Tuesday, 5 April 2011
Prof Anatoli Kheifets, Institute of Advanced Studies, Research School of
Physical Sciences, The Australian National University, Canberra, Australia
Atomic photoionization: When does it actually begin?
Among other spectacular applications of the attosecond streaking technique,
it has become possible to determine the time delay between subjecting an
atom to a short laser pulse and subsequent emission of the photoelectron [1].
This observation opened up a question as to when does atomic photoionization
actually begin [2]. We address this question by solving the time dependent
Schröodinger equation and by carefully examining the time evolution of
the photoelectron wave packet. In this way we establish the apparent
"time zero" when the photoelectron leaves the atom. At the same time, we
provide a stationary treatment to the photoionization process and connect
the observed time delay with the quantum phase of the dipole transition
matrix element, the energy dependence of which defines the emission timing.
As an illustration of our approach, we consider the valence shell
photoionization of Ne [3] and double photoionization of He [4]
[1] M. Schultze et al. Delay in Photoemission. Science, 328(5986):1658-1662,
2010.
[2] H. W. van der Hart. When Does Photoemission Begin? Science,
328(5986):1645-1646, 2010.
[3] A. S. Kheifets and I. A. Ivanov. Delay in atomic photoionization.
Phys. Rev. Lett., 105(23):233002, Dec 2010.
[4] A. S. Kheifets, I. A. Ivanov, and Igor. Bray. Timing analysis of
two-electron photoemission. ArXiv e-print 1012.1048v2, December 2010.
Thursday, 31 March 2011
Prof Itzik Ben-Itzhak, J. R. Macdonald Laboratory, Department of Physics,
Kansas State University
Probing molecular ion beams with intense few-cycle laser pulses1 -
two-color controlled
dissociation
We have studied laser-induced fragmentation of molecular-ion beams using
coincidence 3D momentum imaging, with direct separation of all the reaction
products measured simultaneously. These measurements provide detailed
kinetic energy release and angular distributions of the different
fragmentation processes. We mainly focus on the fundamental
H2+ and H3+ molecules
(in 7-50 fs laser pulses having 1012-1016
W/cm2 peak intensity)
as models for more complex systems, and at times we do explore more complex
molecules such as O2+ and CO2+.
In this talk, we will discuss electron localization on specific nuclei
during strong-field dissociation of molecular-ion beams which is
controlled by the relative phase between the 790 and 395 nm components
of an ultrashort laser pulse.
In addition, clear experimental and theoretical evidence for the intriguing
zero-photon dissociation (ZPD) process of H2+ will be
presented. The key
role of the laser-pulse bandwidth will be discussed. Moreover, we will
explore control over the final dissociation product of HD+,
either H+ + D or H + D+ - usually referred to a
channel asymmetry.
Others contributing to this work: B. Gaire, M. Zohrabi, J. McKenna,
U. Ablikim, A.M. Sayler, N.G. Johnson, K.D. Carnes, D. Ursrey, F. Anis,
J. Hernandez, J.J. Hua, and B.D. Esry.
1Supported by the Chemical Sciences, Geosciences and
Biosciences Division, Office of Basic Energy Sciences,
Office of Science, US Department of Energy
Tuesday, 15 February 2011
Dr Stefano Pirandola, Department of Computer Science, University of York
Entanglement-assisted discrimination of Gaussian channels
Discriminating between two different Gaussian channels is one
of the most interesting problems in quantum information theory.
In this talk we discuss how quantum entangled light can reduce
the error probability which affects this statistical problem,
providing a dramatic advantage over the use of classical light
in the regime of few photons. Then we show how this advantage
can be exploited in two paradigmic applications: the detection of
low-reflectivity targets and the readout of digital memories.
1. S. Pirandola, arXiv:0907.3398
2. S.-H. Tan et al., Phys. Rev. Lett. 101, 253601 (2008)
3. S. Pirandola, and S. Lloyd, Phys. Rev. A 78, 012331 (2008)
Tuesday, 18 January 2011
Dr Jason Greenwood, Department of Physics and Astronomy, Queen's University
Belfast
Can we do better than Fourier in analysing signal from a novel
electrostatic ion trap mass spectrometer?
