AtMol Research Seminar Archive
Index Current Seminars

Michaelmas 2008

Date Speaker Institution
  Multiparticle entanglement in rotating ultra-cold atomic systems
8th October Dr. David Hallwood Leeds University

Ultra-cold atomic systems provide an ideal playground for experiments on entanglement. Their extremely large coherence times, absence of impurities and high degree of control offers the detailed study of strongly correlated systems. Furthermore, Feshbach resonances, optical lattices and dipole interactions allow the tuning of interactions between atoms from weak short range interactions to strong long range interactions where more complex many body effects can be seen. Finally, we can use atomic systems to better understand solid state systems. The large length scales of ultra-cold atomic systems open up many possible measurement techniques that can measure velocities, densities, and correlations that are not available in solid state systems. My talk will focus on the most famous entangled state, the Schrodinger cat state. I will describe a scheme for making ultra-cold atoms trapped in a torus potential with a barrier form a superpositions of all atoms stationary and all atoms moving with one quantum of flow. Modelling a large number of atoms with many modes cannot be done directly and mean field approaches like the Gross-Pitaevskii equation loose the entanglement properties of the system. Instead I will use the adiabatic approximation to treat the loop semi-classically and the barrier quantum mechanically. This reveals that the barrier is the crucial ingredient in making the superposition.

  Cold Atoms: From the Big Bang to the Ultimate Chicken Soup Machine
15th October Dr. Tom Judd Nottingham University

Mean-field methods often describe cold atom systems with great success but there are interesting examples where extra physics is required, particularly if inter-atomic interactions are large or finite-temperature effects are important. In this talk, I will consider two recent experiments that we have analysed using large simulations where mean-field methods fail to predict the results. We first examine a phase fluctuating atom interferometer, and go on to consider an interacting Fermi gas undergoing collective oscillations. In both cases, we postulate simple additions to the mean-field picture that allow us to recreate the experimental results and explain the underlying physical mechanisms. We find that fluctuations play a crucial role in these systems and I will end with a brief section on fluctuating cold atom methods in a broader context.

  Bright matter wave solitons: formation, dynamics and quantum reflection
22nd October Miss Sylvi Haendel Durham University

We are building a new apparatus to study the formation and the dynamics of bright matter wave solitons during the collapse of a 85Rb BEC. The collapse is induced by using a Feshbach resonance to suddenly change the atomic interactions from repulsive to attractive. Observations reveal the formation of multiple solitons oscillating along the weak direction of the trap colliding repeatedly at the trap centre. Simulations based upon Gross-Pitaevskii theory show that the nature and stability of these collisions depend critically on the relative phase and velocity of the solitons and suggest the need for a relative phase of p to explain the experimental observations; but this is still under debate. For intermediate phases population transfer between solitons occurs, suggesting potential applications in atom interferometry and precision measurement. We plan to explore this potential application in the study of atom-surface potentials and quantum reflection.

  Testing time reversal symmetry with cold molecules
29th October Dr. Jony Hudson Imperial College, London

I will describe how sensitive measurements of molecular properties can be used to test fundamental physical principles. In our laboratory we are looking for violations of time-reversal symmetry --- searching for a microscopic arrow of time, if you like. I will describe what time-reversal symmetry is, how it could be broken, and why it is centrally related to the very existence of a material universe. I'll go on to describe our experimental approach: measuring interactions of the esoteric molecular radical YbF with mind-bogglingly high resolution. I'll try to give an idea of the challenges faced when making such a sensitive measurement, outlining the spectroscopic and molecular manipulation techniques that we have developed to overcome them. Finally, I'll talk about data analysis and how we can be sure that what we think we're measuring is really what we are measuring!

  Measuring the time-variation of fundamental constants using cold molecules
5th November Dr. Rick Bethlem Vrije Universiteit Amsterdam

The recent demonstration of cooling and manipulation techniques for molecules offer new possibilities for precision measurements. At the LCVU, we are constructing a molecular fountain based on a Stark decelerated molecular beam. In this fountain, ammonia molecules are decelerated, cooled, and subsequently launched upwards some 10-50 cm before falling back under gravity, thereby passing a microwave cavity twice - as they fly up and as they fall back down. In this way the inversion frequency in ammonia can be measured at unique precision. This measurement may be used as a test of the time-variation of fundamental constants.

12th November Prof. Kai Bongs Birmingham University
  Stark-shift induced resonances in multiphoton ionization
19th November Dr. Robert Potvliege Durham University

Multiphoton resonances between the ground state and Stark-shifted excited states play an important role in the detachment of electrons from atoms exposed to strong laser pulses, even at peak intensities as high as 100 to 500 TW/cm2. The seminar will be a tutorial introduction to this topic and to multiphoton processes in intense fields in general.

  Laser Cooling of Calcium
26th November Prof. Erling Riis University of Strathclyde

An update will be given on the laser cooling activities at Strathclyde. Our current approach in the aim of observing interference in our existing magnetic ring trap is described. This work has led to ideas being developed for a new generation of ring traps based on electro-magnetic induction. These traps will provide a smooth toroidal trapping potential, that can be transformed adiabatically into a standard quadrupole potential for ease of loading. The technique is scalable and suitable for micro-fabrication opening tantalizing prospects for manufacturing several ring interferometers on an atom chip. Finally our new experiment on laser cooling and trapping of atomic calcium will be described. New techniques have been developed for laser stabilization as well as repumping.

3rd December Dr. Sebastian Slama University of Teubingen

I will speak about the interaction of ultracold atoms with surfaces. This topic is very interesting for several reasons, for example from a fundamental point of view for the measurement of van der Waals like surface potentials and the understanding of surface effects like adsorption. From a quantum optical point of view it is interesting to coherently couple atomic matter waves with evanescent light waves for nondestrcuctive atom detection with the prospect of QND measurements of atom numbers. Evanescent light waves are also interesting for technological reasons because by the use of surface plasmons it might be possible to produce nanostructured surface traps. In Tuebingen we have made a first step towards these goals and have set up an experiment in which we bring Bose-Einstein condensates very close to the surface of a glass prism. We are able to position the atoms in a controlled way to distances from the surface below one micrometer by loading them into a combined magnetic and evanescent wave surface trap. I will present first measurements with our setup and discuss our plans for the future.

10th December Dr. Dieter Jaksch Oxford University
  Universal scaling and coherence properties of an ultracold Rydberg gas
17th December Dr. Ulrich Raitzsch Durham University

I will present two recent experiments on the properties of ultracold Rydberg gases. The first part of the talk will be discussing the universal scaling of characteristic quantities of the Rydberg excitation dynamics. It has theoretically been shown that the excitation into a Rydberg state exhibits a second order quantum phase transition and, thus, shows a universal scaling, if the system is strongly blockaded due the interaction between Rydberg atoms. The second part of the talk is discussing the measurements of dephasing rates of a sample of ultracold Rydberg atoms. For this purpose two complementary experiments are shown. The first one will use the `rotary echo' sequence known from the research field of nuclear magnetic resonance, whilst the second experiment uses electromagnetically induced transparency involving a Rydberg state to obtain a measure for the dephasing caused by the interaction between Rydberg atoms.