The Queen's University of Belfast | School of Maths & Physics | Theor. & Comput. Physics

Theoretical and Computational Physics

Seminars in 2000-2001, 2001-2002, 2002-2003, 2003-2004, 2004-2005, 2005-2006, 2006-2007, current year

SEMINARS in 2006-2007

Tuesday, 21 August 2007

Isao Shimamura, RIKEN, Japan

Time-Delay Matrix Analysis of Resonance Processes

Resonances have long been known to occur quite often in atomic, molecular and nuclear processes due to the formation of quasi-bound states as intermediate states. They may greatly enhance or suppress particular processes. Much attention has been paid to them recently in relation to the resonance control of Bose-Einstein condensation, resonant damage to bio-molecules, resonance control of processes in a strong field, etc.

An isolated resonance in an N-channel process may be described by the Breit-Wigner formula for the scattering (S) matrix, which consists of the resonance part involving the resonance energy and the N partial widths, and the background part varying slowly with the scattering energy. When two or more resonances overlap each other, the S matrix elements and the cross sections take very complicated forms, and the resonance analysis becomes extremely difficult. This is because the interactions between these resonances can affect different channels in various different ways.

The time delay in single-channel scattering, or the collision time spent additionally (compared with the free passage) by the projectile because of the interaction with the target, may be related to the phase shift, and hence to the S matrix. It can be generalised into "time-delay matrix" (F. T. Smith, 1960) for multichannel scattering and may be related to the multichannel S matrix. This time-delay matrix is found to be very useful, especially for analysing overlapping resonances in a visually transparent way, without missing resonances that can be easily missed by the conventional resonance analysis method. The theory will be outlined and some numerical examples will be shown.

Tuesday, 3 July 2007

Prof Béla Sulik, Institute of Nuclear Research (ATOMKI), Debrecen, Hungary

Guiding of few keV ions in insulator nanocapillaries

Recently, studying the interaction of slow, highly charged ions with the walls of insulator nanocapillaries, Stolterfoht and co-workers [1-3] discovered a phenomenon known as "capillary guiding". They observed that a significant amount of few-keV highly charged ions traveled along the direction of the axis of even strongly tilted nanochannels without significant collisions with the capillary walls. This phenomenon has been found for 10 μm long and 100 nm diameter capillaries created in polyethylene terephtalate (PET) membranes. Even for a deliberate misalignment between the incident ion direction and the nanochannels axis, a significant fraction of keV Ne7+ ions passed through, and left the nanochannels following their axis with a narrow angular distribution. This guiding effect has been explained by a self-organizing charge-up process [1-4]. According to this picture, the ions, which enter the capillary will charge up parts of the cap-illary surfaces in such a way that finally some of them will be guided to the exit of the capillary. In equilib-rium, only a fraction of the incoming ions will pass, while the rest provide the electrostatic field necessary for the guiding. More recently, similar guiding effect has been reported for SiO2 capillaries [5]. In the pre-sent talk, the first results on capillaries formed in anodized alumina (Al2O3) [6] will be reported. Experiments recently performed in ATOMKI, Debrecen, indicate that the guiding effect is probably universal for good insulators.
  1. Stolterfoht N, et al, 2002 Phys. Rev. Lett. 88 133201
  2. Stolterfoht N, et al, 2004 Vacuum 73 31
  3. Stolterfoht N, et al, 2004 Nucl. Instr. Meth. B 225 169
  4. Schiessl K, et al, 2005 Phys. Rev. A 72 062902
  5. Sahana M B, et al, 2006 Phys. Rev. A 73 040901
  6. Mátéfi-Tempfli S, et al, 2006 Nanotechnology 17 1

Tuesday, 5 June 2007

Prof Jonathan Sherratt, Department of Mathematics Heriot-Watt University, Edinburgh

Mathematical modelling of cell adhesion in developmental biology and cancer

Many processes in the early development of embryos are regulated by the extent to which cells stick to one another. This "cell adhesion" is also critical in cancer biology, with cancer cells typically adhering less to one another than their normal counterparts, enabling them to break away from a solid tumour and enter the blood stream. Cell adhesion is one of the more difficult aspects of cell biology to model mathematically because it is intrinsically non-local. I will describe a new integrodifferential equation model for the process. I will discuss some basic mathematical properties of the model, and will describe its application to simple laboratory experiments on cell sorting. I will then present two applications of the model: the process of somite formation in early development, and the invasive phase of cancer metastasis. The work I will discuss is a collaboration with Nicola Armstrong and Kevin Painter (Heriot-Watt University).

Tuesday, 29 May 2007

Prof. W. T. Coffey, Department of Electronic & Electrical Engineering, School of Engineering, Trinity College, Dublin

Quantum Brownian Motion

Recent progress in our understanding of quantum effects on the Brownian motion in an external potential is reviewed. This problem is ubiquitous in physics and chemistry particularly in the context of decay of metastable states, for example, the reversal of the magnetization of a single domain ferromagnetic particle, kinetics of a superconducting tunnelling junction, etc. Emphasis is laid on the establishment of master equations describing the diffusion process in phase space analogous to the classical Fokker-Planck equation. In particular, it is shown how Wigner's [E. P. Wigner, Phys. Rev., 1932, 40, 749] method of obtaining quantum corrections to the classical equilibrium Maxwell-Boltzmann distribution may be extended to the dissipative non-equilibrium dynamics governing the quantum Brownian motion in an external potential yielding a master equation for the Wigner distribution function in phase space. The explicit form of the master equation so obtained contains quantum correction terms and in the classical limit, reduces to the classical Klein-Kramers equation. For a quantum oscillator, the method yields an evolution equation coinciding in all respects with that of Agarwal [G. S. Agarwal, Phys. Rev. A, 1971, 4, 739]. In the high dissipation limit, the master equation reduces to a semiclassical Smoluchowski equation describing noninertial quantum diffusion in configuration space. The Wigner function formulation of quantum Brownian motion is further illustrated by finding quantum corrections to the Kramers escape rate, which in appropriate limits reduce to those yielded via quantum generalizations of reaction rate theory.

Tuesday, 15 May 2007

Mag. Robert Prevedel, Faculty of Physics, University of Vienna

Experimental one-way quantum computing with linear optics

In recent years, one-way quantum computing has become an exciting alternative to existing proposals for quantum computers. In this specific model, coherent quantum information processing is accomplished via a sequence of single-qubit measurements applied to an entangled resource known as cluster state. During my talk, I will introduce, review and discuss the unique properties of this model and present recent experiments that have realized one-way quantum computational tasks.

Tuesday, 8 May 2007

Dr Jimena Gorfinkel, Department of Physics and Astronomy, Open University

Recent theoretical studies of electron-molecule collisions

In my talk I will briefly describe the R-matrix method as applied to the study of electron-molecule collisions. I will discuss its successes and limitations when dealing with processes of applied relevance.

I will then talk about some of my current work on the treatment of near threshold ionisation and collisions with molecules of biological relevance (and why theoretical work on electron molecule collisions is relevant in this field). Finally, I will discuss some initial attempts at devising a method to treat electron collisions with molecular clusters and molecules immersed in media.

Tuesday, 24 April 2007

Prof Nigel Badnell, Department of Physics, University of Strathclyde, Glasgow

Calculations for electron-ion collisions and photoionization processes for plasma modeling

We review our studies of atomic collision processes relevant to the spectroscopic diagnostic modelling of astrophysical and fusion plasmas.

We consider dielectronic recombination (DR) of the Fe M-shell, its benchmarking [1] by ion storage ring experiments at Heidelberg, and its importance for the modelling of absorption features in active galactic nuclei [2]. We also comment on the effect of our new L-shell DR data on coronal ionization balance for all elements up to Zn [3], and on results from CLOUDY for photoionized plasmas.

For photoionization, we look at the revised opacities from the Opacity Project (OP) which include inner-shell transitions and so extend the validity of the OP data into the stellar interior [4]. We note recent applications of it to the study of gravity mode stellar pulsations which suggest a preference for OP data over OPAL opacities for the `Z-bump' [5].

We consider large-scale R-matrix electron impact excitation calculations along isoelectronic sequences, up to Zn, which provide a new level of `baseline data' for spectroscopic diagnostic modelling codes such as CHIANTI and ADAS.

Finally, we look to the future and the demands of ITER on theory for describing collision processes involving heavy species such as W, Xe.
  1. Badnell, J. Phys. B, 39 4825 (2006)
  2. Badnell, Ap. J. Lett., 651 L73 (2006)
  3. Bryans et al, Ap. J. S., 167, 343 (2006)
  4. Badnell et al, MNRAS, 360, 459 (2005)
  5. Jeffery & Saio, MNRAS, 371, 659, (2006)

Tuesday, 17 April 2007

Dr Christina Cobbold, Department of Mathematics, University of Glasgow

A mathematical model of insect development: Spatial and temporal consequences

Host-parasitoid systems are common in the insect world. Parasitoids are typically flies or wasps while caterpillars are a typical example of a host. A key aspect to modelling host-parasitoid interactions is describing the timing of parasitism events. We present a generic difference equation model and show that the timing of parasitism can have a significant impact on the dynamics of the model. In particular, we show how the period and amplitude of temporal cycles are affected by paramtisim.

Habitat structure has also been shown to effect the insect population dynamics. Thus we also present an extension to the model whereby integro-difference equations are used to describe the probability of insect dispersal. In this case habitat patch size is found be important in determining host-parasitoid persitence.

Tuesday, 27 March 2007

Dr Emma Sokell, School of Physics, University College Dublin

Resonant Photoelectron Spectroscopy in the Gas Phase

Photoelectron spectroscopy is a well established technique for investigating atoms and molecules. However, two-dimensional photoelectron spectroscopy, in which the yield of photoelectrons is measured as a function of both electron energy and incident photon energy, is still able to reveal unexpected behaviour.  The requisite sources of tuneable photons are synchrotrons. Two-dimensional spectra are ideally suited to exploring the decay of photoexcited resonance states, as the encompassing nature of the technique often reveals features that are overlooked or misinterpreted if only a small number of spectra are recorded at selected photon energies. After briefly introducing the technique of two-dimensional photoelectron spectroscopy, the talk will focus on specific cases where the study of the decay of photo-excited resonant states has provided new and often unexpected information. These examples will range from one of the simplest atoms, helium, through simple molecular systems including H2 and D2, to larger molecules such as HCl. The comprehensive nature of the experimental measurements has meant that complete theoretical models have often not been available to fully explain all of the results.

Tuesday, 13 March 2007

Prof Vincenzo Aquilanti, Department of Chemistry, University of Perugia, Italy

Few-body quantum and many-body classical hyperspherical dynamics for elementary reactions and phase transitions of neutral and ionic nanoaggregates

The hyperspherical method is a successful approach for the quantum treatment of elementary chemical processes. It has been mostly applied to three-atomic systems and current progress is on the basic theoretical framework for the extension to four-body bound state and reactive scattering problems.

Most applications only exploit the advantages of the hyperspherical coordinate systems, but its full power is in the representations explicitly involving quantum hyperangular momentum operators as dynamical quantities and hyperspherical harmonics as basis functions.

In terms of discrete analogues of the harmonics one has a universal representation for the kinetic energy and a diagonal representation for the potential (hyperquantization algorithm).

Recently, advances have been on classical dynamics, based on "classical" definitions of the hyperangular momenta and related quantities. After a sketch of progress on the general quantum approaches for three- and four-body systems, specifically on the basis set issue; I will present formulation and implementations to molecular dynamics of simple nanoaggregates.

Tuesday, 13 February 2007

Prof Tania Monteiro, Department of Physics and Astronomy, University College London

AC-driven cold atoms

I will look at the dynamics of a range of quantum systems subjected to time-periodic potentials. The unifying feature of these systems is that their dynamics can be analysed by the structure of their Floquet states, which for a time-periodic system play a similar role to the eigenstates of the Hamiltonian of a conservative system. I will review coherent matter-wave experiments done at UCL and elsewhere with pulsed and sinusoidally driven standing waves of light and show that they may be understood by analysing the form of their Floquet states; I will look at possibilities for improving quantum state transfer in spin chains sujected to fields which localise the Floquet states. I will review recent results showing that (within the theoretical framework of a simple sinusoidally-driven Bose-Hubbard model and the Floquet states) the phenomenon of 'coherent destruction of tunnelling' may have applications for manipulating the Mott-Insulator transition.

Tuesday, 23 January 2007

Dr Elinor Irish, Department of Applied Mathematics and Theoretical Physics, Queen's University Belfast

The Theory of Quantum Electromechanics

"Quantum electromechanics" combines a superconducting qubit and a nanofabricated mechanical resonator into a system similar to an atom in an optical cavity. Many fascinating quantum optical effects should be realizable in this solid-state system. Additionally, new effects may appear due to the possibility of very strong coupling even at large detunings. I will talk about my work on the theory of quantum electromechanical systems, motivated in particular by the search for ways to observe the quantum behavior of nanoscale mechanical resonators.

Tuesday, 5 December 2006

Michael Lysaght, School of Mathematics and Physics, Queen's University Belfast

Tin based laser-produced plasma source development for EUV lithography

Laser produced plasmas containing tin are currently being researched as possible extreme ultraviolet (EUV) sources for next generation nanolithography tools. The optimisation of conversion efficiency (CE) (i.e. the ratio of EUV energy output to laser input energy) is still currently one of the outstanding issues confronting the EUV lithography (EUVL) community, where a minimum CE of 3% is required by industry. This requirement forces the developers of EUV sources to obtain a better understanding of the theoretical limits of plasma emission. One of the key aspects of any attempt to accurately model the plasma is the need for reliable fundamental atomic data on Sn ions, which has been considerably lacking to date. This talk will focus on the recent efforts of the Atomic and Molecular Physics Research Group at University College Dublin to provide some of this core data to the EUVL source modelling community.