I will present an overview of a new instrument we have developed to
study the interaction of femtosecond lasers with large molecules. In
this device, ions are generated in-situ by a femtosecond laser and
oscillate stably in an electrostatic potential in a manner analogous
to light in a cavity. As the trapping potential is time-invariant (at
least in the lab frame), stability is mass independent allowing, for
example, protons and proteins to be simultaneously trapped. Signal
from trapped ions is recorded by an image charge detector and the
oscillation frequencies yield the ion masses.
The signal is a series of narrow impulses which can be analysed by a
standard Fourier transform. However, sinusoids are not a great match
to the signal, resulting in multiple harmonics and lower frequency
precision than one might anticipate from the data. I will demonstrate
an alternative frequency analysis method we have developed which uses
a comb function to sample the data. By making use of the well defined
phase of the signal impulses relative to the initial ion generation
time, I will show that harmonics can be suppressed and considerably
higher precision achieved. This combination of novel instrument and
new analysis method facilitates the generation of high resolution
mass spectra over a wide mass range.
Tuesday, 14 December 2010
Dr Vitali Averbukh, Department of Physics, Imperial College London
Theory of inter-atomic electronic decay processes in clusters
Since their theoretical prediction a decade ago [1], inter-atomic
(inter-molecular) Coulombic decay (ICD) and related processes have been in
the focus of intensive theoretical [2] and experimental [3] research. The
spectacular progress in this direction has been stimulated both by the
fundamental importance of the discovered electronic decay phenomena and by
the exciting possibility of their practical application, for example in
spectroscopy of interfaces [4].
Inter-atomic decay phenomena take place in inner-shell-ionized clusters due to
electronic correlation between two or more cluster constituents. These
processes lead to the decay of inner-shell vacancies by electron emission and
in many cases also to disintegration of the multiply ionized cluster. The
primary objective of the theory is, thus, to predict the kinetic energy
spectra of the emitted electrons and of the resulting cluster
fragments. These spectra are determined by an interplay between the electronic
decay process and the nuclear dynamics [5]. Key to the reliable prediction
of the observable quantities is, thus, the knowledge of the time scale of the
inter-atomic decay. In this talk, I will review the recent developments in the
ab initio quantum chemical methods for the calculation of
inter-atomic decay rates in excited [6], singly ionized [7] and doubly
ionized [8,9] systems as well as some of their applications to various types
of inner-shell-ionized clusters, e.g. rare gas systems and endohedral
fullerenes.
[1] L. S. Cederbaum, J. Zobeley, and F. Tarantelli, Phys. Rev. Lett. 79,
4778 (1997).
[2] R. Santra et al., Phys. Rev. Lett. 85, 4490 (2000); R. Santra, and
L. S. Cederbaum, Phys. Rev. Lett. 90, 153401 (2003); V. Averbukh,
I. B. Müller, and L. S. Cederbaum, Phys. Rev. Lett. 93, 263002 (2004);
V. Averbukh and L. S. Cederbaum, Phys. Rev. Lett. 96, 053401 (2006).
[3] S. Marburger et al., Phys. Rev. Lett. 90, 203401 (2003); T. Jahnke et al.,
Phys. Rev. Lett. 93, 163401 (2004); G. Öhrwall et al., Phys. Rev. Lett.
93, 173401 (2004); T. Aoto et al., Phys. Rev. Lett. 97, 243401 (2006);
T. Jahnke et al., Phys. Rev. Lett. 99, 153401 (2007); T. Jahnke et al., Nature
Physics 6, 139 (2010); M. Mucke et al., Nature Physics 6, 143 (2010).
[4] S. Barth et al., PCCP 8, 3218 (2006).
[5] S. Scheit et al., J. Chem. Phys. 121, 8393 (2004); J. Chem. Phys. 124,
154305 (2006).
[6] K. Gokhberg, V. Averbukh, and L. S. Cederbaum, J. Chem. Phys. 124, 144315
(2006).
[7] V. Averbukh and L. S. Cederbaum, J. Chem. Phys. 123, 204107 (2005).
[8] P. Kolorenč et al., J. Chem. Phys. 129, 244102 (2008).
[9] V. Averbukh and P. Kolorenč, Phys. Rev. Lett. 103, 183001 (2009).
