AtMol Research Seminar Archive
Index Current Seminars

Epiphany Term 2008

Date Speaker Institution
Microscopic optical devices on atom chips
January 16th Dr. Benoit Darquié Imperial College, London

A high degree of control in the manipulation of atomic quantum states can today be achieved by means of small-scale, integrated devices known as atom chips. These offer the advantage of miniaturisation and flexible control of cold atoms, for applications in atom interferometry, in quantum information and in the study of low-dimensional quantum gases. These applications require the ability to detect and manipulate low atom numbers. For this purpose, we try to integrate microscopic optical components on atom chips.

I will first describe the kind of atom chips we are currently using in the lab to trap, transport or split cold neutral atom clouds or Bose-Einstein condensates near the surface of the chip using magnetic fields.

I will then detail the different microscopic optical tools we are now starting to integrate into atom chips. We have recently performed experiments with atoms in high-finesse microfabricated cavities. A plano-concave resonator consists of an etched spherical mirror and a Bragg stack glued on the tip of a single mode optical fibre. I will show measurements characterising the optical properties of the cavities, as well as the detection of atoms dropped from magneto-optically trapped clouds. The presence of less than one atom on average in the cavity is seen through changes in both the intensity and the noise characteristics of probe light reflected from the cavity input mirror. An excitation laser passing transversely through the cavity triggers photon emission into the cavity mode and hence into the single-mode fibre. We are also working on a future generation of atom chips containing optical waveguides in order to bring light as close as possible to the atoms and collect photons from them with a high efficiency. Waveguides can either be tapered standard single mode optical fibres glued on the surface of the chip, or integrated waveguides manufactured by standard micro-fabrication techniques such as lithography.

An ultracold mixture of diamagnetic and paramagnetic atoms
January 23rd Prof. Axel Görlitz Heinrich-Heine-Universitat Dusseldorf

Studies on mixtures of ultracold atoms are currently attracting significant attention. Among the most prominent results achieved so far are the creation of two-species quantum gases, the discovery of interspecies Feshbach resonances and the production of heteronuclear molecules in the vibrational ground state.

In our experiment, we investigate a mixture of paramagnetic rubidium (Rb) and diamagnetic ytterbium (Yb). The different magnetic and electronic properties make it possible to design a combined trap, in which the two species can be manipulated independently. The trap consists of a Ioffe-Pritchard type magnetic trap for Rb and a bichromatic optical dipole trap for Yb employing. In this novel type of combined trap, we have successfully trapped mixtures of 87Rb and five different Yb isotopes and realized sympathetic cooling of Yb by evaporatively cooled Rb. We have determined the interspecies elastic scattering cross sections and observed a strong dependence of the scattering corss section on the mass of the Yb isotope.

In a second line of experiments, we investigate the possibilities to create RbYb molecules by photoassociation. The first step towards this goal is the investigation of one-photon spectroscopy from the atomic ground state of the two atomic species to an electronically excited state of the heteronuclear RbYb molecule. We have recently been able to observe photoassociation resonances near the Rb D1-transition at 795 nm in a combined magneto-optical trap.

The Application of Quantum Chemistry to Large Systems
January 25th Professor Trygve Helgaker University of Oslo

Friday 25 January 2008 at 12pm

Department of Chemistry, Room CG60

Stereodynamics of elementary reactions: effect of the reagent's rotational angular momentum polarisation
January 30th Dr Jesus Aldegunde Department of Chemistry

Two of the most persistent goals of scientific investigations of the dynamics of molecular collisions are understanding and control. On the one hand, collision dynamicists strive for a detailed understanding of collision mechanisms and of the role of energetic and directional factors in scattering events. On the other hand, they attempt to devise techniques for the control of molecular collisions and, in particular, for the selection of desired collision outcomes. Naturally, the two endeavours go hand in hand. Understanding of collision mechanisms facilitates the development of control schemes, while analysis of the dynamics of controlled collisions can offer important clues about collision mechanisms.

Both issues are directly related to the subject matter of this seminar: to study the stereodynamics of atom-diatom reactions
A + BC --> AB + C
through the consideration of the role of the BC rotational angular momentum polarization in the dynamics of the process. This analysis is performed by means of theoretical tools useful (i) for the analysis of the dependence of reaction mechanisms on reactant polarization, and (ii) for the selection of optimum reactant polarization schemes for the control of reaction probabilities and product state distributions.