Tuesday, 7 November 2006

Prof Massimo Palma, Dipartimento di Scienze Fisiche ed Astronomiche, Universita degli Studi di Palermo

Entanglement controlled electron transport

We will show the interplay which exists between entanglement and single electron transport properties in a 1D quantum wire in the presence of two scattering magnetic impurities or dots. We consider a system consisting of single electrons moving along a 1D wire in the presence of two magnetic impurities. Such system shows strong analogies with a Fabry - Perot interferometer in which the impurities play the role of two mirrors with a quantum degree of freedom: the spin. We discuss how the electron transmittivity of the wire is affected by the presence of entanglement between the impurity spins. In particular we will show that for suitable values of the electron momentum, there are two maximally entangled state of the impurity spins the first of which makes the wire transparent whatever the electron spin state while the other strongly inhibits the electron transmittivity. We will also consider the Aharonov-Bohm (AB) interference oscillations of electron transmission through a mesoscopic ring in which two non-interacting magnetic impurities are embedded. Finally we will discuss how maximally entangled states of the impurity spins can be generated via scattering with a conduction electron.
Tuesday, 10 October 2006

Prof Rachid Mohallem, Universidade Federal de Minas Gerais, Brazil

Molecular Isotope Symmetry Breaking and Dipole Moments


Wednesday, 4 October 2006

Prof. Wolfgang Lange, Department of Physics and Astronomy, University of Sussex

Cavity-QED: entangling ions with light

In a cavity-QED system, the quantum states of single atoms or ions can be coupled deterministically to those of single photons, as long as the coherent interaction dominates any decay processes. In this way, quantum information may be reversibly transferred between ions and photons. An important application is the realization of a quantum network, linking distant ion trap quantum processors through photonic channels.

We are currently implementing an ion-photon interface, based on the controlled single-photon emission from an ion in an optical resonator. This provides a method to distribute entanglement over long distances. But also local ion-entanglement may be created, by coupling two ions to the same cavity mode. The successive optical entanglement of ion pairs in a long chain generates a cluster state and thus a resource for one-way quantum computation.

In contrast to these deterministic schemes, we also investigate the probabilistic entanglement of ions, conditioned on the detection of photons emitted from the cavity. The technique exploits the fact that it is impossible to distinguish two ions through their interaction with the cavity mode. In the talk, I will discuss the status of experiments and prospects for the realization of theoretical schemes.

SEMINARS in 2005-2006

Tuesday, 13 June 2006

Prof. Ronald McCarroll, Université Pierre et Marie Curie, Laboratoire de Chimie Physique, 75231 Paris 05

Use of Complex Hamiltonians to Calculate Photo Ionization and Photo Dissociation Cross Sections

A review will be given of the various procedures currently used to construct complex Hamiltonians for the investigation of photo absorption processes in atomic and molecular systems: complex scaling, exterior complex scaling, complex absorbing potentials. These procedures allow for a representation of continuum states on a square integrable (L2) basis and reduce the calculations to matrix digitalization. Complex scaling, which involves the transformation of the dissociative coordinate r to re provides the most rigorous approach, but while satisfactory for photoionization in atomic systems, it is not less so dissociative processes in molecules. For molecular systems, complex absorbing potentials prove to be very satisfactory, provided some elementary constraints are imposed.

The main advantage of using an L2 basis, rather than the more conventional description of continuum states is its ease of application to complex systems, such as multichannel di-atoms and tri-atoms with relatively modest computing requirements. Both narrow and wide resonances profiles can be calculated to high precision. The methods are also self consistent. And in all cases we have studied, it has been verified that convergence of the results is obtained for a wide range of absorbing potential parameter.

The methods will be illustrated by a number of applications both to photo ionization in atoms and to simple dissociative systems in molecules. Special emphasis will be given to the description of resonance profiles. A comparison will be presented with some recent experiments the Heidelberg heavy ion storage ring on the dissociation of molecular ions.

Wednesday, 7 June 2006

Prof. W. T. Coffey, FAPS, MRIA, Docteur honoris causa (Perpignan), Department of Electronic & Electrical Engineering, School of Engineering, Trinity College, Dublin

Quantum dissipation and quantum Fokker-Plank equation


Tuesday, 23 May 2006

Dr Terry Rudolph, Department of Physics, Imperial College London

Towards optical quantum computation with realistic devices

The primary technological hurdle facing linear optical quantum computation is commonly thought to be the construction of efficient sources and detectors. I will argue that the primary hurdle is in fact theoreticians who haven't devoted enough time to thinking about whether we can get by with the devices we have. In defense of this thesis I will discuss how, by making use of some neat features of cluster state computation, we can get by with much more noisy devices than one might have hoped, and why I am optimistic that smarter theoreticians then me should be able to relax these fault tolerant thresholds even further.

Tuesday, 11 April 2006

Marco Bellini, Istituto Nazionale di Ottica Applicata - CNR, Florence, Italy

Generation, manipulaton and tomographic analysis of quantum light states

The generation and the analysis of nonclassical light states is of great importance both for a better understanding of the underlying fundamental physics and for the possible applications of such states to the emerging fields of quantum information and communication. In this talk I will briefly discuss some of the latest activities of our group in the field of experimental quantum optics. In particular, I will describe our schemes for the conditional generation of highly nonclassical light states: these include single-photon Fock states, single-photon-added coherent and thermal states (i.e., essentially classic light states which acquire a quantum character thanks to the excitation by a single photon), and time-delocalized single photons (a novel example of single-particle entanglement). A recently-developed, ultrafast, time-domain, homodyne technique allows us to perform complete tomographic analysis of these light states and to characterize and quantify their degree of quantumness from the reconstruction of their density matrix elements and Wigner functions.

Tuesday, 28 March 2006

Dr Wes Walter, Department of Physics and Astronomy, Denison University, Granville, Ohio

Negative ions: correlation and dynamics in inner-shell photodetachment

Negative ions attract great interest for both applied and fundamental reasons. Studies of the structure and dynamics of negative ions provide stringent test cases for atomic theory because the added electron is bound to a neutral core, thus the influence of correlation is greatly enhanced relative to neutral atoms. In this talk, I will give an overview of our recent experiments on laser photodetachment of lanthanide negative ions and electric field effects, then focus on our inner-shell photoexcitation studies performed with synchrotron radiation at the Advanced Light Source at Lawrence Berkeley National Laboratory. Threshold and resonance structures are observed in photoexcitation from the K-shell for He-, Li- and C-, and the L-shell for S-, together with measurements of absolute cross sections for double detachment. The studies provide fundamental information on core-excited states of atoms and negative ions, and have also revealed some surprises including the validity of the Wigner threshold law over unexpectedly long-ranges and the substantial production of high-charge states of positive ions following inner-shell excitation of S-.

Tuesday, 21 March 2006

Prof Alan Dickinson, School of Natural Sciences (Physics), University of Newcastle upon Tyne

Cold Metastable Helium: Scattering and Photoassociation with Nano-Grenades

Metastable helium (He*) first joined hydrogen and the alkalis in forming a Bose-Einstein Condensate a few years ago. Metastable helium offers unique possibilities for single-atom detection, exploiting its 20 eV internal energy. In the talk I shall discuss some atomic and molecular physics problems associated with cold He*, including the scattering length for spin-polarized He* and binding energies of ultra-long range molecules formed in photoassociation in 4He* and in 3He*.

Tuesday, 7 March 2006

Dr Sarah Baker, Laser Consortium, QOLS, Department of Physics, Imperial College, London

Probing proton dynamics in molecules on an attosecond timescale

We demonstrate a new technique using high order harmonic generation (HHG) in molecules to probe nuclear dynamics and structural rearrangement on a sub-femtosecond timescale. The photon emission upon electron-ion recombination in an HHG process is chirped: use of this chirp in the emitted light allows information about nuclear dynamics to be gained with 100 attosecond temporal resolution, from excitation by an 8 fs pulse, in a single laser shot. Measurements on H2 and D2 agree well with calculations of ultra-fast nuclear dynamics in the H2+ molecule, confirming the validity of the method. Guided by this result, we have measured harmonic spectra from CH4 and CD4 to demonstrate a few-femtosecond timescale for the onset of proton rearrangement in methane upon ionization.

Tuesday, 28 February 2006

Dr Fred Currell, QUB and the UK Coordinator and Member of Collaboration Board of SPARC

The SPARC/FLAIR Collaboration, a developing opportunity for a new generation of atomic physics

The future international accelerator Facility for Antiproton and Ion Research (FAIR) under development at Darmstadt in Germany has key features that offer new, challenging opportunities for atomic physics and related fields. The facility promises the highest intensities for relativistic beams of stable and unstable heavy nuclei, combined with the strongest available electromagnetic fields, allowing extension of atomic spectroscopy across virtually the full range of atomic matter. Due to tremendous improvements in the intensity and energy, new fields will open through the greatly enhanced yields of unstable nuclei. Moreover, the new facility will produce the highest flux of antiprotons worldwide and will facilitate the creation of low-energy antiprotons at high intensities and brilliances to be used in a physics programme including atomic collision studies and precision spectroscopy of antiprotonic atoms and of antihydrogen.

Centred on use of FAIR the Stored Particle Atomic Physics Research Collaboration (SPARC) has already become the world's largest atomic physics collaboration with over 200 members drawn from more than 20 countries. Members of SPARC will perform experiments pertaining to two major research themes, fundamental interactions between electrons and heavy nuclei (in particular the interactions described by Quantum Electrodynamics, QED) and collision dynamics in strong electromagnetic fields. Whilst primarily experimental in nature the collaboration includes a theoretical division performing complementary theoretical studies and has the opportunity thereby to influence the experimental program.

In this talk the types of experiments proposed will be broadly reviewed with a view to developing opportunities for members of the Centre for Theoretical and Computational Physics to become involved in the collaboration.

Tuesday, 13 December 2005

Prof Helen Byrne, Centre for Mathematical Medicine, School of Mathematical Sciences, University of Nottingham

Modelling Solid Tumour Growth: What is the Point?

Cancer is a complex disease in which a variety of factors interact over a wide range of spatial and temporal scales with huge data sets relating to the different scales available. However these data do not always reveal the mechanisms underpinning the observed phenomena. In this talk I aim to explain how mathematics can be used to interpret such data by presenting case studies that illustrate the types of insight that realistic theoretical models of solid tumour growth may yield. These range from discriminating between competing hypotheses for the formation of collagenous capsules associated with benign tumours to predicting the most likely stimulus for protease production in early breast cancer.
JOINT SEMINAR WITH TSLS

Wednesday, 7 December 2005

Dmitrii Shalashilin and Mark Child, Physical and Theoretical Chemistry Laboratory, Oxford University

Multidimensional phase space quantum mechanics with the help of coupled coherent states

The method of Coupled Coherent States (CCS) [1-9] is a new technique for direct numerical simulation of quantum many-body problems, which avoids exponential scaling of basis set with dimensionality and therefore allows exact fully quantum treatment of systems with a large number of degrees of freedom. The method is based on a solution of multidimensional Schrödinger equation in the basis of trajectory guided Monte Carlo sampled coherent states. Evolution of the wave function is simulated by running an ensemble of classical trajectories of coherent states together with a system of coupled quantum equations for the coherent states amplitudes. An important feature of our approach is that it uses quantum averaged potentials for running classical trajectories. This efficiently takes into account local zero point energy and results in a number of cancellations in working equations. The CCS method is fully quantum, however, classical mechanics helps to remove quantum oscillation and to decrease coupling, which is essential for making Monte Carlo sampling work. A number of multidimensional applications (with the number of DOF in the order of 10 or 20) to the spectroscopy of polyatomic molecules and small clusters, intramolecular vibrational energy redistribution, proton tunneling have been reported. They include:

(1) simulations of tunneling splitting (up to 20D) [2,8].

(2) simulations of Franck-Condon spectra of Henon-Heiles model (up to 14D) [3] and pyrasine (4D and 24D models) [6].

(3) simulation of infrared absorption specrum of water trimer (3D) [5].

(4) simulation of intramolecular vibrational energy redistribution (9D) [4].

(5) simulation of electronic energy levels (6D) [9].

as well as a number of low dimensionality model calculations. The method has recently been reviewed in [7]. Currently our attention is focused on electron dynamics in strong laser field.

[1] D. V. Shalashilin, M. S. Child, J. Chem. Phys., 113, 10028, 2000.

[2] D. V. Shalashilin, M. S. Child, J. Chem. Phys., 114, 9296, 2001.

[3] D. V. Shalashilin, M. S. Child, J. Chem. Phys., 115, 5367, 2001.

[4] D. V. Shalashilin, M. S. Child, J. Chem. Phys., 119, 1961, 2003.

[5] D. V. Shalashilin, M. S. Child and D. C. Clary, J. Chem. Phys., 120, 5608, 2004.

[6] D. V. Shalashilin, M. S. Child, J. Chem. Phys., 121, 3563, 2004.

[7] D. V. Shalashilin, M. S. Child, Chem. Phys., 304, 130, 2004.

[8] P. A. J. Sherratt, D. V.Shalashilin and M. S. Child, Chem. Phys. in press

[9] D. V. Shalashilin and M. S. Child, J. Chem. Phys. in press.

