Durham Atomic and Molecular Physics 
AtMol Research Seminar Archive

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 highorder quantum accelerator modes, under certain conditions, produce a coherent statedependent momentum transfer to atoms. This can be applied as new kind of beamsplitter 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 BoseEinstein 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 largeperiod, quasielectrostatic optical lattice  
October 5th  Mr Paul Griffin  Durham University 
This talk reports on the design and construction of, and results from an opticaldipole trapping apparatus, developed to confine ultracold rubidium atoms in a conservative, largeperiod, opticaldipole 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 BoseEinstein Condensates requires a stage of rfevaporation 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 insitu 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. 

SMatrix theory of laserinduced nonsequential double ionization  
November 9th  Dr Carla Figueira de Morisson Faria  City University 
We have investigated nonsequential double ionization within the strongfield approximation, considering in particular the process in which the second electron is released by electronimpact ionization from its parent ion. Several issues have been addressed such as the role of the electronelectron interaction (or finalstate electronelectron 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 fewcycle 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 quantumclassical 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. 

Atommolecule 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, 3body 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 longlived molecules that are stable to such collisions by tuning the atomatom 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 spinpolarized Li + Li_{2} and K + K_{2} 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 nonadditive: for Li + Li_{2} 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 lowtemperature inelastic rate coefficients are typically above 10^{10}cm^{3}s^{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 longlived. Molecules produced in any intermediate vibrational state are likely to be ejected from the trap very quickly. We have also carried out calculations for mixedisotope collisions involving alkali dimers. For ^{7}Li colliding with either ^{6}Li_{2} or ^{6}Li^{7}Li, reactive scattering is possible even when the molecule is in its lowest rovibrational state because of the change in zeropoint energy. 
Department of Physics, Durham University  Tel +44 (0)191 33 43520 
Rochester Building, Science Laboratories  Fax +44 (0)191 33 45823 
South Road, Durham DH1 3LE  
United Kingdom  © Simon A Gardiner, Durham University 2005 