AtMol Research Seminar Archive
Index Current Seminars

Michaelmas Term 2005

Date Speaker Home
Special Seminar: Quantum Accelerator in Cold Atoms - approach to quantum chaos and quantum computing
August 24th Dr Zhaoyuan Ma University of Oxford

This talk will first show the recent work on the experimental and theoretical investigation on the quantum chaotic behaviour of cold atoms. This focuses on the experiments and on the theory of quantum accelerator modes. We show that quantum accelerator modes emerge in a systematic way, governed by patterns in apparently abstract number theory. This demonstrates the link between quantum nonlinear resonances and a mathematical structure named Arnol'd tongues. The high sensitivity of quantum accelerator modes to gravitational acceleration is also uncovered in the research. The talk also discusses the application of quantum accelerator modes to quantum information processing. We have found that the high-order quantum accelerator modes, under certain conditions, produce a coherent state-dependent momentum transfer to atoms. This can be applied as new kind of beam-splitter for atom optics. A new type of quantum random walk based on this discovery is then proposed.

Special Seminar: Experiments with optically trapped single atoms
August 31st Dr Matt Jones Institute d'Optique, Orsay

I will present recent results on the coherent control of an optical transition in a single rubidium atom, trapped in an optical tweezer. We excite the atom using resonant light pulses that are short (4 ns) compared with the lifetime of the excited state (26 ns). By varying the intensity of the laser pulses, we can observe an adjustable number of Rabi oscillations, followed by free decay once the light is switched off. To generate the short light pulses we have developed a novel laser system based on frequency doubling a telecoms laser diode at 1560 nm. By setting the laser intensity to make a π pulse, we use this coherent control to make a high quality triggered source of single photons. In addition, I will present preliminary results on the use of Raman transitions to couple the two hyperfine levels of the ground state of our trapped atom. This will allow us to prepare and control a qubit formed by two hyperfine sublevels. The combination of these two techniques offers possibilities for testing protocols for entangling neutral atoms using interference and photon detection.

Special Seminar: Classical Field and Quantum Turbulence
September 21st Prof Crispin Gardiner University of Otago

We apply the classical field method to simulate the production of correlated atoms during the collision of two Bose-Einstein condensates. We predict the existence of quantum turbulence in the field of the scattered atoms - a property which should be straightforwardly measurable.

Laser cooling and loading of Rb into a large-period, quasi-electrostatic optical lattice
October 5th Mr Paul Griffin Durham University

This talk reports on the design and construction of, and results from an optical-dipole trapping apparatus, developed to confine ultracold rubidium atoms in a conservative, large-period, optical-dipole trap.

Strong correlation physics in low dimensional ultra cold atomic gases
October 12th Dr Andrew Ho University of Birmingham

In this talk, I will give an overview of strong correlation physics in one dimensional (1D) systems in the context of ultra cold atoms trapped in optical lattices. I will describe my theoretical modelling of binary mixtures of bosons and fermions, concentrating on their ground state properties, including their experimental signatures. I will also briefly discuss going from 1D to the 3D system.

Light engineered evaporation
October 19th Mr Matt Pritchard Durham University

Traditionally the creation of Bose-Einstein Condensates requires a stage of rf-evaporation in a magnetic trap to produce the required phase space density. Although an effective method, there are limitations to its use which will be outlined in the talk. In recent years light has been used to trap and evaporate atomic species. Different approaches to light engineered evaporation will be discussed, and experimental results presented.

Optical trapping of ice crystals
November 2nd Prof Maki Tachikawa Meiji University

The process of water solidification in air is not fully understood and there still remain challenging issues such as variety in the crystal habit. We are developing an optical trap that levitates a single water droplet and allows in-situ observation of its crystallization process. In this talk, we report on our first attempt to trap supercooled water droplets and ice crystals artificially produced from them. Size, shape, and motion of the trapped particles are analyzed from the scattering pattern of the trapping laser beams. Also the possibility of controlling crystal formation by the radiation pressure is discussed.