Tuesday, 6 December 2005

Dr Carla Figueira de Morisson Faria, School of Engineering and Mathematical Sciences, City University, London

S-Matrix theory of laser-induced nonsequential double ionization: from electron-electron dynamics to absolute-phase diagnosis

Laser-induced nonsequential double ionization (NSDI) is a phenomenon occurring in the context of matter in strong laser fields (intensities of the order of 10 13 W/cm2 or higher) for which electron-electron correlation plays a very important role. We investigate this effect within the strong-field approximation using saddle-point methods, considering a process in which the second electron is released by electron-impact ionization with its parent ion, and address several issues such as the role of the electron-electron interaction, of final-state electron-electron repulsion, of the initial electronic bound states, and the possibility of describing the problem using a classical method. We have also shown that NSDI with few-cycle laser pulses is very sensitive to the so-called absolute phase (the phase difference between the pulse envelope and its carrier frequency). Hence, it can be used as a tool for absolute phase diagnosis. Furthermore, we 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.

Tuesday, 29 November 2005

Prof Yan V Fyodorov, School of Mathematical Sciences, University of Nottingham

Statistics of delay times in mesoscopic systems

We reveal a very general relation between the statistics of delay times in one-channel reflection from a mesoscopic sample of any spatial dimension and the statistics of the eigenfunction intensities in its closed counterpart. This relation opens a possibility to use experimentally measurable delay times as a sensitive probe of eigenfunction fluctuations.

References: Phys.Rev. B 71, 125133 (2005).

Tuesday, 1 November 2005

Prof Paul Coleman, Department of Physics, University of Bath

Probing Matter with Antimatter

Positrons are the most common form of antimatter in our world, being emitted from radioisotopes and used in PET scanners for medical imaging. They also are emitted in great numbers from a source close to the centre of our galaxy. Positron beams have featured in some important fundamental experiments, such as the recent production of antihydrogen at CERN. American scientists have even worked on the use of antimatter to power intergalactic space travel.

If a positron enters a solid it quickly slows down and, after about one hundred picoseconds, annihilates its antiparticle, the electron. By detecting the gamma rays emitted following the annihilation (each having an energy close to mc2) we can learn about the density and momentum distribution of electrons in solids, and in turn gain information on the submicroscopic structure of the material being studied.

This talk will review what can be learned by using beams of positrons as probes of matter on the subatomic scale, and how this relates to modern technological problems.

Tuesday, 27 September 2005

Prof Ronald McCarroll, Université Paris et Marie Curie, Laboratoire de Chimie Physique, 75231 Paris 05

Photodynamics of Molecules in Space

In the first part, a brief survey will be given of the processes leading to the formation and fragmentation of molecules under conditions typical of cold interstellar molecular clouds. The special case of H2 formation and the importance of ion-molecule collisions will be reviewed. The main discussion will be centred on the nature of photodissociative processes induced by the interstellar photons, in particular on the importance of resonant structure resulting form the interaction of discrete and continuum states, may lead to possible isotopic fractionation.

The second part will be devoted to a discussion of some new methods we have developed for calculating photodissociation cross sections based on the introduction of complex absorbing potentials and discrete variable representations. It will be shown how these methods can be easily adapted to calculating cross sections without the need for any explicit determination of continuum wave functions. The methods will be illustrated by some applications to the dissociation of different isotopes of CO. New results on the photodissociation of the important interstellar molecule CH+ will also be presented.

Wednesday, 14 September 2005

Dr. Nobuyuki Nakamura, The Institute for Laser Science, The University of ElectroCommunications, Tokyo

Recent Activities at the Tokyo Electron Beam Ion Trap

Highly charged ions (HCIs) are the ions in which few electrons are left. The electrons in a HCI are moving near the nucleus at a speed near the speed of light under the influence of a strong field, outside the realm of our ordinary laboratory experience. The electronic states in such strong field exhibit both relativistic and quantum electrodynamical features, and thus of much interest from the point of view of atomic physics.

Another notable characteristic of HCIs is that they have huge potential energy even if they do not have kinetic energy even though they can be regarded as "sub-nanometer" in size. Thus HCIs can modify the local area of material surfaces drastically, and as a result, nanometer-sized structures can be created on surfaces with a quantum efficiency of 1.

In order to study such highly charged ions, we are using a device called an electron beam ion trap (EBIT) in Tokyo. In this talk, activities at the Tokyo EBIT will be presented after brief introduction of the device.

SEMINARS in 2004-2005

Monday, 27 June 2005

Dr Sougato Bose, Department of Physics and Astronomy, University College London

Dualism in Entanglement and Testing Quantum to Classical Transition of Identicity


Tuesday, 14 June 2005

Dr Dario De Fazio, Dipartimento di Chimica dell Universita, 06123 Perugia, Italy, and Istituto di Metodologie Inorganiche e dei Plasmi C.N.R., 70126 Bari, Italy

Lifetime matrix analysis of reactive resonances by the hyperquantization algorithm

In the last years a new hyperspherical technique has been developed and implemented to solve exactly the reactive scattering problem. Such time-independent exact tecnique exploit a semiclassical limit of the hyperspherical harmonics obtained for large values of the hyperquantization vector. Some prototypical chemical reactions have been investigated where state-to-state integral and differential cross sections as well as rate constants have been compared with experiment and with previous theoretical investigations.

In addition to these S-matrix applications, the hyperquantization algorithm has been recently exploit for direct calculations of resonance properties of triatomic system. In such applications simultaneous propagation of hyperradial solutions together with its energy derivative permit the direct calculations of the Smith's lifetime Q-matrix of the system. From the analysis of the eigenvalues of this matrix in function of J several properties of the metastable states can be investigated and several insight about resonance effects in reaction dynamics can be otained.

In this talk results for the F + H2 system will be presented. From the resonace theory point of view, this reaction appear very attracting. In fact the resonances of such system are few enough to be clearly investigated but at the same time are a number such to cover several different cases. Although resonance effects are relevant in the reactivity of the system, they are often not trivial to be properly understood. The study of the J-behavior of the resonance properties permit the identification of several unexpected effects such splitting, interaction and crossing of resonances with different local modes.


Tuesday, 31 May 2005

Prof Liviu Ixaru, Department of Theoretical Physics, Institute of Physics and Nuclear Engineering, Bucharest, Romania

CP Methods for the Schrödinger Equation

The CP methods are based on perturbation theory; CP is the abbreviation for Constant based Perturbation. In each interval [xk, xk+1] of the partition the potential function V(x) is written as V(x) = V + ΔV(x) where V called the constant reference potential, is the average value of V(x). Then the solution is propagated from xk to xk+1 by the exact solution for the constant reference potential plus corrections from the perturbation ΔV(x). Since all these ingredients have simple analytic expressions the algorithm is very fast. The accuracy depends on how many perturbation corrections are retained in the algorithm. In the most recent versions this number was sufficient to reach errors which behave as hp, where 12 ≤ p ≤ 18; for comparison many of the well established codes have p between 4 and 6. Other advantages are that the number of steps is small (in many cases of direct interest a few tens of steps are sufficient to obtain results accurate in the roundoff range) and that the accuracy is practically the same irrespective on how low or high the involved energy is. Comparison with other methods will be given.

Tuesday, 24 May 2005

Prof Ian Percival FRS, Queen Mary, University of London

Lecture Two: Bell's Theorem and Local Theories of Quantum Measurement

Bell's theorem depends on two conditions that are not usually stated. Removing either condition invalidates the theory, and might permit a quantum field theory of quantum measurement, bringing it into the mainstream of modern theoretical physics.

Thursday, 19 May 2005

Prof Ian Percival FRS, Queen Mary, University of London

Lecture One: The Strange Role of Quantum Measurement in Modern Physics

Modern theoretical physics follows a programme initiated by Newton in the 17th century, and continued to this day: prediction, dynamical models, locality, quantum fields and quantum gravity. Quantum measurement stands outside this programme. The role of Bell's theorem.

Tuesday, 12 April 2005

Dr Verne L. Jacobs, Center for Computational Materials Science, Materials Science and Technology Division, Naval Research Laboratory, Washington, D. C.

K-α Emission Spectra from Non-Equilibrium Ionizing Plasmas*

K-α X-ray emission spectra from highly charged Fe ions have been theoretically predicted using a detailed and systematic spectral model. Account has been taken of the fundamental atomic radiative-emission processes associated with inner-shell electron collisional excitation and ionization, as well as dielectronic recombination. Particular emphasis has been directed at extreme non-equilibrium or transient-ionization conditions, which can occur in astrophysical and tokamak plasmas. Good agreement has been found in comparisons with spectral observations on the EBIT-II electron beam ion trap at the Lawrence Livermore National Laboratory. We have identified spectral features that can serve as diagnostics of the electron density, the line-formation mechanism, and the charge-state distribution.

*This work has been supported by NASA, ONR, and DOE.

Wednesday, 6 April 2005

Prof Alexander Devdariani, St Petersburg State University

Radiative transitions in the Z1eZ2 quasi-molecules in the context of plasma spectroscopy

The problem of radiation produced in collisions of a bare nucleus Z1 with a one-electron ion eZ2 holds a special position in the spectroscopy of quasi-molecules. It emerges naturally in the physics of inner-shell collision excitation, in plasma physics, and in astrophysics. It is essential from a conceptual point of view that, just as for a hydrogen atom, the energy terms and the transition dipole moments for the Z1eZ2 quasimolecule can be calculated exactly on the basis of the Schrödinger equation in relation to the distance between the nuclei. But in contrast to the energy terms the transition dipole moments in the non-symmetrical case have been calculated quite recently. The main features of the dipole moments are discussed, and some of them are accounted for by avoided crossings of energy terms. All the features found for neutral quasi-molecules can be identified in the Z1eZ2 quasi-molecule too.

The semi-classical approach based on the Fourier transform of the time-dependent dipole transition matrix element is used to study the manifestations of potential energy terms and dipole moment features in the spectral-line wings. The discussion of the influence of non-adiabatic transitions induced by collisions is given in terms of the model approaches widely accepted in the physics of collisions. Some potential applications to plasma spectroscopy rely on the dependence of the position and form of some satellites on plasma parameters.

Tuesday, 29 March 2005

Dr Ronald Murphy, Naval Research Laboratory, Washington, USA

The Physics of Positron Annihilation in the Solar Atmosphere


Tuesday, 22 March 2005

Dr Jaromir Fiurasek, Department of Optics, Palacky University, Olomouc, Czech Republic

Conditional generation of arbitrary single-mode quantum states of light via repeated subtraction of photons

I will present an experimentally feasible scheme for conditional generation of arbitrary (squeezed) finite superpositions of Fock states in a single mode of a traveling optical field. The suggested setup requires only a source of squeezed states, beam splitters, strong coherent beams, and photodetectors with single-photon sensitivity. The quantum state preparation starts with squeezed vacuum state which is subjected to a sequence of displacements and single photon subtractions. The displacement is accomplished by mixing the beam with an auxiliary strong coherent beam on a highly unbalanced beam splitter. A single photon is removed from the beam by sending a tiny portion of the beam on a photodetector using an unbalanced beam splitter and the photon is removed if the photodetector clicks. The method works well even with low-efficiency photodetectors that cannot resolve the number of photons.

Monday, 14 March 2005

Prof Lars Bojer Madsen, Department of Physics and Astronomy, University of Aarhus, Denmark

Atoms and diatomic molecules in intense fields

Recent advances in experimental techniques and in the development of light sources have put focus on certain aspects of the interaction between strong laser pulses and atoms and molecules. One aspect is the study of the importance of nondipole terms in the interaction with high-frequency fields, and for the molecules the question of effects of orientation and nuclear motion is central. In the talk, I will present some recent work on

(i) Vibrational excitation of diatomic molecular ions in strong-field ionization of diatomic molecules. Does the Franck-Condon principle work after all?

(ii) Orientation effects in ionization of the molecular hydrogen ion by short, intense, high-frequency light sources. Is ionization really most likely from the parallel geometry?

(iii) Exact nondipole Kramers-Henneberger form of the light-atom Hamiltonian: An application to atomic stabilization and photoelectron energy spectra.

Tuesday, 1 March 2005

Prof Gerry O'Sullivan, Department of Experimental Physics, University College Dublin

Extreme UV Source Development for Lithography

The wavelength of choice for the next extreme UV lithography (EUVL) step, 13.5 nm, is based on the availability of MoSi multilayer mirrors with excellent reflectivity (Approx. 72%) at this wavelength with a reflectance bandwidth of approximately 0.5 nm. It is envisaged that a suitable exposure tool will be ready for insertion in the 45 nm node alongside excimer laser sources by 2009 and will be sole choice for the 32 nm node which will be reached by 2010. A wide variety of pulsed discharge sources using xenon or xenon/helium mixtures were initially selected because of the lack of debris. However all of the conversion efficiencies reported to date for these devices are close to 0.5%. Considerable work has also been expended on exploring the feasibility of using laser produced plasmas of xenon clusters produced by supersonic jets or gas puffs from nozzles or solid xenon targets. The highest conversion efficiencies (1.2% into 2% bandwidth) have been achieved using solid xenon. In xenon, the transitions responsible are 4p64d8 - 4p64d75p in Xe XI. The required conversion efficiency figure has now been revised upwards to better than 3% conversion into a 2% bandwidth. We have identified tin as potentially the brightest emitter at this wavelength. The transitions responsible in tin arise from 4p64dn - 4p54dn+1 +4dn-14f lines from stages VIII through XIII that merge to form an unresolved transition array (UTA). If the tin concentration is reduced to <10%, the peak brightness increases due to enhanced radiation transport at lower density. Using low density tin plasmas, experimental conversion efficiencies of 2 and 3% have been demonstrated using gas discharge and laser produced plasmas respectively. Higher efficiencies are predicted for laser produced plasmas and may possibly be obtained using appropriate pulse shaping/target configurations. However unlike xenon there is a major problem due to debris and various novel schemes have been proposed to mitigate this problem.