Tuesday, 23 November 2010
Dr. Tim Freegarde, Department of Physics and Astronomy,
University of Southampton
Using light to cool molecules
The Doppler cooling of atoms has opened up a huge field of ultracold
atomic physics, but relies upon repeated cycling of an optically
closed transition and is therefore restricted to just a handful of
species. With a few notable exceptions, molecular species - and,
indeed, many atoms - cannot benefit from this powerful technique.
This talk will address some alternative methods of optical cooling
that do not rely so heavily upon a closed transition. One class
avoids spontaneous emission by stimulated scattering within the
excited state lifetime, and exploits quantum coherence through atom
interferometry and techniques borrowed from quantum computation. A
second class uses the optical dipole force of a non-resonant laser
beam, which can be given a dissipative component when the optical
field has a retarded dependence upon the atomic or molecular
position.
Tuesday, 14 September 2010
Dr Gerardo Adesso, School of Mathematical Sciences, University of Nottingham
Ground state factorization versus frustration in spin systems
We investigate the existence of factorized ground states in Heisenberg-like
quantum spin models with antiferromagnetic interactions of arbitrary range
and exhibiting a varying degree of frustration. After reviewing a method
developed for frustration-free systems based on tools from quantum
information theory, we extend it to characterize the competition between
frustration and ground state factorization. Low frustration is shown to be
signaled by the existence of factorized ground states at specific values of
the external field, while for higher frustration degrees the factorized
eigenstates do not minimize the energy, leaving necessarily room for
entanglement in the ground state. The compatibility threshold, signaling the
frustration-driven transition between order (factorization) and disorder
(correlation), is investigated and exact analytical factorized solutions are
obtained for short-range as well as infinite-range frustrated quantum
magnets. Ground state factorization is thus revealed as an effective tool
to probe quantum frustration in cooperative systems.
Tuesday, 7 September 2010
Prof Vadim Ivanov, St Petersburg State Polytechnic University
Calculation of structure and photoabsorption in metal clusters and fullerenes
The results of calculations of electronic structure and photoionization
processes in various metal cluster systems are discussed. The main attention
is paid to the role of many-electron interactions in stability of clusters,
photoabsorption and photoionization cross sections and photoelectron angular
distributions.
The calculations of structure are performed within the Hartree-Fock (HF)
and Local Density Approximations (LDA) for the electronic subsystem using
the jellium model for distribution of positively charged ions. The stability
of hollow clusters depending on cavity size is discussed. It is found that
the total energy of cluster obtained both within the HF and LDA has two minima
as a function of the radius of the hole. The self-consistent HF approximation
is applied to describe an atom confined within a fullerene shell of finite
width (the endohedral atom) and define their joint electron structure.
The photoabsorption and photoionisation spectra in the vicinity of plasmon
resonances for metal clusters, hollow clusters and fullerenes and photoelectron
angular distributions are calculated within the Random Phase Approximation
and compared with existing experimental data.
Tuesday, 30 March 2010
Prof Edward Armour, School of Mathematical Sciences, University of Nottingham
Zeff and vibrational resonances in scattering of a heavy positron
by H2
For a positron with wave number k, the rate of annihilation when
scattered by an atom or molecule is proportional to
Zeff(k), the effective number of electrons in the
target that are available to the positron for annihilation. There is currently
great interest in the very large positron annihilation rates, and hence values
of Zeff(k), that have been observed in low-energy
positron scattering by some organic molecules. These are observed
experimentally to occur at energies just below the energies of excited
vibrational states of the molecule concerned. This has been explained by
Gribakin [1] and Gribakin and Lee [2] as being due to Feshbach resonances
involving excited quasi-bound vibrational states. These treatments make
skilful use of approximate methods. It is of interest to determine how the
expression obtained for the resonant contribution to
Zeff(k) from a quasi-bound state using a very
accurate method is related to the expressions obtained in the above papers.
In view of this, in this paper I carry out a detailed ab initio treatment of
positron scattering by H2 using the Kohn variational method.
H2 is the simplest molecule, which makes it easier to take into
account all the interactions involved. However, a positron does not form a
bound state with H2. To investigate resonant behaviour in
Zeff(k), I increase the mass mp
of the 'positron' so that it forms a weakly bound state with H2.
