LP2N - Laboratoire Photonique, Numérique et Nanosciences
Talence, France

 - not available -

The MIGA experiment, towards sub-Hz GW detection with atom interferometry

The concepts of rotation and angular momentum are ubiquitous across quantum physics, whether In the last decades, several ground-based gravitational wave detectors based on optical interferometry were built and operated. Under few tens of Hz, such experiments are limited by several sources of cavity length noise that mimic the effect of gravitational waves (seismic noise, radiation pressure noise, thermal noise...). We are building a new, hybrid detector called MIGA (Matter-wave laser Interferometer Gravitation Antenna) that couples atomic and optical interferometry to study the strain tensor of space-time and gravitation at lower frequencies. The MIGA interrogation scheme will allow to read GW signals free of cavity length noise which opens new perspective for ground based GW detection. This underground detector, that will be installed in Rustrel (France), will consist in a set of atomic interferometers simultaneously manipulated by the resonant optical field of a 200 m cavity. The new experimental concept of MIGA will allow applications in fundamental physics but also in geoscience.

In this talk, I will present the experimental concept and MIGA and put it in perspective with respect to purely optical GW detectors.

 


Colloquium 11. November 2014, 10:00 am st, Room 110, Paschen-Bau (Braunschweig)

Dr. Silvio Koller
Joint Quantum Institute
University of Maryland, NIST Gaithersburg, 

11 November 2014 Spin exchange mediated Dynamics of Anti-Ferromagnetic Order PDF

Spin exchange mediated Dynamics of Anti-Ferromagnetic Order in an Extended 2D Optical Lattice

We study the dynamics of staggered magnetisation of bosons in an extended doublewell 2D optical lattice initially in anti ferromagnetic order. After quenching the lattice depth and applying a checker board staggering that can be spin dependent and spin independent, we measure the staggered magnetisation and transport. We find time scales spanning over 3 orders of magnitude by varying the lattice potential in depth and staggered offset. We find two distinct timescales which are shown be direct tunneling and spin exchange. By tuning the staggering we find distinct resonances for tunneling and spin exchange.

 

Colloquium 10. November 2014, 12:00 am st, Room D326

Prof. Dr. Mikhail Lemeshko
Institute of Science and Technology
IST, Austria

 - not available -

Dynamics of quantum rotation in the presence of a many-body environment

The concepts of rotation and angular momentum are ubiquitous across quantum physics, whether one deals with the lifetimes of unstable nuclei, accuracy of atomic clocks, or electronic structure of defect centers in solids. Pioneered by the seminal works of Wigner and Racah, the quantum theory of angular momentum evolved into a powerful machinery, commonly used to classify the states of isolated quantum systems and perturbations to their structure due to electromagnetic or crystalline fields. In “realistic” experiments, however, quantum systems are almost inevitably coupled to a many-particle environment and a field of elementary excitations associated with it, which is capable of fundamentally altering the physics of the system.

We present the first systematic treatment of quantum rotation coupled to a many-particle environment. By using a series of canonical transformations on a generic microscopic Hamiltonian, we single out the conserved quantities of the problem. Using a variational ansatz accounting for an infinite number of many-body excitations, we characterize the spectrum of angular momentum eigenstates and identify the regions of instability, accompanied by emission of angular Cerenkov radiation.

The developed technique can be applied to a wide range of systems described by the angular momentum algebra, from Rydberg atoms immersed into BEC’s, to cold molecules solvated in helium droplets, to ultracold molecular ions.

 

 

 

 

 

 

Colloquium 06. Novemberber 2014, 1:30 pm st, Room D326

Prof. Dr. Andreas Hemmerich
Institute for Laser Physics
University of Hamburg, Germany

 - not available -

When bosons condense in excited states

Bosons at very low temperatures are known to undergo Bose-Einstein condensation and gather in the ground state, often with intriguing consequences like superfluidity. The many-body wave function thus formed is positive real under most general circumstances and hence topologically trivial as has been early pointed out by Feynman. However, under certain conditions, condensation can also occur in metastable excited states, which can lead to highly non-trivial superfluid order with unusual properties. I will introduce the experimental arena of optical lattices, where atomic gases crystallized in a web of light are used to simulate ultracold condensed matter, and discuss our recent observations of unconventional forms of superfluidity in metastable higher bands.

 

 

Colloquium 23. October 2014, 1:30 pm st, Room D326

Hanns-Christoph Nägerl
Institute for Experimental Physics
Innsbruck, Austria

23 October 2014 Dynamics in one-dimensional chains of Bosons, PDF

Dynamics in one-dimensional chains of Bosons

Ultracold atoms are an ideal setting to study non-equilibrium quantum many-body dynamics in a very controlled way. I will present a series of experiments in the context of strongly correlated atomic bosons in one-dimensional geometry.

Specifically, we study the dynamics of one-dimensional chains after a sudden quench of the system’s Hamiltonian, for which we independently control J, the (coherent) tunneling rate, U, the strength of the interaction, and E, a tilt along the longitudinal direction of the chains. For a quench to U≈E we couple to nearest neighbors collectively and observe characteristic oscillations in the number of double occupancies that we analyze in the many-body context [1]. For U/2≈E, U/3≈E etc. we observe collective long-range tunneling to next-nearest neighbors and beyond. In particular, for U/3≈E we observe dynamics due to the higher-order super-exchange interaction scaling as J^3/U^2 [2]. For J≈U<<E we observe interaction-induced quantum phase revivals, and for J≈U≈E we find evidence for the transition to the quantum chaotic regime [3].

If time allows, I will give an outlook on our endeavor to realize bosonic systems with "real" long-range interactions.