Tuesday, 22 February 2005

Prof Martin Plenio, Blackett Laboratory, Imperial College, London

Entanglement in Nano-mechanical oscillators

We consider the dynamical properties of interacting systems of harmonic oscillators for the propagation and generation of entanglement in these systems. We apply these ideas to arrays of nano-mechanical oscillators to generate entanglement without the need for detailed local control.

Tuesday, 30 November 2004

Dr Leonid Gerchikov, IRCEP Distinguished Visiting Fellow, St Petersburg State Polytechnic University, Russia

Theory of resonant photoabsorption of metallic clusters in a strong laser field

Clusters are systems intermediate between atoms and bulk material. In particular, metallic clusters have much in common with atoms, molecules and solids, but even more, with atomic nuclei! Metalic clusters possess a giant plasmon resonance which completely dominates their photoabsorption spectra. In this talk I will outline a new approach developed to study highly excited electron states beyond the linear response theory. It allows one to analyze the anharmonicity of dipole plasmon excitations and to investigate non-linear effects in cluster photoabsorption in intense laser fields.

Tuesday, 16 November 2004

Dr Andrew Barlow, AWE

Computational Physics at AWE

The main role of the Computational Physics Group at AWE is to develop 2D and 3D multi-physics simulation codes that are capable of modelling shock hydrodynamics, radiation and neutron transport. Research is performed to improve the physics, accuracy and performance of these codes. This requires expertise in maths, physics and parallel computing. The talk will provide an overview of some of the current research activities within the Computational Physics Group at AWE.

Tuesday, 9 November 2004

Dr Dmitri Sokolovski, Department of Applied Mathematics and Theoretical Physics

'Superluminal' tunnelling, quantum measurements and the speed of information transfer

We exploit the analogy between the transfer of a pulse across a scattering medium and Aharonov's weak measurements to resolve the long standing paradox between the impossibility to exceed the speed of light and the seemingly 'superluminal' behaviour of a tunnelling particle in the barrier or a photon in a 'fast-light' medium. We demonstrate that 'superluminality' occurs when the value of the duration τ spent in the barrier is uncertain, whereas when τ is known accurately, no 'superluminal' behavior is observed. In all cases only subluminal durations contribute to the transmission which precludes faster-than-light information transfer, as observed in a recent experiment.

Tuesday, 2 November 2004

Erika Andersson, Department of Physics, University of Strathclyde, Glasgow

Joint measurements of spin, operational locality and uncertainty

It has long been appreciated that joint measurements of non-commuting observables are possible within quantum mechanics. In a joint measurement, we have to accept an increase in the variances of the jointly measured observables, above the Heisenberg limit. In this talk, we derive a tight bound on the accuracy of the joint measurement of two incompatible components of spin for a spin-1/2 particle. This derivation requires only the existence of entangled states and operational locality. The latter ensures that no operation performed on one of a pair of entangled systems can be detected by observation of its entangled partner. The present derivation is valid independently of how the measurement is described, and there is an interesting connection to Bell inequalities. We give a direct interpretation of the measurement bound in terms of an uncertainty relation.

Tuesday, 26 October 2004, 4:00 pm, Lecture Theatre 3002, David Bates Building

John Gillaspy, National Institute of Standards and Technology, Gaithersburg

From Nanometers To Light Years: Studies With Highly Charged Ions At Two Extremes

This talk will discuss two diverse types of experiments underway at the NIST Electron Beam Ion Trap (EBIT) lab, and the associated modeling work that it has stimulated in other groups. At one extreme are studies aimed at understanding the neutralization of individual highly charged ions on surfaces. In this work, we use scanning probe microscopy to study the surface modifications caused by the deposition of large amounts of potential energy into small regions of the surface. At the the other extreme, we are studying the x-ray emission from trapped highly charged Ne-like Fe ions, because of the relevance to astrophysical models. The need for further modeling in each of these areas will be emphasized.

Tuesday, 19 October 2004

Prof Ray Flannery, School of Physics, Georgia Institute of Technology

Interactions and collisions in ultracold Rydberg plasmas

A new branch of atomic physics -- the interactions, dynamics and collisions in ultracold (T<< 1 K) systems -- has naturally evolved from recent advances in the cooling and trapping of neutral gases. Giant helium dimer molecules have been recently produced via photo-association of ultracold metastable atoms. Such long-range molecules attract considerable attention, with possible application to quantum computing and dipole blockade. The long-range molecules considered up to now are those formed from atoms with low-l electron-core penetrating states, as He (23S1) and He (23P0), appropriate to photo-association experiments.

We will consider the interaction between two Rydberg atoms with internal electronic angular momentum spread over a broad distribution of l values, characteristic of Rydberg atoms placed in a weak electric field. These Rydberg atoms can be called "polar" because they possess permanent multipole moments. We present the physics of the long-range interaction between these polar Rydberg atoms and investigate the possibility of long-range Rydberg-Rydberg molecules. We shall also discuss radiative cascade and collisions involved in Rydberg plasmas. A classical theory of radiative cascade is presented. Cross sections for ionization of Rydberg atoms in n,l states by collision with electrons are presented and the full dependence of the cross sections on the initial angular momentum state l of the target is revealed.


Thursday, 30 September 2004

Kieron Burke, Department of Chemistry and Chemical Biology and Department of Physics and Astronomy, Rutgers University

Electron-atom collisions using time-dependent density functional theory

Ground-state density functional theory has had enormous impact in quantum chemistry, solid-state physics, and materials science. Time-dependent density functional theory is rapidly becoming the method of choice for treating electronic excitations in molecules, and is being applied to systems in strong laser fields. I will review the basics and the state of the field, with an emphasis on the EXACT theory. We have recently shown how to calculate electron scattering cross-sections for atoms, and performed the first such calculations.

SEMINARS in 2003-2004

18 May 2004

Prof Philip Maini, Centre for Mathematical Biology, Mathematical Institute, University of Oxford

Modelling aspects of vascular cancer

The modelling of cancer provides an enormous mathematical challenge because of its inherent multi-scale nature. For example, in vascular tumours, nutrient is transported by the vascular system, which operates on a tissue level. However, it effects processes occuring on a molecular level. Molecular and intra-cellular events in turn effect the vascular network and therefore the nutrient dynamics. Our modelling approach is to model, using partial differential equations, processes on the tissue level and couple these to the intercellular events (modelled by ordinary differential equations) via cells modelled as automaton units. Thusfar, within this framework we have modelled structural adaptation at the vessel level and we have modelled the cell cycle in order to account for the effects of p27 during hypoxia. These preliminary results will be presented.

27 April 2004

Dr Sandro Wimberger, Max Planck Institute for the Physics of Complex Systems, Dresden

Decay rates and survival probabilities in open quantum systems

For the realistic and paradigmatic system of microwave driven hydrogen Rydberg atoms, we establish a quantitative analogy between the excitation/ionisation process and the charge transport across 1D solid state models. The statistical properties of the ionisation probability and the decay rates of the driven atom follow the prediction given for 1D Anderson models, and hence relate intrinsic disorder in such models with the deterministic, but largely chaotic transport in decaying atomic systems.

Tuesday, 9 March 2004

John T. Costello, National Centre for Plasma Science and Technology (NCPST) and School of Physical Sciences, Dublin City University, Dublin 9

Photoabsorption and Photoionization Imaging and Spectroscopy with Laser Plasma Light Sources

The talk will outline the development of laser plasmas as broad band / line free continuum light sources in the extreme-ultraviolet and their application as pulsed photoionizing probes of atoms and ions. In part 1, following a historical retrospective[1], the origin of the continuum will be outlined, some non-spectroscopic applications will be mentioned and recent work on generating picosecond continuum presented [2]. Part 2. comprises of a short discourse on "Dual Laser Plasma" (DLP) experiments in which one laser plasma is used as a light source while a second laser plasma acts as a "sample" of atoms and ions [e.g., 3]. A couple of illustrative DLP spectroscopic experiments and a new VUV "photo-absorption imaging facility" - VPIF [4] will be presented. The talk will conclude with a perspective on the future for photoionization experiments with VUV lasers, specifically the VUV FEL, TTF2 under development at Hasylab.

1. J. T. Costello et al., Physica Scripta T34, 77 (1991)

2. O. Meighan et al., J.Phys.B:At.Mol.Opt.Phys 33, 1159 (2000)

3. A. Neogi et al., Phys. Rev. A 67, Art. No. 042707 (2003) /Recanatini P, Nicolosi P, Villoresi P, Phys. Rev. A 64, art. no. 012509 (2001)

4. J. S. Hirsch, E. T. Kennedy, A. Neogi, P. Nicolosi, L. Poletto and J. T. Costello, Rev. Sci. Instrum. 74, 2992 (2003)

Tuesday, 24 February 2004

Prof Andrew Briggs, Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH

Nanomaterials for Quantum Computing

Quantum computing is not simply a way of increasing the speed of future computers. It is part of a radically new way of processing information that takes advantage of distinctively quantum properties such as superposition and entanglement. It will make possible certain calculations that would be utterly impractical on any foreseeable classical computer. There are many candidate schemes for building a quantum computer, and many people consider that in the long term a solid state scheme will be used. Nanomaterials such as endohedral fullerenes and single-walled carbon nanotubes are proving to have many of the properties that are required for successful quantum computing [1]. Individual atoms can be inserted in fullerene cages, and these in turn can be inserted into single-walled nanotubes to make so-called peapod structures that can be characterised with atomic resolution [2]. The endohedral fullerenes have coherence times that can exceed 0.24 ms, and the spins can be manipulated with a precision that will allow extended gate operations in a quantum computation.

[1] www.nanotech.org

[2] Nanoscale solid-state quantum computing. Phil. Trans. R. Soc. Lond. A 361, 1473-1485 (2003). A. Ardavan, M. Austwick, S.C. Benjamin, G.A.D. Briggs, T.J.S. Dennis, A. Ferguson, D.G. Hasko, M. Kanai, A.N. Khlobystov, B.W. Lovett, G.W. Morley, R.A. Oliver, D.G. Pettifor, K. Porfyrakis, J.H. Reina, J.H. Rice, J.D. Smith, R.A. Taylor, D.A. Williams, C. Adelmann, H. Mariette and R.J. Hamers.

Tuesday, 17 February 2004

Dr Jacob Dunningham, Junior Research Fellow, Merton College and Clarendon Laboratory, University of Oxford

Made to measure: Using condensates to surpass the standard quantum limit

One of the most spectacular recent advances in atomic physics has been the ability to trap and manipulate Bose-Einstein condensates in an optical lattice. These systems allow unprecedented control over the parameters of trapped condensates and have enabled experimentalists to demonstrate the superfluid to Mott-insulator phase transition. The Mott state is of interest since each lattice site has precisely the same number of atoms, and so provides a potentially significant route to quantum computing. The phase transition is reversible and enables large numbers of particles to be entangled and disentangled at will. Such a process is at the very heart of quantum information processing schemes. In this talk we will explore how this feature can be used to perform tasks beyond the classical limit. We will focus particularly on practical ways that condensates may be used to make measurements with a resolution beyond the standard quantum limit.

Wednesday, 28 January 2004

Prof. Theo J.M. Zouros Department of Physics, University of Crete & Institute of Electronic Structure and Lasers, Heraklion

Quasi-free electron scattering from highly charged ions

Differential electron-ion scattering measurements probe the delicate atomic structure and interferences between the short-range scattering potential due to the electronic structure of the ion and the long range Coulombic potential due to the ion's charge. These types of measurements can be readily performed only in the high-luminosity environment provided by ion-atom collisions in which the lightly bound (quasi-free) target electrons simulate an electron "beam" in the frame of the ion. After a brief introduction to ion-atom and electron-ion collision techniques, recent developments [1,2] in quasi-free electron scattering from highly charged ions in ion-atom collisions will be presented.

[1] T.J.M. Zouros et al., Phys. Rev. A68, 010701(R) (2003).

[2] E.P. Benis et al., J. Phys. B36, L341 (2003).

Tuesday, 9 December 2003

Prof Vincenzo Aquilanti, Department of Chemistry, University of Perugia, Italy

The hyperquantization algorithm for reactive scattering


Tuesday, 25 November 2003

Dr M Kozlov, Petersburg Nuclear Physics Institute, IRCEP Distinguished Visiting Fellow

High accuracy calculations for atoms and atomic tests of fundamental laws

Quite often atomic physics provide uniquely accurate measurements. Sometimes these measurements can give an important information about fundamental physics. Study of the violation of the discrete symmetries (parity P and time reversal T) is a well known example. More recently atomic methods were used to study the possible time variation of fundamental constants, such as the fine structure constant and the mass ratio for proton and electron. Usually fundamental information is extracted from the experiment with the help of complicated atomic calculations. That requires high accuracy and reliability of atomic theory. In particular, electron correlations should be accurately accounted for. We will discuss the method of effective operators for valence electrons and its applications to the studies of parity non-conservation and variation of the fine structure constant.

Wednesday, 12 November 2003

Prof W T Coffey, Department of Electronic and Electrical Engineering, School of Engineering, Trinity College, Dublin

Stochastic dynamics of the reversal of the magnetization of fine particles over potential barriers

The theory of the Brownian motion as applied to the calculation of the reversal time and complex susceptibility of single domain ferromagnetic particles is reviewed including anomalous relaxation effects.

Friday, 7 November 2003

Prof Eva Lindroth, Department of Atomic Physics, Stockholm University

Resonant states and their importance in atomic processes

Doubly or multiply excited states in atoms or ions are frequently encountered in atomic processes such as ionization (by photons or particle impact), charge transfer or electron-ion recombination. Their signature is a resonance, i.e. a dramatic change of the probability for a process under special conditions, e.g. for electron-ion recombination at certain relative velocities.