The technique will be illustrated by means of examples corresponding to the H+D2 and F+H2 reactions, covering a range of collision energies that goes from several eV to the ultracold regime

Coherent laser spectroscopy of quantum dots
February 6th Dr. Brian Gerardot Heriot-Watt, Edinburgh

Strong quantum confinement has led to the observation of discrete, atom-like energy levels in solid-state quantum dots (QDs). However, for a typical self-assembled QD the confinement potential of a particle spans more than 104 atoms. A natural expectation for such a mesoscopic system is that many-body interactions will dominate and lead to unwanted decoherence. In this talk I will present two experiments which confirm that the quantum coherence is in fact maintained for particles in a QD, both for two and three level systems, even with intense optical excitation. In the first example, a single valence-band hole is trapped in the QD. Using a resonant laser, optical pumping of the hole-spin is achieved. Due to a very long spin-relaxation time, high fidelity (~99%) hole-spin preparation is realized. Furthermore, the spin preparation is achieved even when the spin states are degenerate at zero external magnetic field. This verifies that hole spins not only have reduced interactions with phonons but also negligible hyperfine interaction with the nuclear spins of the QD's constituent atoms. Secondly, I will discuss a two-color pump-probe experiment on the exciton and bi-exciton states in a QD. In the strong-field excitation regime, the dressed-transition states (Autler-Townes splitting) are observed. These results confirm that solid-state mesoscopic systems are a suitable for quantum optical techniques and promising for applications in quantum information processing.

Factorising Numbers with Cold Atoms
February 13th Mr. Paul Halkyard Atomic and Molecular Physics

Number factorisation plays a key role in deciphering encrypted messages. Advancing our capability in this area is an extremely active area of research and is a main theme in the quantum computing community. In this talk I will be discussing a recently published paper[1] where a large number is successfully factorised using a physical system. The system does not rely upon entanglement but instead uses currently available technologies in a relatively simple configuration. Although the time taken to perform such a factorisation scales exponentially, as must be the case, the scheme has great potential to be extended to include state entanglement and, therefore, serves as a useful proof of principle.

[1] "Gauss Sum Factorization With Cold Atoms", M.Gilowski, T.Wendrich, T. Muller, Ch. Jentsch, W.Ertmer, E.M.Rasel and W.P. Schleich, PRL 100, 030201 (2008).

Optical lattice clocks
February 20th Dr. Anne Curtis Imperial College, London
Three-body Efimov states and recombination calculations in ultra-cold gases
February 27th Dr Mark Lee UCL

Since the early days of quantum physics the complex behavior of three interacting particles has been the subject of numerous experimental and theoretical studies. This is of practical interest due to the importance of three-body recombination in limiting the lifetime of ultra-cold gases. Novel three-body bound states known as Efimov states can exist when there is no corresponding two-body bound state. These quantum states refer to an infinite series of energy levels of three identical Bose particles, accumulating at the threshold for dissociation as the scattering length of each pair is tuned to infinity.

In this talk, I will discuss our numerical calculations of the three-body recombination rates of such gases, which incorporates a detailed description of the two-body physics. Signatures for Efimov states emerge from these calculations, and will be compared to experimental evidence for such states. We find good agreement, without having to perform the fitting generally required of other universal treatments.

Designer Atoms: Engineering Rydberg Atoms using Pulsed Electric Fields.
March 5th Prof. F. Barry Dunning Rice University, Houston, Texas

Advances in experimental technique allow application of pulsed unidirectional electric fields, termed half-cycle pulses (HCPs), to Rydberg atoms whose characteristic times are much less than the classical electron orbital period. In this limit each HCP simply delivers an impulsive momentum transfer, or "kick", to the excited electron. A number of protocols for controlling and manipulating Rydberg atom wavepackets using carefully tailored sequences of HCPs will be described with emphasis on the production of quasi-one-dimensional and near-circular Rydberg states, and on navigating electron wavepackets in phase space. A technique based on electric dipole echoes following field reversal that is used to probe reversible and irreversible dephasing will also be described. Insights provided by this work into atomic engineering, classical-quantum correspondence, and decoherence in mesoscopic quantum systems will be discussed.

Pair-Breaking Scattering Resonances in a Degenerate Fermi Gas
March 12th Dr. Katharine Challis University of Aarhus, Denmark

We consider single-particle scattering from a trapped two-component degenerate Fermi gas, when the incoming projectile particle is identical to one of the confined species. Our theoretical treatment is based on a one-dimensional mean-field approach and we predict pair- breaking scattering resonances. These resonances are of the Fano/ Feshbach type, but are only possible because of the many-body pairing in the system. We describe the main features of the scattering resonances and provide a physical interpretation of the phenomena.