S-Matrix theory of laser-induced nonsequential double ionization
November 9th Dr Carla Figueira de Morisson Faria City University

We have investigated nonsequential double ionization within the strong-field approximation, considering in particular the process in which the second electron is released by electron-impact ionization from its parent ion. Several issues have been addressed such as the role of the electron-electron interaction (or final-state electron-electron repulsion) of the initial electronic bound states, and the possibility of describing the problem using a classical method. We have shown that nonsequential double ionization with few-cycle laser pulses can be used as a tool for absolute phase diagnosis. We have also extended our model to systems with an arbitrary number of electrons. In this context, we have been able to infer thermalization times for the multielectron system, from experimental data, which lie in the attosecond regime.

Thermodynamical witnesses of macroscopic entanglement
November 16th Prof Vlatko Vedral University of Leeds

Thermodynamics is a macroscopic theory of physical systems, independent of the underlying structure and details of dynamics. Two systems can share the same macroscopic property, e.g. temperature, even though they are of completely different compositions. In much the same way, entanglement pertaining to two different physical states can have the same value. In my talk I will show that entanglement can be witnessed using standard thermodynamical state variables such as the internal energy, magnetic susceptibility and heat capacity. I will illustrate this by obtaining a temperature bound for some systems below which entanglement definitely exists, and compare this with some existing solid state experiments. Implication of theser results to quantum computation and the "quantum to classical transition" will be discussed.

Chaos, decoherence and the quantum-classical boundary
December 7th Andrew Martin Durham University

To reconcile the seemingly opposing descriptions of the world given by quantum and classical mechanics, classical mechanics is expected to be derivable as a limiting case of quantum mechanics when a system becomes large and interacts with its environment. In this talk, classical and quantum mechanics are reviewed, and methods of comparing their predictions are outlined. Recent papers have discussed quantum and classical treatments of Hyperion (a satellite of Saturn with a chaotic orbit) and have investigated the necessary conditions for emergent classical behaviour in the quantum regime.

Atom-molecule collisions in atomic Bose and Fermi gases
December 14th Prof Jeremy Hutson Durham University

It is now possible to form diatomic molecules from pairs of alkali metal atoms in both bosonic and fermionic gases in several ways. These include photoassociation, 3-body collisions and Feshbach resonance tuning. The molecules formed so far have all been in highly excited vibrational states, often the highest state supported by the potential well. It has been found experimentally that diatomic molecules formed from bosonic atoms are quickly ejected from the trap, possibly because they undergo fast inelastic collisions that release kinetic energy. However, for fermions it has been possible to form long-lived molecules that are stable to such collisions by tuning the atom-atom scattering length to large positive values. The suppression of inelastic collisions for such molecules has been attributed to "Pauli blocking".

We have obtained new potential energy surfaces and carried out full quantum dynamics calculations for spin-polarized Li + Li2 and K + K2 collisions for both bosonic and fermionic isotopes of Li and K. These are "reactive" scattering collisions because they include all possible arrangement channels. They are carried out in hyperspherical coordinates, which allow the full boson or fermion symmetry to be imposed. The potential energy surfaces are highly non-additive: for Li + Li2 the well depth of the trimer potential is 4 times the value based on pairwise additivity.

Our calculations give very high quenching rates for alkali dimers in excited vibrational states. For the low vibrationally excited states that we can handle at present, we do not see any suppression of inelastic scattering for fermionic atoms, even when the scattering length is large and positive. The low-temperature inelastic rate coefficients are typically above 10-10cm3s-1. We conclude that Pauli blocking occurs only for molecules formed in the highest vibrational state in the potential well.

Our results have important implications for experiments aimed at transferring molecules to lower vibrational states. We expect that it will be necessary to transfer them directly to the ground vibrational state for them to be long-lived. Molecules produced in any intermediate vibrational state are likely to be ejected from the trap very quickly.

We have also carried out calculations for mixed-isotope collisions involving alkali dimers. For 7Li colliding with either 6Li2 or 6Li7Li, reactive scattering is possible even when the molecule is in its lowest rovibrational state because of the change in zero-point energy.