Since resonances can change the probability for a process with orders of magnitude there is an obvious interest to be able to describe them and make quantitative predictions of their effects. The requirements on the computational approaches to be used for this are high since the excited states causing resonances are highly correlated systems embedded in the continuum.

In the seminar I will discuss resonances in some different systems and in connection with both photon and electron impact. Special emphasis will be given to the process of dielectronic recombination. I will briefly cover the computational method and show several examples of unexpected behaviour.

Tuesday, 28 October 2003

Dr Tom Gorczyca, Western Michigan University

Inner-Shell Photodetachment Dynamics

Recent theoretical and experimental work on the photodetachment of Li-, C-, and B- will be reviewed, with emphasis on the dominant shape resonance behavior above the K-shell thresholds. Of particular importance, the interaction between low-energy photoelectrons and Auger electrons is found to deplete the measured yield of positive ions near threshold. A new method for incorporating this post-collision interaction into existing R-matrix methods will be presented, and previous discrepancies between theoretical and experimental results for the photodetachment of Li- [1] are resolved.

[1] N. Berrah, J. D. Bozek, A. A. Wills, G. Turri, H.-L. Zhou, S. T. Manson, G. Akerman, B. Rude, N. D. Gibson, C. W. Walter, L. VoKy, A. Hibbert, and S. M. Ferguson, Phys. Rev. Lett. 87, 253002-1 (2001)

Tuesday, 30 September 2003

Jens Eisert, Junior Professor, University of Potsdam

Towards feasible distillation of continuous-variable entanglement

The key requirement in essentially all applications of quantum information science is to have strategies at hand to protect the involved quantum systems against decoherence. In particular, entanglement-based schemes rely on the possibility of distilling highly entangled states from a supply of weakly entangled noisy states. Finite-dimensional schemes for distillation have been proposed and a proof-of-principle experiment has even been performed. In practical schemes, however, there are crucial limitations, as one for example requires photon counters in an optical full scale distillation scheme.

This talk will be concerned with an alternative to this setting: entanglement distillation with continuous-variable systems. Firstly, an introduction will be given to the study of Gaussian states and Gaussian operations. Secondly, it will be shown that indeed, with Gaussian operations alone Gaussian states can not be distilled at all. Thirdly, and this will be the main emphasis of the talk, a procedure will be presented that is capable of distilling Gaussian two-mode states from a supply of weakly entangled mixed states. This procedure makes use of passive optical elements and photon detectors only, the latter distinguishing the presence and the absence of photons, without the need for photon counters. Both mathematical abstract issues such as the weak convergence of the protocol to Gaussians will be discussed, as well as practical issues concerning the actual implementation using imperfect devices. The use of this method as a starting point for quantum key distribution will be outlined.

Tuesday, 23 September 2003

Prof S. C. Mukherjee, IACS, Calcutta, India

Collisions of Antimatter with Matter

The most fundamental antimatter-matter collision system is antiproton-atomic hydrogen collisions. To study the low energy (keV) collision in antiproton-hydrogen ionization calculation, we are interested to solve the time dependent Schrodinger equation in spheroidal co-ordinates in a two center approximation. Before going into details of the above calculation, as a preliminary investigation we have solved the Schrodinger equation in one center expansion, where the wave functions are defined in terms of B-spline basis function. In this approach the results are found to be encouraging. In addition we like to present some of our previously calculated results in intermediate and high energy region. In antihydrogen-hydrogen collisions we propose to use the coupled-pseudostate approach.

SEMINARS in 2002-2003

Tuesday, 8 July 2003

Prof Klaus Bartschat, Drake University, Des Moines, IA 50311, USA

Simultaneous Ionization-Excitation of Quasi-Two-Electron Targets

As a highly correlated process, simultaneous electron-impact ionization-excitation presents a major challenge to both experimentalists and theorists. With the rapid advance of computational power, it has recently become possible to account for both first-order and second-order effects in the interaction of a "fast" projectile with the target, combined with a convergent close-coupling-type description of the initial bound state and the interaction between a "slow" ejected electron and the residual ion. Results from our work on ionization-excitation of helium will be presented and compared with available experimental data and predictions from other theoretical approaches, and the accuracy of the various data sets will be assessed.

In addition to the triple-differential cross section (TDCS), information about the ionization-excitation process may sometimes also be obtained by observation of the light emitted in optical transitions of the excited ion. The symmetry properties of the process are analyzed to explore what complementary information can be obtained using different experimental setups. Finally, the possible advantages of replacing the helium target by other quasi-two-electron systems, such as Mg or Ca, will be discussed.

Tuesday, 3 June 2003

Prof David J. Pegg, Department of Physics, University of Tennessee Knoxville, Tennessee, USA

Correlated processes in the detachment of electrons from negative ions

Electron correlation plays a major role in determining the structure and dynamics of a negative ion. Due to the short-range nature of the binding force, negative ions typically possess only a single bound state, in sharp contrast to the infinite spectrum of states characteristic of atoms and positive ions. Consequently, traditional spectroscopic methods are not applicable to negative ions. Experimental investigations of negative ions necessarily involve detachment processes in which the final continuum state consists, in general, of one or more detached electrons and a residual heavy particle such as an atom or positive ion. There is often structure in the detachment continua that is associated with multiply excited or core excited states of the negative ion. These states are short lived and reveal themselves as resonances that modulates detachment cross sections. Measurements of correlation-sensitive quantities associated with thresholds and resonances provide stringent tests of the ability of theory to go beyond the independent electron model. In the talk I will discuss some recent experimental results in the areas of photodetachment and electron-impact detachment.

Tuesday, 27 May 2003

Dr R. Sala Mayato, Universidad de La Laguna, Tenerife, Spain

Structures in phase-space distributions

In a two recent papers, W. Zurek has claimed two things: first, that for some specific systems it is possible to find sub-Planck structures confined to a phase space volume characterized by the classical action A. Second, and most important, the decay of the overlap between some state quantum state and its shifted version, is determined primarily by the size of such sub-Planck structures in the corresponding phase space distribution. Our claim is that the first statement is true and very well understood and the second one is basically false.

Tuesday, 20 May 2003

Dr Jorge Kohanoff, Atomistic Simulation Group, School of Mathematics and Physics, Queen's University Belfast

Squeezed hydrogen: state-of-the-art and present theoretical and computational challenges

An old conjecture in solid state physics states that any material becomes metallic if sufficient pressure is applied. Since hydrogen is, in principle, the simplest material in nature, it seems the natural place to start looking for it. Metallization has not yet been observed but, in its quest, a surprisingly rich phase diagram emerged, thus generating an interest in understanding the nature of the different phases and transitions. Experimental information is available up to 300 GPa, but the phase diagram is interesting in the whole pressure range for several reasons. Firstly, the issues of metallization and dissociation remain open. Secondly, it is arguably the simplest system, and the phase diagram is still largely unknown. Finally, due to the small mass of the protons, several energy scales come into play simultaneously: zero-point energies, phonon excitations and electronic excitations. A robust theory of solid hydrogen should then include the following ingredients:

(a) quantization of the nuclear degrees of freedom

(b) a proper treatment of electronic excitations

(c) non-adiabatic electron-phonon couplings

Clearly, this represents a major theoretical and computational challenge.

In this presentation I will first review the present understanding of the low-temperature phase diagram of hydrogen as it emerges from optical experiments and first-principles electronic structure calculations. Next I will describe a methodology, based on the vibrational self-consistent field (VSCF) method, that we are currently developing in order to introduce quantum nuclear effects in electronic structure calculations. This reformulation of the VSCF method extends over its present capabilities, including cases in which the quantum (average) "geometries" are significantly different from the classical ones, e.g. tunneling coordinates. Finally, I will briefly comment on a possible strategy to introduce non-adiabatic electron-phonon couplings.

Tuesday, 13 May 2003

Dr Celal Harabati, Max-Planck-Institute for Physics of Complex Systems, Dresden, Germany

Semiclassical propagation of wave packets

The calculation of matrix elements of the time evolution operator in different representations (position, momentum, etc.) is a fundamental task in quantum physics. Its semiclassical version, applying variants of the stationary-phase approximation, is motivated by several reasons: (i) to connect to the underlying classical physics, which is particularly interesting if the classical system is nonlinear and behaves chaotically, (ii) to use classical phase space information and trajectories for a better understanding of the dynamics, (iii) in some cases, to allow for a more efficient calculation.

Compared to the original formulations in the 1920ies, van Vleck-Gutzwiller (VVG) propagator is the state of the art propagators (Hermann-Kluk in connection with the initial value representation (IVR) introduced by Miller). Yet, alternative formulations have been proposed, e.g., the non-linear-wavepacket dynamics, introduced by Tomsovic and Heller, and more recently, a coherent state propagator by Baranger et al.

In the talk the difference of these formulations will be analyzed and explained, using simple and transparent examples to underline the results, e.g., the interaction with Coulomb and Morse potentials. Analytical tools as well as numerical integration of semiclassical matrix elements with phase space Monte Carlo techniques will be used.

Tuesday, 6 May 2003

Prof Ian Walmsley, Physics Department, University of Oxford

Engineering Photons for Quantum Technologies

Quantum interference is predicated on the absence of information distinguishing the paths from source to detector. In many quantum technologies, extremely high interference visibility is needed. Postselection cannot usually provide the necessary erasure of information without severely compromising the rate of detection of particles, and this one must seek to suppress all distinguishing characteristics at the source. we discuss how this may be accomplished for photons generated via parametric downconversion, and its implications for certain protocols in quantum information processing.

Tuesday, 15 April 2003

Prof Kanika Roy, Department of Theoretical Physics, IACS, Jadavpur, Calcutta, India

Ionization via capture in slow ion-atom collision

Recent developments in recoil - ion momentum spectroscopy have added new dimensions in our present knowledge of ionization dynamics. Measurements of ejected electron spectra, projectile angular distributions and the monentum distributions of recoiling ions offer a wealth of informations and can serve as a stringent test for the theory.

Our aim is to obtain more informations about the final state momentum distribution for ejected electron, projectile and recoiling ion using a method which considers the influence of coupling with important bound states in direct as well as in re-arrangement channels. The final state wave function describes the continuum electron is represented by the product of two Coloumb wavefunctions and it considers the distortion due to the Coulomb fields of both the projectile and the residual target in equal footing.

Tuesday, 8 April 2003

Dr. Alex Schuchinsky, Reader, School of Electrical & Electronic Eng., Queens University Belfast

Numerical-analytical modelling of microwave electromagnetic structures

The recent advances in nanotechnology, micromachining and micro-fabrication techniques have enabled dramatic reduction in size of electronic components and implementation of artificial electromagnetic media with properties unavailable in natural substances. New "metamaterials" such as photonic band gap structures and media with negative refractive index (DNG) have recently attracted much attention due to their potential applications to processing of electromagnetic signals. Significant research efforts are currently directed toward developing the high precision simulation tools and investigating EM wave phenomena in novel media.

Numerical-analytical techniques are of particular significance for understanding the wave interactions with peculiar media because they provide the combined power of numerical analysis and qualitative insight into essential physical mechanisms. The features of rigorous numerical-analytical techniques will be illustrated in this presentation with three examples: Synthetic conformal mappings for canonical structures containing right-angle conductor wedges and steps. Modified mode-matching technique for the problems of step discontinuities in waveguides. Spectral domain Galerkin method for dual integral equations with the basis functions accounting for the field singularities. The computational issues of the solution convergence, root searching and evaluating elliptic functions and integrals will be discussed from the viewpoint of high-precision calculations and numerical stability of the entire models.

The features and applications of the developed numerical-analytical techniques will be illustrated for the specific structures such as single and coupled lines of rectangular cross-sections and resonators of complex shapes, a dielectric step in a plane waveguide, strip dielectric waveguide formed by thin-film dielectric layer on ferrite-dielectric substrate.

Tuesday, 11 March 2003

Dr J D Cresser, Physics Department, Macquarie University, Sydney, Australia

The quantum trajectory method: Some properties and applications

The quantum trajectory method is a technique used both for numerically solving master equations for Markovian open systems and as a means of providing a measurement interpretation to these master equations. In this talk, the quantum trajectory method is described, some of the properties of the quantum trajectories and the associated measurement detection records such as the ergodicity of these records is considered. Some applications of the method to the micromaser, particle paths in bubble chambers, and extensions to non-Markovian systems may also be considered.

Tuesday, 25 February 2003

Melvyn Folkard, Gray Cancer Institute, Mount Vernon Hospital, Northwood, Middlesex

New insights into the mechanisms and consequences of DNA damage by ionizing radiation

One of the goals of radiation biophysics is to develop a mechanistic model of how ionizing radiations interact with living tissues, from the initial energy deposition event at the molecular level, through to the longer-term consequences for the whole organism. At the Gray Cancer Institute, a number of strategies are being used to address this goal. For example, we have devised experiments to quantify the amount of energy involved in the induction of strand-breaks in DNA. The formation of strand-breaks is known to be the critical step that can lead to observable effects at the cell and tissue level. Using novel techniques involving vacuum UV sources, we have obtained some surprising results, most notably that energy depositions as low as 7-10 eV can readily induce double-strand breaks in DNA. Theoretical models predict that much higher energies are required. We are also at the forefront of studies that use microbeams of ionizing radiation to irradiate individual cells, or selected targets within cells. Studies involving microbeams are challenging established models of the mechanisms that can cause DNA damage. For many years, it was believed that DNA damage by ionizing radiation arose solely though direct ionization of the DNA itself, (or the water adjacent to the DNA). However, using microbeams it is possible to show that DNA damage can also arise in unirradiated cells if they happen to be close to an irradiated cell (the so-called bystander effect). This, and other recent discoveries, challenge our understanding of the mechanisms of DNA damage and have very significant consequences for current models of radiation risk at low-doses.