The expression I obtain for the resonant contribution to
Zeff(k) has some similarity with the expressions
obtained by Gribakin and Lee. This gives some support to their explanation
of the very large values of Zeff(k). However, they
neglect corrections to the Born-Oppenheimer approximation, whereas these play
an important role in the enlargement of the resonant contribution to
Zeff(k) from a quasi-bound state in my treatment.
My treatment could be applied to positron scattering by molecules such as
methyl halides in which very high Zeff(k) values are
observed, though using the Kohn variational method would be considerably more
complicated than in the case of H.
[1] G F Gribakin, Phys. Rev. A 61, 022720 (2000).
[2] G F Gribakin and C M R Lee, Phys. Rev. Lett. 97, 193201 (2006).
TALK CANCELLED
Tuesday, 23 March 2010
Dr. Dimitris Tsomokos, Department of Mathematics, Royal Holloway,
University of London
Topological order following a sudden quench
In this talk I discuss the dynamical response of a topologically-ordered
quantum state to a sudden change of evolution. In particular, I consider
that a quantum spin lattice is initialized in the ground state of the Toric
Code Model and, subsequently, it is let to evolve under a different
Hamiltonian H. Firstly, we determine conditions on the form of H under which
the entanglement entropy and the topological entropy of the initial state
remain unaffected by the sudden quench. Furthermore, we provide numerical
evidence for the influence of other quench Hamiltonians on the topological
order using the topological entropy and a new measure, which is based on the
similarity between partial states from different topological sectors.
http://sites.google.com/site/tsomokos
Tuesday, 9 March 2010
Dr Marco G. Genoni, Università degli Studi di Milano
Quantifying non-Gaussianity for quantum information
In this talk I will review the properties of two recently
proposed measures of non-Gaussianity based, the first on the
Hilbert-Schimidt (HS) distance, the second on the quantum
relative entropy (QRE), between the state under examination
and a reference Gaussian state. I will apply them to
different families of non-Gaussian quantum states,
showing that the two measures have the same basic properties
and also show the same qualitative behaviour on most of the
examples taken into account. Anyway they introduce a different
relation of order, i.e. they are not strictly monotone.
By using the QRE based measure, I will show how to rephrase the
extremality of Gaussian states for some entropic quantities
such as conditional entropy, coherent information and Holevo
bound. I will also study the behaviour of our measures under
Gaussification and de-Gaussification processes, and in particular
I will investigate the role played by non-Gaussianity in
some relevant quantum information protocols such as entanglement
distillation. Finally I will show some recent results where the
QRE based measure is related to the quantum central limit theorem.
Tuesday, 23 February 2010
Dr. Tony J. G. Apollaro, Department of Physics, University of Florence
Dynamics of entanglement in the presence of spin environments
Entanglement is one of the most fragile, and promising, properties of a
quantum state: several quantum information protocols rely on it;
nevertheless, entanglement between physical systems is rapidly destroyed
because of the unavoidable interaction with the surrounding environment. In
this talk, the dynamics of two maximally entangled qubits interacting with
indipendent environments composed of spin chains is shown to have quite
different regimes, ranging from entanglement preservation to ESD
(entanglement sudden death) depending on the environmental properties.
Tuesday, 17 November 2009
Dr. Matthias Keller, Lecturer of Atomic, Molecular and Optical Physics,
University of Sussex
Cooling and Non-Destructive State Detection of Molecular Ions:
An Important Tool for Physics and Chemistry
Cold molecules have been identified as attractive systems for quantum
information processing and ultrahigh-resolution spectroscopy, with the aim
to develop new time standards, and to test fundamental physics. Furthermore,
the investigation of ultra-cold molecular collisions opens new insight into
quantum mechanical effects in chemical reactions.
Due to their complicated level structure it is not possible to directly
laser-cool molecules, or to measure their internal states directly. These are
major road blocks which limit possible applications despite the favorable
properties of molecules.
In my presentation I will propose novel schemes which can be employed to
overcome these roadblocks: By using sympathetically cooled molecular ions,
techniques from quantum information technology and cavity QED can be employed
to cool and detect the internal states of molecules non-destructively.
In order to cool the internal degrees of freedom of molecular ions their
motion in the external trapping potential or the mode of an optical cavity
can be employed as a reservoir for the laser induced cooling process.
