AtMol Research Seminar Archive
Index Current Seminars

Michaelmas Term 2007

Date Speaker Institution
BECs in time-periodic fields
September 6th Dr. Karl-Peter Marzlin Calgary, Alberta, Canada

Thursday; 4:15pm; Ph30.

Over the past few years several proposals have been made how to realize artifical Abelian and non-Abelian gauge potentials for ultracold atoms. I will give a pedagical introduction to this field and present our own proposal to achieve an analogous effect for quantized light (photons) propagating through an atomic gas.

In this case, the dynamical equation for photons is equivalent to the Schroedinger equation for charged particles in two dimensions. The corresponding vector potential is completely determined by a set of classical light fields that are used to manipulate the atomic state. We suggest classical field configurations that generate potentials corresponding to a constant electric and a constant magnetic field. Furthermore we devise a scheme of fields that generates a vector potential of Aharonov-Bohm type which induces a topological phase shift for slow photons.

An CO2 optical dipole trap for ultracold 85Rb
September 27th Sylvi Handel Bonn, Germany

Thursday; 12:00 noon; Ph30.

BECs in time-periodic fields
October 10th Prof. Tania Monteiro University College London

Even without interatomic interactions, coherent matter wave dynamics in the presence of time-periodic fields can lead to new behaviour: for example, atoms subjected to short pulses 'kicks' from standing waves of light represents a clean experimental test of quantum chaos; new possibilities arise for manipulating cold atoms coherently, including the generation of directed motion. Recent experimental and theoretical work has probed the role of interatomic interactions. In particular, the question of the dynamical stability of 'kicked' condensates is of particular significance.

I will discuss the mechanism for destabilization of a BEC under time-periodic driving and analyse the origin of a recently proposed "critical stability border" for kicked BECs. I will also discuss the dynamics of the 'double-kicked' cold atoms which were investigated experimentally and theoretically at UCL. The presence of a small additional time-scale has a strong effect on the dynamics of the system, both on the classical limit as well as the quantum dynamics with or without interactions.

Topological vortex rings in BECs and ferromagnets
October 17th Prof Paul Sutcliffe Mathematical Sciences

Numerical simulations are used to compute vortex rings carrying topological charges. Two different systems are studied. The first is a Gross-Pitaevskii equation describing a 2-component BEC and the second is a Landau-Lifshitz equation which models the dynamics of a ferromagnet.

Bose gas in Flatland
October 31st Dr. Zoran Hadzibabic University of Cambridge

Physics of a Bose gas in 2D is quite different from the usual 3D situation. In a homogeneous 2D fluid of identical bosons long-range order is always destroyed by long wavelength thermal fluctuations, but the system can nevertheless become superfluid at a finite critical temperature. This phase transition does not involve any symmetry breaking and in the Berezinskii-Kosterlitz-Thouless (BKT) paradigm it is explained in terms of binding and unbinding of pairs of vortices with opposite circulations. Above the critical temperature, proliferation of unbound vortices is expected.

Using optical lattice potentials we can create two parallel, independent 2D atomic clouds with similar temperatures and chemical potentials. When the clouds are suddenly released from the trapping potential and allowed to freely expand, they overlap and interfere. This realizes a matter wave heterodyning experiment which gives direct access to several features of the phase distributions in the two planes. Long wavelength phase fluctuations create a smooth and random variation of the interference fringes and free vortices appear as sharp dislocations in the interference pattern. Both the temperature study of these effects, and the measurements of the critical point support the BKT picture of the development of quasi-long-range coherence in these systems.

[1] Zoran Hadzibabic, Peter Kruger, Marc Cheneau, Baptiste Battelier, and Jean Dalibard, Nature 441, 1118 (2006).

[2] Peter Kruger, Zoran Hadzibabic, and Jean Dalibard, Phys. Rev. Lett. 99, 040402 (2007).

Matter wave optics : "supersonic" metastable atoms scattered by van der Waals interactions with solid surfaces
November 2nd Dr. Jean-Christopher Karam Universite Paris

Friday; 12:00 noon; Ph30.