Tuesday, 11 February 2003

Dr Ramin Daghigh, University of Minnesota

Microscopic Black Holes and Hot Matter

The relativistic viscous fluid equations describing the outflow of high temperature matter created via Hawking radiation from microscopic black holes are solved numerically for a realistic equation of state. We focus on black holes with initial temperatures greater than 100 GeV and lifetimes less than 6 days. The spectra of direct photons and photons from neutral pion decay are calculated for energies greater than 1 GeV. We calculate the diffuse gamma ray spectrum from black holes distributed in our galactic halo. However, the most promising route for their observation is to search for point sources emitting gamma rays of ever-increasing energy. We also calculate the spectra of all three flavors of neutrinos arising from direct emission from the fluid at the neutrino-sphere and from the decay of pions and muons from their decoupling at much larger radii and smaller temperatures for neutrino energies between 1 GeV and the Planck energy. The results for neutrino spectra may be applicable for the last few hours and minutes of the lifetime of a microscopic black hole.

Tuesday, 28 January 2003

Prof V N Ostrovsky, The University of St Petersburg

Multistate generalizations of the Landau-Zener model

The Landau-Zener model (1932) is the simplest exactly solvable model to describe non-adiabatic transitions between two states. This model is widely employed in atomic physics and beyond. It is natural to seek multistate generalizations of the Landau-Zener model (with linear time dependence of the Hamiltonian) which retain analytical solvability. One of such generalizations stems from the work by Majorana (1932) who was in fact a co-discoverer of the Landau-Zener model. This particular generalization (sometimes called SU(2) model) allows one to consider an arbitrary number of states, but effectively contains only one variable paramter. Another generalization, including an arbitrary number of states as well as an arbitrary number of parameters, was suggested about 30 years ago by Demkov and Osherov. We discuss a new, recently found generalization, known as the bow-tie model and its futher generalized form. It also includes an arbitrary number of states and parameters. We also consider a multistate Coulomb model with ~1/t time-dependence of the Hamiltonian.

Friday 10 January 2003

Prof Colm T Whelan, Department of Physics, Old Dominion University

(e,2e) and the Scanning Transmission Electron Miscroscope - two ways of looking at atomic matter

In this talk I will contrast the sort of information one gets from a STEM and that which you can get from an (e,2e) experiment. I will look at the possibility of using inelastic imaging using a very high energy electron beam (300 keV) in the STEM and discuss the basic scattering theory for such a process.

Tuesday, 17 December 2002, 4:00 pm

John Ludlow, Queen's University

Development of many body theory for positron-atom interactions

The scattering and annihilation of low-energy positrons on atoms will be described via many-body theory. It allows one to identify and include the dominant effects, namely, polarisation of the atom by the positron, and the formation of virtual Ps (electron-positron pair). They are essential for the correct description of positron-atom scattering and greatly enhance the probability of positron annihilation with an atomic electron. The present theory reproduces the "exact" variational results for hydrogen, but has the advantage that it is easily applicable to bigger targets. Preliminary results for the noble gases will be presented.

Tuesday, 10 December 2002, 4:00 pm

Prof Massimo Palma, University of Milan

Geometric phases and quantum computation

When a quantum system undergoes an adiabatic cyclic evolution the energy eigenstates acquire, on top of the usual dynamical one, a phase which depends only on the geometric details of the cyclic evolution. Such phase, known as Berry phase, has received renewed attention in recent years since it could be potentially useful in the implementation of new form of fault tolerant quantum computation.

The concept of geometric phase will be introduced together with some generalizations and its potential use in quantum computation will be illustrated. Particular attention will be devoted to the possibility to generate and detect such phase in superconducting nanostructures.

Tuesday, 3 December 2002, 4:00 pm

Prof Donald Lynden-Bell

Exact interacting N body problems in quantum mechanics

When N equal particles interact via a potential V(r), where r^2 is the sum of the squares of the separations between all the particles, the N body problem is exactly soluble. When the particles are fermions or bosons the degeneracies of the energy levels are determined by generalising Hardy and Ramanujan's theory of partitions.

Tuesday, 26 November 2002, 4:00 pm

Prof Nigel Mason, Department of Physics and Astronomy, The Open University

Electron Induced Processing at the Molecular Level

The ability to understand, manipulate and control physico-chemical processes at the molecular level is one of the great challenges of modern research and underpins the development of vibrant new technologies of the 21st century, for example the development of nanolithography. Such single molecule engineering requires selective bond cleavage in target molecules to allow subsequent management of the local site chemistry. Recent research has revealed that it is possible to influence the excitation and dissociation of molecules through the manipulation of electron interactions at the individual molecular level. Since electrons are ubiquitous in nature and electron induced reactions (in the gaseous phase, on surfaces and in the condensed phase) initiate and drive the basic physical-chemical processes in many areas of science and technology from industrial plasmas to living tissues our ability to control electron interactions provides exciting new opportunities that can now be exploited by both the research and technological communities.

Wednesday, 6 November 2002, 2:00 pm

Dr Andrei Korol, IRCEP Distinguished Visiting Fellow, Department of Physics, St Petersburg State Maritime University

Electron-ion recombination: who emits the photon?

Traditionally, two main mechanisms of electron-ion recombination are considered. The first one is radiative recombination (RR), where the incident electron accelerates in the field of the ion and looses energy by radiating a photon. In the second mechanism, recombination proceeds via capture of the electron in a doubly excited state, which is then stabilised by photoemission (hence dielectronic recombination, DR). This process is strongly resonant and difficult to calculate. In contrast, RR has a smooth energy dependence, and is rather easy to evaluate. Hence, RR is often used to normalise experimental data.

The talk focuses on a third recombination mechanism, in which the photon is emitted by the ion, when its electron "cloud" is distorted by the incident electron. This mechanisms (polarisation recombination, PR) is indistinguishable from RR. In the talk I will present simple estimates of the size of PR, show an example when they grossly overestimate the effect, and demonstrate that for a variety of ions and charge states the contribution of PR changes the magnitude of RR by 10 to 20%.

Tuesday, 29 October 2002, 4:00 pm

Dr Sudhakar Sahoo, Department of Applied Mathematics and Theoretical Physics, Queen's University Belfast

Configuration mixing and enhancement of electron recombination with complex multiply charged ions

It is known that some multiply charged ions are characterised by electron recombination rates 102 times greater than that of the simple direct radiative recombination. We perform an extensive study of configuration mixing between the doubly excited (doorway) states belonging to different configurations and the multiply excited states which account for the large recombination rate on Au25+. A detailed anyalysis of spectral statistics and statistics of eigenstate components shows that in Au24+ the dilectectonic doorways are completely "mixed" in complicated chaotic multiply excited eigenstates. In U28+, we find that most of doubly excited states doorways mix weakly with each other. However, they show a substantial mixing with multiply excited states, which accounts for the observed recombination rates and explains the mechanism of recombination. The energy avaraged cross sections calculated as a sum over the doorways are found to be in accord with experimental data.

Friday, 11 October 2002, 11:00 am

John G McWhirter FRS FREng, Senior Fellow, QinetiQ Ltd, Malvern Technology Centre

The Mathematics of Independent Component Analysis

Independent component analysis (ICA) is a powerful new technique for signal and data processing. It extends the scope and capability of principal component analysis (PCA) by exploiting higher order statistics in circumstances where the statistics of the data or signal samples are non-Gaussian. The development of effective techniques for ICA leads to some interesting and challenging mathematical problems.

For example, the use of fourth order statistics to separate independent signals which have been mixed in an instantaneous manner involves approximate diagonalisation of a fourth order (i.e. four index) tensor. Whereas the problem of matrix diagonalisation is well understood, the diagonalisation of tensors of order greater than two poses some very challenging problems.

The use of independent component analysis to separate signals which have been mixed in a convolutive manner poses a particularly interesting challenge. The problem may be formulated in terms of polynomial matrices (i.e. matrices with polynomial elements) and involves identifying the elements of a paraunitary unmixing matrix.

In this seminar, I will introduce the basic concept of ICA, explain how some of these interesting mathematical problems arise and outline some of the progress which has already been made. I will then present some results obtained using ICA in practical applications such as HF communications and foetal heartbeat analysis.

Tuesday, 1 October 2002, 4:00 pm

Hugo van der Hart, Queen's University Belfast

A direct measure of electron-electron repulsion: double photoionization

Double ionization provides a measure for the importance of electron-electron interactions. In the absence of these interactions, electrons cannot share energy, and photoabsorption can only change the behaviour of a single electron. Double ionization therefore provides insight into the many-electron dynamics of atoms.

Over the last 2 years, we have developed a B-spline approach to describe double photoionization. This B-spline approach has been combined with R-matrix Floquet theory in order to describe double ionization in laser fields that are expected to be generated by free-electron laser sources. In addition, this approach allows us to study double ionization from autoionizing states.

We have further applied the R-matrix Floquet approach for double ionization to study two-photon double ionization of He. When the frequency is chosen carefully, two-photon double ionization cannot occur in a sequential process He -> He+ -> He2+. Double ionization therefore indicates again the importance of dielectronic interactions.

Friday, 20 September 2002, 11:00 am

Michael Kuchiev, University of New South Wales, Sydney

QED radiative corrections to parity nonconservation in heavy atoms and the Standard Model

Precision experiments of C.E.Wieman and his colleagues on parity nonconservation in Cs (Wood et all (1997)) indicated that there is a pronounced deviation (2.3 sigma) of their results from predictions of the Standard Model. This claim greatly stimulated interest in the field. The talk gives a review of recent activity in the area showing, in particular, that the previously neglected QED radiative corrections to the parity nonconservation are much bigger than it was anticipated. The most difficult for calculation self-energy (plus vertex) radiative correction evaluated recently to be the largest giving -0.9(2)% for the parity nonconservation amplitude in Cs. For heavier atoms it is larger, reaching the value -1.6% for Tl,Pb, and Bi. This correction brings the experimental results in line with the Standard Model.

Thursday 19 September 2002 CANCELLED

Prof Vipin Srivastava, Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE

Of magnets, memories and the art of bicycle repair

We put forward a hypothesis that the brain in the course of discriminat- ing one information from another performs an operation similar to the mathematical operation of 'orthogonalization' typically used to construct an orthonormal basis set from a given set of linearly independent vectors. Working in the framework of spin-glass like neural networks we propose that a new information that comes to be recorded in the brain is first orthogonalized with respect to the information already in store and is then stored in its orthogonalized form. We use Gram-Schmidt procedure for orthogonalization and find that it enables the brain to do three operat- ions which are intuitively appealing too: it enables the brain to compare the new information with those in store, isolate its similarities and differences with old information and store these in an economical manner.

The Gram-Schmidt procedure is appropriate for cognitive processes that are sequential in nature. We have applied our scheme to address an old problem in language acquisition -- whether words are stored in mental lexicon in their full glory or as word-parts -- and have found interesting results.

We also propose that to handle the cognitive processes that are non- sequential in nature, the brain might be employing the so called 'democratic' orthogonalization processes due to Lowdin, which we find possess certain curious geometrical properties.

Friday 13 September 2002, 11:00 am

Dr. B. Ham, Electronics and Telecommunications Research Institute, South Korea

Quantum coherent control of dark resonance

Quantum coherent control has been broadly interested in many research areas. Of them are information communications such as quantum switching, quantum processing, and quantum communications. Dark resonance observed in mid 1970s has been strongly studied in many fields from nonlinear optics to quantum optics for future information communiations. Several promising researches based on dark resonance are reviewed and discussed for potential applications.

SEMINARS in 2001-2002


Tuesday 18 June, 4:00 pm

Charles Courtney and Leszek Frasinski, The University of Reading

De Broglie-Bohm trajectories in photoionisation - insight or distraction?


Thursday 13 June, 11:00 am

P. Lambropoulos, Department of Physics and Institute of Electronic Structure and Lasers, FORTH, Heraklion, Crete, Greece

Coherent control of photoabsorption products.

I review the basic ideas of coherent control through the manipulation of the relative phase of two electromagnetic fields, so as to maximize pro- ducts of photodisintegration into specific channels. I discuss examples of photoionization and/or dissociation, as well as the issue of molecular phase. The discussion is cast in the context of specific experiments in atoms and molecules.

Wednesday 15 May, 11:00 am

Dr. Majid Ebrahimzadeh, School of Physics and Astronomy, University of St Andrews

Elastic Light.

Since its invention more than forty years ago, the laser has become an indispensable optical tool capable of transforming light from its naturally incoherent state to a highly coherent state in space and time. The impact of this "optical coherence transformer" on the field of Optics can be likened to that of the transistor on the field of Electronics. At the same time, due to fundamental limitations, operation of the laser remains confined to restricted spectral and temporal regions, rendering it an "inflexible" light source for many applications. Nonlinear optics can overcome this limitation by allowing access to new spectral and temporal regimes through the exploitation of suitable dielectric materials in combination with the laser. In particular, optical parametric oscillators are versatile sources that can produce coherent radiation with unique spectral and temporal "elasticity" across an entire spectral range from the ultraviolet to the far-infrared and across all time-scales from the steady-state to the ultrafast femtosecond domain.

Tuesday 7 May, 4:00 pm

Dr. Dmitri Sokolovski, Queen's University

Understanding reactive angular distributions.

Modern computer codes can calculate atom-diatom differential cross-sections (DCS) to a very high accuracy. The DCS' contain a wealth of information about the physics of collision, in particular, about the presence (or otherwise) of long-lived resonances. This talk is about methods which can be used to extract this information from pre-calculated numerical data. The currently employed technique involves the Pade' analysis of the S-matrix element combined with a semiclassical reconstruction of the angular distribution. Examples include the H+D2->HD+D reaction recently reviewed by Stewart Althorpe at one of our seminars.