Similarly, the detection of the internal state of molecules can be detected
by coupling a particular molecular state to an atomic ion, trapped alongside
the molecule through their common motion in the trapping potential.
Tuesday, 10 November 2009
Dr Gabriele De Chiara, Grup d'Optica, Universitat Autonoma de Barcelona
Critical behavior of one dimensional ion crystals
A one dimensional ion chain, confined by a harmonic potential, exhibits a
sudden transition to a zigzag configuration when the radial potential falls
below a critical value. Here we show that this structural change is a phase
transition of second order, whose order parameter is the crystal
displacement from the chain axis. We study analytically the transition using
Landau theory and find full agreement with numerical predictions. Our theory
[1] allows us to determine analytically the system's behaviour at the
transition point. In addition, we show that the statistical properties of a
Coulomb crystal can be measured by means of a Ramsey interferometry
procedure performed on the spin of one ion in the chain [2]. The ion spin,
constituted by two internal levels of the ion, couples to the crystal modes
via spatial displacement induced by photon absorption. The loss of contrast
in the interferometric signal allows one to measure the autocorrelation
function of the crystal observables. Close to the linear-zigzag transition
point, the signal reveals information on the critical properties of the
crystal.
[1] S. Fishman, G. De Chiara, T. Calarco, and G. Morigi: Structural phase
transitions in low-dimensional ion crystals, Phys. Rev. B 77, 064111
(2008).
[2] G. De Chiara, T. Calarco, S. Fishman, and G. Morigi: Ramsey
interferometry with a spin embedded in a Coulomb chain, Phys. Rev. A
78, 043414 (2008).
Tuesday, 20 October 2009
Prof Artur Ekert, Professor of Quantum Physics, Mathematical Institute,
University of Oxford, Lee Kong Chian Centennial Professor,
National University of Singapore
Less reality, more security
Recent research shows that security of communication can be
guaranteed by peculiar "non-local" correlations, no matter whether
they are of quantum origin or not. Bell's inequality alone makes
seemingly insane scenario possible --- devices of unknown or dubious
provenance, even those that are manufactured by our enemies, can be
safely used for secure communication, including key distribution. This
is a truly remarkable feat, also referred to as the "device
independent cryptography". I will provide a brief overview of the
intriguing connections between Bell's inequality and cryptography.
Recommended reading: Semi-popular article titled "Less reality, more
security" available at
http://www.arturekert.org/Site/Varia.html -
abbreviated version published in Physics World, September 2009.
Thursday, 30 April 2009
Prof F Barry Dunning, Department of Physics and Astronomy, Rice University,
Houston, Texas
Designer atoms: Engineering Rydberg atoms using pulsed electric fields
Atoms in high-lying (n~300) Rydberg states provide a valuable
laboratory in which to explore the engineering of electronic
wavefunctions using carefully-tailored sequences of short electric
field pulses whose characteristic times (duration, rise/fall times)
are much less than the classical electron orbital period. The level
of control that can be exercised is illustrated with reference to
quasi-one-dimensional atoms and the production of localized wave
packets that move in near-circular orbits. Although such Bohr-like
wave packets slowly dephase and disperse their localization can be
maintained for many hundreds of orbits by external driving using a
periodic train of pulses and they can be further manipulated by
slowly "chirping" this drive field. The physics underlying these
control schemes is explained using classical trajectory Monte Carlo
simulations. Even in the absence of external driving strong
relocalization is observed at late times due to quantum revivals
demonstrating that quantum effects can be seen even in mesoscopic
very-high-n atoms.
Tuesday, 24 March 2009
Dr Jaeyoon Cho, Centre for Theoretical Atomic, Molecular and Optical Physics,
School of Mathematics and Physics, Queen's University Belfast
Cavity-assisted energy relaxation for quantum many-body simulation
Advances in laser cooling and trapping technologies have opened up
intriguing possibilities of using atomic systems to simulate quantum
many-body phenomena in strongly correlated regimes. Concerning such
quantum simulation, diverse studies have been carried out with the
main emphasis placed on how to manipulate atomic systems to mimic
particular many-body model Hamiltonians. However, there is no general
way of simulating energy relaxation in such systems despite special
interest being drawn to the properties of the ground states in most
condensed-matter problems. In this talk, I introduce a cavity-assisted
energy relaxation mechanism whereby strongly correlated many-body
systems decay into their ground states. I also discuss the remarkable
possibility of exploiting the mechanism as a means to probe the
ground-state energy gap.