We have realised a bright source of mestastable rare gas atoms that are produced by metastability exchange collisions within a supesonic beam scattered by electrons from an electron gun. It allows to resolve diffraction peaks by 100 nm-period transmission grating.

Van ver waals interaction between an atom and a solid surface results in both an global energy shift and a quadrupolar tensor potential acting on m sublevels. In a scatering experiment at velocities of a few 100m/s, I have made a simple model to estimate the transition probabilities.

We have observed experimentally Magnetic field dependent atom surface scattering at angles oaf a fex mrad, which range is in good agreement with the developped model. Several peaks are observed which show how Zeeman enery is transferred to the motion in transverse direction.

Novel interactions in quantum gases
November 7th Prof. Tilman Pfau Universität Stuttgart

5:15pm; Ph30.

Interactions in atomic quantum gases make them a model system for many branches of physics including condensed matter, and nonlinear dynamics. So far all the impressive phenomena (like superfluidity, soliton and vortex formation, BEC-BCS crossover etc.) in atomic Bose and Fermi gases are caused by a contact interaction, originating from s-wave scattering off the van der Waals potential.

Here we report on our experiments on Bose Einstein condensation in a gas of chromium atoms [1] with a sizable magnetic dipolar interaction. By the use of Feshbach resonances [2] the contact interaction can be suppressed such that the dipolar interaction becomes equally important. Here we report on the investigation of the resulting quantum ferro fluid [3] and the observation of the dipolar collapse.

We also report on our experiments on interacting ultracold Rydberg atoms excited from a Rb Bose-Einstein condensate or a thermal cloud. We observed a blockade of the Rydberg excitation due to the strong repulsive van der Waals interaction of Rydberg atoms in the 43 s state. We confirm the coherent collective nature of the excitation by the measurement of the scaling law for the excitation time [4]. We confirm the coherence of excitation by reversing the excitation by a ¹ phase-shift in the driving laser field, a so called rotary echo sequence [5].

[1] A. Griesmaier, J. Werner, S. Hensler, J. Stuhler, and T. Pfau, Phys. Rev. Lett. 94, 160401 (2005)
[2] J. Werner, A. Griesmaier, S. Hensler, J. Stuhler, T. Pfau, A. Simoni and E. Tiesinga, Phys. Rev. Lett. 94, 183201 (2005)
[3] Th. Lahaye, T. Koch, B. Fršhlich, M. Fattori, J. Metz, A. Griesmaier, S. Giovanazzi, T. Pfau, Nature 448, 672 (2007)
[4] R. Heidemann, U. Raitzsch, V. Bendkowsky, B. Butscher, R. Lšw, L. Santos, T. Pfau, Phys. Rev. Lett. 99, 163601 (2007)
[5] U. Raitzsch, V. Bendkowsky, R. Heidemann, B. Butscher, R. Lšw, T. Pfau arXive: quant-phys / 0706.2639 (2007)

Bose-Einstein Condensates in Optical Lattices and Superlattices
November 14th Dr. Mason A. Porter California Institute of Technology
University of Oxford

3:00pm; Ph30

Bose-Einstein condensates (BECs), formed at extremely low temperatures when particles in a dilute gas of bosons condense into the ground state, have generated considerable excitement in the atomic physics community, as they provide a novel, experimentally-controllable regime of fundamental physics. In this talk, I will discuss my research on the macroscopic dynamics of coherent structures in BECs loaded into optical lattice and superlattice potentials, for which I employ methods from dynamical systems and perturbation theory. Among other topics, I will discuss "multiple-period" and "fractional-period" spatially extended solutions, solitary wave manipulation, Feshbach resonance management, and spatially dependent scattering lengths.