Tuesday 23 April, 4:00 pm

Dr. W. J. Munro, Hewlett Packard

Quantum Computation and Communication with Light.


Tuesday 26 March, 4:00 pm

Dr. Stuart Althorpe, Department of Chemistry, University of Durham

Direct and time-delayed mechanisms for the H + D2 --> HD + D reaction.

Recently we reported the first full quantum simulation of the time-evolution of a bimolecular reaction [Nature v416, p67 (2002)]. The simulation was done for the H + D2 reaction, and direct comparisons were made with scattering experiments. We see that there are two reaction mechanisms in H+D2, which scatter the HD product in opposite directions in space. One mechanism is direct; the other is indirect and is delayed by 25 femtoseconds.

Tuesday 12 March, 4:00 pm

Dr. Gleb Gribakin, Department of Applied Mathematics and Theoretical Physics, Queen's University Belfast

Application of zero-range potentials to the problem of positron interaction with molecules.

It has been known for over fourty years that positron annihilation rates on many polyatomic molecules are anomalously large. In a naive picture the annihilation rate per molecule should be proportional to the number of molecular electrons. However, experimental values exceed such estimates by up to four orders of magnitude. Recent theoretical and experimental advances make it clear that the high annihilation rates are caused by positron capture in vibrationally excited states of the positron-molecule complex, also known as vibrational Feshbach resonances (VFR).

Solving the full positron-molecule problem with account of nuclear vibrational motion is a formidable task. However, the problem can be treated successfully in the zero-range potential approximation. The talk will contain a brief introduction into this useful theoretical atomic physics tool. Then I will show that it allows one to examine all essential features of positron-molecule annihilation including the VFR.

Thursday 21 February, 10:00 am, Room G001

Prof. Gerard J. Milburn, Centre for Quantum Computer Technology The University of Queensland, Australia

Linear optics quantum computation.

Quantum computers are expected to increase the efficiency of algorithms for solving problems such as factoring large integers. However an experimental implementation of even relatively simple quantum information processing, with a few to a dozen qubits, presents an enormous technological challenge. One of the earliest proposals for quantum computation is based on implementing a quantum bit with two optical modes containing one photon. The proposal is appealing due to the ease with which photon interference can be observed. However these early proposals required huge single photon optical nonlinearities. Unfortunately currently available optical nonlinearities are too weak and too noisy to be useful. Recently however we have discovered that efficient quantum computation is possible using only beam splitters, phase shifters, single photon sources and photo-detectors [E.Kill, etal. Nature vol409, 46 (2001)]. Our methods exploit feed forward from photo-detectors and are robust against errors from photon loss and detector inefficiency. In this talk I will outline this scheme, highlighting the technical challenges required to implement it, and current experimental progress at The University of Queensland.

Tuesday 19 February, 4:00 pm

Prof. Sergei Sheinerman, Department of Physics, St.Petersburg State Maritime Technical University, Russia

Post-collision interaction and dynamics of two-electron decay from the Xenon 4d hole.

Ejection of two Auger electrons, one very slow, one fast, is considered, following near threshold 4d-photoionization of the Xe atom. The line shape of fast Auger electron measured in electron/electron coincidences reveales the presence of post collision interaction (PCI) effect. Analysis of the PCI distorted line shapes is carried out in order to clarify dynamics of the two Auger electron decay.

Tuesday 12 February, 4:00 pm

Dr. Marcus Appleby, Department of Physics, Queen Mary, University of London

Observability of non-self adjoint operators.

There is a long-standing and widespread belief that non self-adjoint operators do not correspond to physical observables. It is argued that this belief is unjustified. The talk will begin with a discussion of imprecise joint measurements of non-commuting observables. It will then be shown that a solution to this problem leads to a solution of the problem of making an imprecise measurement of a non self-adjoint operator. It will be argued that there is nothing to prevent one from measuring non self-adjoint operators: it is just that there is an upper bound on the degree of achievable precision. To put it another way: the observables corresponding to non self-adjoint operators are intrinsically unsharp. It will be suggested that this has a bearing on the role of time in quantum mechanics.

Tuesday 29 January, 4:00 pm

Dr Peijun Hu, School of Chemistry, QUB

What chemistry can we learn from computer simulations? -Insight into Reactions in Heterogeneous Catalysis.

Catalysis is one the most important subjects in chemistry: Most chemical reactions are catalytic and it has been estimated that 20-30% of GNP is generated through catalytic processes. Despite its importance, catalysis is still the subject of much speculation. In this talk, I will show that density functional theory is a very useful approach to address many problems that are extremely difficult to be studied experimentally in catalysis. Specifically, I will discuss how density functional theory calculations can be used to study two fundamental issues in catalysis: physical origins of (i) reaction barriers; and (ii) reaction pathways.

Wednesday 21 November 2001, 4:00 pm

Prof. Artur Ekert, Centre for Quantum Computation, University of Oxford

A unified approach to quantum compuation.

I will discuss quantum computation in terms of multi-particle quantum interferometry. Quantum algorithms provide description of the same physical phenomenon but in terms of logical operations: a superposition of computational paths is prepared by the Hadamard (or the Fourier) transform, followed by a quantum function evaluation which effectively introduces phase shifts into different computational paths, followed by the Hadamard or the Fourier transform which acts somewhat in reverse to the first Hadamard/Fourier transform and combines the computational paths together. I will use this approach and discuss the quantum speed-up of quantum computers, the origin of decoherence, and the basic ideas behind recoherence and quantum error correction.

Tuesday 20 November 2001, 4:00 pm

Prof. Roberto D. Rivarola, Instituto de Fisica Rosario, Universidad Nacional de Rosario and CONICET, Rosario, Argentina

One and two active electron reactions in collisions between charged particles and atomic and molecular targets.

Collisions between light and heavy charged particles impacting on atomic and simple molecular targets are studied. Distorted wave models are employed to describe the different mechanisms proposed to explain one and two active electron processes, within first and second order-four body series and independent electron and independent event models. Stopping power calculations for bare heavy projectiles are presented, paying particular attention on molecular targets of biological interest.

Tuesday 13 November 2001, 4:00 pm

Dr Thomas Konrad, Fakultat fur Physik der Universitat Konstanz

The detection of quantum oscillations by means of consecutive unsharp measurements.

The normalized state $\ketpsi{t}=c_1(t)\ket{1}+c_2(t)\ket{2}$ of a single two-level system performs oscillations under the influence of a resonant driving field. It is assumed that only one realization of this process is available. We show that it is possible to approximately visualize in real time the evolution of the system as far as it is given by $|c_2(t)|^2$. For this purpose we use a sequence of particular unsharp measurements separated in time. They are specified within the theory of generalized measurements in which observables are represented by positive operator valued measures (POVM). An experimental realisation of the detection scheme in Quantum Optics is proposed.

Tuesday 6 November 2001, 4:00 pm

Dr Elena Bichoutskaia, School of Chemistry, University of Nottingham

On the Semiclassical Approach to Cold Atomic Collisions.

We extend the semiclassical description of two-state atomic collisions to low energies for which the impact parameter treatment fails. The problem reduces to solving a system of first-order differential equations with coefficients whose semiclassical asymptotes experience the Stokes phenomenon in the complex coordinate plane. Primitive semiclassical and uniform Airy approximations are discussed.

Tuesday 30 October 2001, 4:00 pm

Dr V Vedral, Imperial College

Quantum Computation, Geometry and Entanglement.

This talk will be a brief introduction to quantum computation, both from the theoretical and the practical perspective, and its relationship with some fundamental aspects of quantum mechanics. I will begin by explaining the basic difference between the quantum and classical computation and then exemplify powers of quantum computation through the well known Grover search algorithm which achieves a quadratic speed-up over its classical counter-part. Although theoretically quantum computers are provably more efficient, they are very difficult to make because of decoherence - the process of destroying superpositions though interactions with the environment. In the second half of my talk I wish to present an alternative way of performing quantum computation which uses geometrical phases to perform quantum gates. This promises to offer a fault-tolerant quantum computation, resistant to noise, and I discuss various possibilities for its implementation.

Tuesday 23 October 2001, 4:00 pm

Dr Fred Currell, Queen's University

Here come the HCIs!

Electron Beam Ion Traps (EBITs) are renowned for their ability to investigate the physics of highly charged ions (HCIs). The ability to interact a freely tunable, quasi monoenergetic electron beam with trapped HCIs, coupled with observation of either product ions or photons gives them a unique place in experimental atomic and molecular physics. Some of these capabilities will be illustrated through a brief review of experimental results from the Tokyo EBIT.

Building upon this review, plans for a new EBIT-like machine will be presented, along with a tentative roadmap for its construction and use. Whilst not aiming at producing ions of the very highest charge states, this machine will have several new features facilitating new measurements. In particular, the speaker is interested in getting feedback from the theoretical community regarding measurements of interest whilst the new machine is still on the drawing board.

Monday 8 October 2001, 4:00 pm

Prof Ray Flannery, School of Physics, Georgia Institute of Technology

Quantal and Classical Stark Mixing at ultralow energies.

Solution to a long-standing problem in atomic physics is provided. A 40 year-old problem of nl-nl' transitions (Stark Mixing) in a Rydberg atom induced by the time-dependent (dipole) electric field generated by (adiabatic) collision with a slow ion is solved exactly in a very elegant and novel way. The exceptional rich dynamical symmetry of the hydrogen atom enables development of a radically new theory of the process and construction of both the exact classical and quantal solutions in a unified way. The classical-quantal correspondence transcends the well-known Erenfest's theorem just because of the SO(4) group symmetry of the hydrogen atom. An advantage of the (complementary) classical treatment is that it is able to expose essential physics, which remains obscured in the quantal treatment. The exact classical and quantal analytical solutions expose the analytical beauty of the problem, in addition to its pragmatic value. A classical transition probability is defined. Probabilities for the full array of transitions within the n-shell are derived in excat analytical forms. As n is increased, convergence of the quantal onto the classical transition probabilities (as a function of l' and a "collision parameter" chi) is illustrated. Agreement with measurements is obtained. Movies of the Stark Mixing process and the variation of the probabilties with time will be shown.

Tuesday 2 October 2001 CANCELLED

Prof G S Agarwal, Physical Research Laboratory, Ahmedabad-38009

Controlling Light by Dispersion Management - Distortionless Sub- and Super-Luminal Propagation.

It is now becoming increasingly clear that coherent fields can control the fundamental properties of a medium like dispersion and absorption. In particular, one could get distortionless propagation of pulses even through an absorbing medium. An appropriate management of the dispersive and absorptive properties of a medium has led to remarkably very large number of applications. For example it is now possible to produce ultraslow light and even stop the light. One could even have propagation in frequency regions where it is normally not permitted and manage dispersion in desired spatial regions of the medium. Furthermore the coherent control can also produce superluminal propagation of light as well as cloning of light pulses. In this talk many of these new ideas and experiments would be reviewed.

SEMINARS in 2000-2001


Wednesday, 20 June 2001

Prof Edward Armour, School of Mathematical Sciences, Nottingham University

Hydrogen-antihydrogen interactions at low energies.

A large amount of effort is currently going into devising a method of preparing antihydrogen and trapping it at very low temperatures,[1-4]. The aim is to carry out experiments on it to test the CPT invariance of quantum field theory and the equivalence principle that is the corner stone of Einstein's general theory of relativity. In view of this, we have carried out calculations of the interaction between H and AH at low energies. Our starting point has been the detailed calculation of the potential between them by Kolos et al. [5]. We have been able to show that for internuclear distances of 0.8 bohr and above, the electron and the positron are bound to the nuclei [6].

Uptil now, all calculations carried out on H-AH, including the most accurate calculation so far of low energy scattering [7], have been within the Born-Oppenheimer (BO) approximation. We have examined the effect of including the terms in the Hamiltonian representing coupling between the light particle and nuclear motion that are neglected in the BO approximation and found this to be small [8]. We are in the process of carrying out a variational calculation to determine the s-wave scattering parameters for H-AH scattering, using the Kohn method. We are currently working on the problem of including a channel to represent the formation of positronium and protonium, i.e. a bound state of a proton and an antiproton.

[1] M. Charlton, J. Eades, D. Horvath, R.J. Hughes and C. Zimmermann, Phys. Rep. 241, 65 (1994).

[2] M.H. Holzscheiter et al., Nucl. Phys. B (Proc. Suppl.) 56 A, 338 (1997).

[3] M.H. Holzscheiter and M. Charlton, Rep. Prog. Phys. 62, 1 (1999).

[4] G. Gabrielse, Adv. At. Mol. Phys. 45, 1 (2001).

[5] W. Kolos, D.L. Morgan, D.M. Schrader and L. Wolniewicz, Phys. Rev. A 11, 1792 (1975).

[6] E.A.G. Armour, J.M. Carr and V. Zeman, J. Phys. B 31, L679 (1998).

[7] P. Froelich, S. Jonsell, A. Saenz, B. Zygelman and A. Dalgarno, Phys. Rev. Lett. 84, 4577 (2000).

[8] E.A.G. Armour and V. Zeman, Int. J. Quant. Chem. 74, 645 (1999).

Tuesday, 12 June 2001

Dr Francois Bardou, Institut de Physique et Chimie des Materiaux de Strasbourg

Levy flights in ultracold atoms and quantum tunnelling.