Tuesday, 3 March 2009
Mr John Goold, Ultracold Quantum Gases Group,
Department of Physics, National University of Ireland, UCC, Cork
Detection and engineering of spatial mode
entanglement with ultra-cold bosons
We outline an interferometric scheme for the detection of bi-mode and
multi-mode spatial entanglement of finite-temperature,interacting Bose gases
of fixed particle number. Whether entanglement is present in the gas depends
on the existence of the single-particle reduced density matrix between
different regions of space. We apply the scheme to the problem of a
harmonically trapped boson pair and show that while entanglement is rapidly
decreasing with temperature, a significant amount remains for all
interaction strengths at zero temperature.
Tuesday, 17 February 2009
Dr Jorge Kohanoff, Atomistic Simulations Centre, School of Mathematics and
Physics, Queen's University Belfast
Radiation damage of biomolecular systems:
a first-principles molecular dynamics approach
In this talk I will first present results of first-principles molecular
dynamics simulations of carbon projectiles shooting through water in the
liquid state in the adiabatic regime, where the electrons are always in the
instantaneous ground state. We studied a range of projectile velocities up
to the estimated upper limit for the adiabatic approximation and analysed
the different types of collision events. I will show that for high
projectile velocities collisions are mostly binary, but at lower velocities
most trajectories exhibit a continuous energy loss to the medium, which
cannot be properly described as a sequence of independent binary collisions.
For the slowest projectiles we observe the formation of new chemical
species such as hydronium, H5O2+ and hydrogen peroxide.
When the C-atoms are completely stopped, we also observe the formation of
species like formic acid. By analyzing the generation of secondary
fragments, we see that these are mostly hyperthermal and their spatial rate
of generation increases with decreasing projectile energy. The two most
numerous species are H and OH. We also show preliminary results for
guanine solvated in water.
In the second part I will present results of electronic dynamics with fixed
nuclei in ice. In this regime, and under channeling conditions, swift
protons produce only electronic excitation. Firstly, we observed the
existence of a threshold velocity for electronic excitation of about
0.2 a.u. Next, by monitoring the rate of increase of the total energy, we
computed the electronic stopping power, and reproduced the experimentally
observed maximum in electronic stopping at proton energies of about 70 keV.
We also analysed the charge state of the projectile, the extent and
localization of the deformation of the charge density, and the time scale
for transient processes as a function of the projectile's velocity.
Tuesday, 13 January 2009
Dr Dmitry Savin, Department of Mathematical Sciences, Brunel University
Scattering and impedance of waves in chaotic systems with absorption
Scattering in quantum chaotic systems (e.g. cavities) is considered.
Under real laboratory conditions there is a number of different sources which
course a part of a flux to get irreversibly lost in the environment. In this
talk, I will discuss two viewpoints on the arising problem of absorption by
looking at it both from the "outside" (non-unitary S matrix, reflection
coefficient and scattering phase) and from the "inside" (impedance matrix,
local density of states and reactance). Within the random matrix approach, we
establish the interrelation between the corresponding quantities that amounts
to studying statistics of the local Green function. We derive the general
relationship between the distribution of the latter at finite absorption and
its correlations in ideal system without absorption that can be viewed as a
kind of the fluctuation-dissipation relation. This allows us to find further a
number of exact analytic results at arbitrary absorption. The comparison to
available experimental data is also discussed.
Tuesday, 16 December 2008
Dr John Ludlow, Department of Physics, Auburn University, Auburn, AL, USA
Time-dependent close-coupling calculations of electron-impact
ionization of atoms and ions
The time-dependent close-coupling (TDCC) method solves the time-dependent
Schrodinger equation for the problem of ionization of few body systems.
An overview of the method and its history will be given, along with a
comparison to other ab-initio methods. Results will be presented for the
electron-impact single ionization of C+, Mg and Al+.
Future applications and extensions of the TDCC method to diatomic molecules
will be discussed.
