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.
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 reiθ
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.
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.
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.
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.
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.
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