One usually thinks of random phenomena in terms of moderate fluctuations around some average value. However, there also exist cases in which intrinsic fluctuations are so large that the very notion of average value becomes irrelevant. Unusual behaviours (like anomalous scaling laws and non-ergodicity) emerge in these phenomena, called `Levy flights', that are dominated by fluctuations. This talk will concentrate on two quantum phenomena in which Levy flights play a crucial role: subrecoil laser cooling of atomic gases and tunnelling through nanometric tunnel junctions.

Tuesday, 5 June 2001

Prof Stephen M. Barnett, Department of Physics and Applied Physics, University of Strathclyde, Glasgow

Retrodiction.

Much of physics and indeed of other human endeavour is aimed at prediction, that is making statements about the future on the basis of present knowledge. These statements are usually in the form of probabilities. Some problems, however, are intrinsically retrodictive in that they attempt to make statements about the past based on present knowledge. Simple examples include archeology, forensic science and also communications. I will show that the connection between prediction and retrodiction is closely associated with conditional probabilities through Bayes' theorem. Quantum theory is usually used as a predictive theory. I will show that it can also formulated as a retrodictive theory and will give examples of its application.

Tuesday, 22 May 2001

Dr Helen Fielding, King's College London

Electron wavepacket dynamics in molecules.

The dynamics of autoionising and predissociating Rydberg electron wavepackets are studied in molecular NO. Predominantly radial motion of the electron wavepacket is observed which is similar to that previously reported in atomic systems. Interference effects similar to those observed in unperturbed Rydberg series are evident and third and fourth order partial revivals are identified. Some peculiarly molecular features are also observed. When the classical period of electronic motion is close to the classical period of rotation of the molecular ion, the molecular dynamics dominates the electronic dynamics. Interesting beat patterns are evident at specific excitation energies which we interpret classically in terms of coupled oscillators.

Tuesday, 15 May 2001

Professor David C Clary, Department of Chemistry, University College London

Quantum simulation of water clusters, hydrated molecules and proteins.

Many molecular systems are controlled by weak bonds such as hydrogen bonds. These bonds often have strongly anharmonic motions and quantum effects can be important. We have been using the diffusion Monte Carlo method to simulate weakly-bound molecular systems, including water clusters, hydrated molecules and proteins. This rigorous simulation method has given new insight into quantum effects such as tunnelling and delocalised nuclear motion in these systems. This general technique for finding quantum solutions to multidimensional problems also enables detailed predictions to be made for comparison with experiment and allows for potential energy surface to be tested.

Tuesday, 8 May 2001

Professor Jonathan Tennyson, Department of Physics & Astronomy, University College London

Electron-molecule scattering using the R-matrix method.

Low energy electron impacts can lead to both excitation and dissociation of molecules. These processes are important in astrophysics, where excitation rates for molecular ions are criticial, in the upper atmosphere where molecular are destroyed by dissociative recombination, at the edge of fusion plasmas where thermal electrons can dissociate molecular hydrogen, in etching plasmas where the etching process goes via unstable molecular species activated by electron collisions, and in may other locations. Sophisticated codes developed as part of a UK collaboration can treat electron collisions both diatomic and polyatomic molecules. The codes will be described and results for the above processes discussed.

Tuesday, 1 May 2001

Dr. Bernold Feuerstein, Fakulat fur Physik, Universitat Freiburg, Germany

Recollision mechanisms in nonsequential strong field double ionisation of neon and argon. The role of (e,2e) and excitation-tunneling.


Tuesday, 24 April 2001

Dr Andrew Murray, Schuster Laboratory, The University of Manchester

Laser preparation & probing of electron impact interactions.

Tuneable laser radiation is becoming an invaluable tool for the preparation and probing of targets undergoing electron impact excitation and ionization. The advantages of laser radiation are very high resolution (micro-Volts to nano-Volts) together with a high degree of coherence and polarization. These properties can be exploited in experiments combining laser radiation with electron impact excitation and ionization, allowing new information to be obtained from the reaction. In this presentation experiments currently underway at Manchester which exploit these techniques will be described. Electron impact excitation of molecules are probed using tuneable pulsed laser radiation, allowing the excitation properties of individual ro-vibrational states to be ascertained. New experiments are also underway where an atom is prepared in an excited state using CW laser radiation prior to electron impact ionization. By varying the power, polarization & direction of the radiation, the initial state of the atom can be controlled prior to ionization. A detailed 3 dimensional picture of the ionization cross section from this state will then be revealed using the computer controlled (e,2e) spectrometer at Manchester.

Monday, 23 April 2001

Professor V. Buzek, Slovak Academy of Sciences

Universal quantum processors.

A classical computer can be to a first approximation represented as a device with a processor that is a piece of hardware that performs operations on a data register according to a program encoded initially in the program register. The action of the processor is fully determined by the program. In principle, the processor is universal in a sense that we can ``run'' on it an arbitrary program. In this lecture I try to present a quantum version of this picture. I show how "quantum" program can be stored in a program register and how it can be applied to a data register initially prepared in an unknown quantum state.

Tuesday, 3 April 2001

Dr. Jesus F. Castillo, Departamento de Quimica Fisica I Facultad de Ciencias Quimicas, Universidad Complutense de Madrid

QUANTUM AND QUASICLASSICAL STUDIES OF ELEMENTARY CHEMICAL REACTIONS.

This talk will describe theoretical and computational research aimed at understanding of the dynamics of chemical reactions at state-to-state level. The first part of the talk will be focussed on Quantum time-independent reactive scattering of atom-diatom reactions. A brief description of the close coupling hyperspherical coordinates method will be given. Quantum mechanical calculations for the F+H_2, O(1D)+H_2, Cl+H_2 and H+D2 reactions will be presented and compared to experimental and quasiclassical trajectory results. Resonant scattering effects, non-adiabatic multisurface effects, and stereodynamical features will be discussed for these prototypic reactions. Finally, some recent Quasiclassical trajectory results for H+H2O reaction will be exposed.

Tuesday, 6 March 2001

Prof. Dr. A. Solovyov, A.F. Ioffe Physical-Technical Institute, St Petersburg

MANY-BODY PHENOMENA IN ELECTRON SCATTERING ON METAL CLUSTERS AND FULLERENES.

Many-body phenomena manifesting themselves in electron scattering on fullerenes and metal clusters are considered. Focus is made on the following problems: manifestation of the electron diffraction in elastic and inelastic collisions with clusters, the role of multipole surface and volume plasmon excitations in the formation of electron energy loss spectra (differential and total, above and below ionization potential) as well as the total inelastic scattering cross sections, importance of the polarization effects in electron attachment process, mechanisms of the electron excitation width formation and the relaxation of electron excitations in clusters. The solutions of the outlined problems are given on the basis of the consistent many-body theory developed with the Hartree-Fock jellium model wave functions. Many electron correlations in the system are taken into account, where it is necessary, using the random phase approximation with exchange and the Dyson equation method.

Tuesday, 27 February 2001 CANCELLED

Dr Peter van der Burgt, National University of Ireland, Maynooth

Collective Effects in the Multiphoton Ionisation of Atomic Deuterium.

A transient laser plasma is formed by 3-photon ionisation of atomic deuterium with a pulsed dye laser. The photoionisation is resonant with the metastable 2s state. An electric field is turned on after the laser pulse to extract the ions and to quench the metastable atoms. The ion and photon yields are measured as a function of laser intensity and wavelength. At high laser intensities collective effects due to the mutual interaction of the photo-electrons and the ions affect the time evolution of the plasma. The resonant multiphoton ionisation is influenced by the Stark mixing of the 2s and 2p states in the collective field of the plasma. A discussion will be presented of collective effects in the measurements. A brief description of a theoretical model will also be given.

Monday, 9 October 2000

Dr Michael Jamieson, University of Glasgow

Calculations of Scattering of Cold Hydrogen Atoms.

The current interest in cooling and trapping atoms, and the quest for Bose-Einstein condensation, leaves a need for theoretical studies of ultra-cold atomic ensembles. Knowledge of collisions is essential in describing the behaviour of such ensembles. I will describe some recent calculations including that of the H(1S)-H(2S) scattering length, which was used recently in evidence of the achievement of a Bose-Einstein condensate by experimentalists at MIT.

Tuesday, 3 October 2000

Prof Ian Williams, Queen's University

Ions in intense femtosecond laser fields.

There is considerable current interest in the study of atoms and molecules in intense ultrashort laser pulses. To date experiments have concentrated solely on neutral targets. First experimental results from molecular ions and progress with atomic ions will be discussed.

Tuesday, 12 September 2000

Dr Andrei Korol, St Petersburg Maritime University and Institute of Theoretical Physics, University Frankfurt am Main

Spontaneous and stimulated photon emission by an ultra-relativistic positron channelling in a periodically bent crystal.

A possibility to create a new tunable source of X-ray and gamma-radiation is discussed (Greiner, Korol & Solovyov, 1998-2000). The photons are emitted by ultrarelativistic positrons channelling through a crystal which is periodically bent by a transverse acoustic wave transmitted along a crystallographic plane. In such a system there appears, in addition to the well-known channelling radiation, the radiation due to the periodicity of the trajectory of the projectile which follows the bendings of the channel centerline. The characteristics of this radiation depend on the parameters of the acoustic wave, on the type of crystal and crystallographic plane, and on the energy of positrons. It is established that there exists a wide range of the parameters where the undulator radiation dominates over the channelling radiation. The proposed mechanism allows to generate undulator radiation within the 10 keV - 1 MeV range and, under certain conditions, it leads to the emission of the stimulated radiation of the free-electron-laser type.

Thursday, 3 August 2000

Dr Daniel Bessis, Clark-Atlanta University

Partial-wave dispersion relations in the presence of exchange forces.

Within the static exchange approximation, the EXACT left-hand cut discontinuity, in the complex energy plane, of the s-wave scattering amplitude is calculated for the triplet electron-hydrogen scattering. As a test of the correctness of our results, the dispersion relations for the triplet scattering length are numerically fully validated.

Wednesday, 28 June 2000

Prof. J. N. Das, Calcutta University

Hyperspherical partial wave approach for electron hydrogen atom ionization collision.

Calculating ionization collision cross section in the low-energy domain down to threshold is still a challenging problem. There are many experimental results for triple differential cross sections but theoretically these are not fully understood. Some approaches like hyperspherical close coupling method (HSCC), R-matrix pseudo-state method (RMPS) or convergent close coupling method work to some limited way. There are some inherent difficulties in each of these. Recently the approach put forward by Das is perhaps most straightforward. It is basically dependent on an expansion in terms of hyperspherical harmonics (in five angular variables) and on the expansion of plane wave for two free electrons in these harmonics. The whole problem then lies on accurate solution of the relevant coupled radial wave functions beginning from the origin. Their asymptotic solutions are very simple. Once the equations are accurately calculated the whole of the kinematic domain from little above throshold to intermediate energy range may be easily studied. The method may be used for ionization calculations at low energies, for ionization from excited states, or from hydrogenic ions in a very similar manner. This may be extended to similar three-body ionization cross section calculations.

Tuesday, 27 June 2000

Dr Radoslaw Szmytkowski, Technical University of Gdansk

R-matrix method for the Dirac equation.

A fixed-boundary-condition R-matrix theory for the Dirac equation is examined. It is shown that its earlier formulations, due to Goertzel (1948) and Chang (1975), contained an error. The origin of the error is explained and the correct theory is presented.

Thursday, 15th June 2000

Dr Juliet Pickering, Imperial College London

Atomic Spectroscopy and Laboratory Astrophysics.


Friday, 10th March 2000

Prof. Vadim Ivanov, St Petersburg State Technical University

Many-Body theory calculations of photodetachment from negative ions.


Tuesday, 8th February 2000

Prof. Vladimir Gaiduk, Institute of Electronics and Radio Engineering, Moscow

Modelling of wideband (dielectric and far-infra-red) spectra of H2O and D2O, using inhomogeneous potentials.


Tuesday, 7th December 1999

Dr Hugo van der Hart, Queen's University of Belfast

How does a single photon kick several electrons out of an atom?

The description of electron repulsion is essential for the understanding of many-electron atoms. The electron repulsion can be investigated experimentally by examining a collective response after sending in a probe that acts on only a single electron. Light is such a probe, and the collective response is the emission of more than a single electron, double and triple photoionization. I will discuss some of the recent progress in the description of double and triple photoionization processes. In particular, I will demonstrate why double photoionization is much more important in Li (3.2% of the total photoionization) than in He (1.6% of the total photoionization).

Tuesday, 23rd November 1999

Dr Gleb Gribakin, Queen's University of Belfast

Resonant enhancement of electron-ion recombination and positron-molecule annihilation.

In this talk I would like to speak about two apparently different physical processes: the recombination of low-energy electrons on multicharged many-electron ions and positron annihilation on molecules. This will give an overview of a part of my recent work. Apart from this reason, considering the two topics together can be justified, because in both cases the energy-averaged contribution of resonances exceeds the non-resonant background by orders of magnitude.

It is known from a storage-ring experiment that the rate of electron recombination with Au25+ ions at or below 1 eV has the same energy dependence as the single-particle radiative recombination, while its magnitude is 150 times greater. Another experiment, with room temperature positrons in a trap, shows that the annihilation rate for positrons on moderate-sized organic molecules, such as C4H10 or C6H12, is characterised by the effective number of electrons, Zeff, orders of magnitude greater than the actual number of electrons in the molecule. Moreover, the increase of Zeff with the size of the molecule is close to exponential, and it shows a very strong sensitivity to the chemical composition of the molecule.

The aim of my work is to estimate the resonant enhancement produced in the first case by electron capture into multiply-excited states of the compound ion, and in the second case, by positron capture into vibrationally excited states of the positron-molecule complex. Both problems are characterised by a very large degree of complexity, verging on quantum chaos, which hampers direct numerical calculations.


You may email comments about this page to g.gribakin@am.qub.ac.uk