Tuesday, 2 December 2008
Dr Nial Friel, School of Mathematical Sciences, University College Dublin
Statistical inference for posterior distributions with intractable normalising
constants
This talk will examine situations in statistics where the distribution
of interest is specified up to a normalising constant, which depends on
parameters of the distribution. In this case, inference, from both a
likelihood and Bayesian perspective, is difficult. I will explore examples
from spatial statistics, classification and social network analysis where
this occurs, and present various different approaches to overcoming this
problem.
Tuesday, 18 November 2008
Dr Radim Filip, Department of Optics, Palacky University, Olomouc,
Czech Republic
Experimental CV entanglement distillation from non-Gaussian noise
The distribution of entangled states between distant parties in an optical
network is crucial for the successful implementation of various quantum
communication protocols. However, owing to the unavoidable loss in any
real optical channel, the distribution of loss-intolerant entangled
states is inevitably inflicted by decoherence, which causes a degradation
of the transmitted entanglement. To combat the decoherence, entanglement
distillation, which is the process of extracting a small set of highly
entangled states from a large set of less entangled states, can be used.
Here we report on the mesoscopic distillation of deterministically
prepared entangled light pulses that have undergone non-Gaussian noise.
The entangled light pulses are sent through a lossy channel, where the
transmission is varying in time similarly to light propagation in the
atmosphere. By employing linear optical components and global classical
communication, the entanglement is probabilistically increased.
Monday, 3 November 2008
Prof Man-Duen Choi, Department of Mathematics, University of Toronto
My Adventures in the Quantum Wonderland
In the early 70's, I started off my mathematical journey in the
wonderland of completely positive linear maps. Now, in an unexpected era
of quantum computers, the time runs backwards in an alternate world. As
I have to come back to the same scene, I shall report what I found there,
through the Looking-Glass.
Tuesday, 7 October 2008
Prof David Manolopoulos, Physical and Theoretical Chemistry Laboratory,
University of Oxford
Quantum diffusion of hydrogen and muonium atoms in liquid water
and hexagonal ice
The ring polymer molecular dynamics (RPMD) method has been used to study
the diffusion of muonium, hydrogen, and deuterium atoms in liquid water
and hexagonal ice over a wide temperature range (from 8 to 361 K). Quantum
effects are found to dramatically reduce the diffusion of muonium in water
relative to that predicted by a classical simulation. This leads to a simple
explanation for the lack of any significant isotope effect in the observed
diffusion coefficients of these species in the room temperature liquid.
Our results indicate that the mechanism of the diffusion in liquid water
is similar to the inter-cavity hopping mechanism observed in ice, supplemented
by the diffusion of the cavities in the liquid. Within the same model, we
have also been able to simulate the observed crossover in the c-axis
diffusion coefficients of hydrogen and deuterium in hexagonal ice, and
to obtain good agreement with experimental data on the diffusion of muonium
in hexagonal ice at 8 K, where the process is entirely quantum mechanical.
Thursday, 26 June 2008
Dr Camilo Ruiz Mendez, Imperial College London
Modeling the Non Sequential Double Ionization of Helium beyond one
dimension
The interaction between Helium and a strong laser field at visible
wavelength (800 nm) can strip the two electrons from the Helium atom.
As the two electrons are ejected, by momentum conservation, the doubly
charged ion aquire momentum, mostly in the direction of the polarization
of the laser field. In this talk we will describe the main
characteristics of these momentum distributions using a new model.
Recently we have introduced a model for this interaction beyond the
one-dimensional approximation. In the model the electron correlation
is included in its full dimensionality, while the center-of-mass motion
is restricted along the polarization axis. Our results exhibit a rich
double ionization quantum dynamics in the direction transversal to the
field polarization, which is neglected in the previous models based on
the one-dimensional approximation.
As an application of the model, we apply and analyze the concept of
mapping ionization time on to the final momentum distribution of the
correlated electron dynamics in the nonsequential double ionization of
helium in a strong laser pulse (800 nm).
This mapping provides insight into the double ionization dynamics as it
explains how the momentum distribution is built. To this end, we study,
by means of numerical integration of the time-dependent Schroedinger
equation the temporal evolution of the center-of-mass momentum in a
short laser pulse.