In Pursuit of Pulsars
November 14th Prof. Jocelyn Bell Burnell Oxford University

4:15pm; Lecture Theatre 202, Calman Learning Centre

Professor Dame Jocelyn Bell Burnell, Visiting Professor of Astrophysics at Oxford University and a Fellow of Mansfield College, is best known for her involvement in the discovery of pulsars, which opened up a whole new branch of astrophysics, and her tireless work to encourage more women to consider a career in science. Her career highlights include 10 years as Professor of Physics at the Open University, managing the James Clerk Maxwell Telescope in Hawaii, and a spell as President of the Royal Astronomical Society. She was made a CBE in 1999 and a Dame in June 2007 and was a recipient of one of Durham University's 175th Anniversary honorary degrees at the special congregation in the Summer.

Molecular Astronomy
November 21st Prof Peter Bernath York

Venue: CG60
Time: 4:15pm

In recent years the study of cool stellar and sub-stellar objects such as brown dwarfs and extrasolar planets has become an important area of research. In contrast to typical stars like our Sun (which show mainly atomic absorption lines), the spectra of these cool objects are dominated by molecular absorption. Various molecules including CN, VO, CaH, CrH and H2O are found in these objects and need to be included in models that simulate the observed spectral energy distributions. New laboratory spectra of astrophysically important molecules are recorded and combined in an optimal way with theoretical predictions to produce the molecular opacities needed for stellar models. Our work, for example, led to the observation of .Water on the Sun.. In addition, new astronomical observations of the spectra of cool objects such as brown dwarfs are carried out at observatories in Chile, Hawaii and Arizona. (See: Science 268, 1155 (1995); ApJ 594, 651 (2003))

Building Planets and the Ingredients of Life between the Stars
November 21st Professor Ewine F. van Dishoeck Leiden Observatory

Grubb Parsons Lecture
Venue: Appleby Lecture Theatre
Time: 4:30pm

One of the most exciting developments in astronomy is the discovery of planets around stars other than our Sun. More than 200 exo-planets have now been detected. But how do these planets form, and why are they so different from our own solar system? Which ingredients are available to build them? Thanks to powerful new telescopes, astronomers are now starting to address these age-old questions. In this talk, an overview will be given of how stars and planets are born in the extremely cold and tenuous clouds between the stars in the Milky Way. These clouds also contain a surprisingly rich variety of organic material. Which chemical compounds do we find in space? Can they end up on new planets and form the basis for pre-biotic material?

November 28th Prof. Carlo Barenghi Newcastle University

The motion of quantised vortices, either in superfluid helium or in an atomic Bose - Einstein condensate, is not dissipation-free. At finite temperatures, their energy is reduced by the interaction with the normal fluid and the thermal cloud respectively. To illustrate this effect, I shall present recent results of the motion of vortices in a trapped condensate at finite temperature.

Molecules at Nanokelvin Temperatures
December 5th Prof. Hanns-Christoph Nägerl Universität Innsbruck

At ultralow temperatures in the nanokelvin range, the wave nature of particles plays a dominant role. Atomic gases, under suitable conditions, condense into the state of a Bose-Einstein condensate (BEC). The interaction strength in a BEC can be controlled as scattering of particles is resonantly enhanced or, alternatively, suppressed when an external magnetic field is applied. In our experiments with BECs of Cs atoms we study resonant scattering processes ranging from the formation of ultracold dimer molecules to the observation of three-particle and four-particle scattering resonances. For instance, we investigate the signature of exotic three-body Efimov states in atom-atom-atom and in atom-dimer scattering. In the presences of periodic optical lattice potentials, we use our control over the strength of the interaction to drive the superfluid-to-Mott-insulator quantum phase transition and to observe long-lasting Bloch oscillations. We will briefly discuss our newest results on mixtures of quantum gases and on high-precision molecular spectroscopy.

Optical quantum computing with photonic and matter qubits
December 12th Dr. Pieter Kok Sheffield

Venue: James Knott Library

In this talk I will give an overview of optical quantum computing with photons and matter qubits. Whereas in principle it is possible to do scalable quantum computing with only photons, linear optics, and photo-detection, there are huge benefits to using matter qubits with optical transitions. This leads naturally to the use of the one-way model of quantum computing. Recent experiments in Michigan and Paris indicate that this is a feasible route to quantum computing.