Our results show that in the high intensity regime (1.15×10^15 W cm-2),
the mapping is in good agreement with a classical model including binary
and recoil rescattering mechanisms. In the medium intensity regime
(5×10^14 W cm-2), we identify additional contributions from the
recollision-induced excitation of the ion followed by subsequent field
ionization (RESI).
Tuesday, 10 June 2008
Mr Mathias Albert, Laboratoire de Physique Theorique et Modeles Statistiques,
Universite Paris Sud
Superfluidity vs Anderson Localization in a dilute Bose
gas
Phase coherence is a key ingredient of many characteristic quantum
effects in transport phenomena, some of the most striking ones being
superfluidity (SF), conductance quantization, or the quantum Hall
effect. In particular, interference effects have a prominent role in
presence of disorder, resulting in weak or strong Anderson localization
(AL).
Recent advances in creating and manipulating guided cold atoms and atom
lasers opens up the prospect to re-examine such transport phenomena in
Bose-Einstein condensates (BEC).
BEC systems are of particular interest in that respect because they are
almost perfectly phase coherent, and interaction between their
components can be easily modified and modeled, opening new directions to
experimental and theoretical studies.
In this talk I will discuss transport properties of a one dimensionnal
Bose-Einstein condensate moving through a disordered region of finite
extent. I will show that the interplay between disorder and interactions
leads to a quite rich physics where superfluidity and Anderson
localization are in competition. Motivated by recent experiments, I will
also discuss the effect of disorder on dipole oscillations.
References: T. Paul et al PRL 98, 210602 (2007);
M. Albert et al cond-mat/0803.4116v1 (2008)
Tuesday, 3 June 2008
Prof Vladimir Minogin, Walton Researcher, Tyndall National Institute, Lee
Maltings, Photonics Building, Cork, Ireland, and Institute of Spectroscopy,
Russian Academy of Sciences, Troitsk, Moscow oblast, 142190, Russia
Dynamics of atoms in far-off-resonance and near-resonance laser fields
We discuss two basic theoretical approaches to translational dynamics of
atoms in laser fields which are of interest for various applications,
including atom optics, atomic frequency standards and clocks, micro- and
nanofabrication of materials, as well as atom lithography with a nanometer
resolution. One of them is a semi-classical approach based on classical
description of the atomic centre-of-mass motion and quantum-statistical
description of the internal atomic states. The other, fully
quantum-statistical approach takes into account the quantum-mechanical
nature of the linear momentum exchange between the atoms and light fields.
The above approaches are illustrated by two examples. One of them describes
focusing of atomic beams by the atom near-field microlenses formed by the
optical fields existing in the vicinity of small apertures in a metal
screen. This approach opens a way for fabricating a large set of atom
microbeams from a single initial atomic beam. As another example we
describe 3D model of a magneto-optical trap (MOT) for a simplest case of
(1+3)-level atoms and compare predictions of 3D theory with experimental
observations. 3D analysis of the motion of (1+3)-level atoms in the MOT
shows that the friction force, momentum diffusion tensor, and accordingly
atomic temperature are strongly influenced by the coherences between the
atomic magnetic sublevels.
Tuesday, 29 April 2008
Dr Alessio Serafini, Quantum information theory group,
Department of Physics and Astronomy, University College London
An ion trap-based optical entanglement generator
Recent years have witnessed, in the context of quantum information
processing, remarkable advances both in the coherent control of the
oscillations of trapped ions and in the implementation of quantum
information protocols with optical systems in the continuous variable
regime. Nonetheless, the accuracy of such continuous variable protocols,
with the prototypical example of quantum teleportation, is still
critically limited by the degree of entanglement that can be currently
achieved in "all optical" set-ups. Entanglement in such systems is in
fact generated by parametric processes in non linear crystals, and is
limited by the strength of the exploited nonlinearities. In this talk,
an alternative way to generate continuous variable quantum optical
entanglement is presented: two trapped ions are first entangled via
their Coulomb interaction by controlling their trapping frequencies and,
then, their entanglement is swapped to two light modes. This strategy
would grant unprecedented degrees of entanglement between the two modes.
Even if several technological hurdles will have to be faced towards the
optimal implementation of the proposed scheme, we will show that, quite
surprisingly, such an entanglement generator is likely to outperform
parametric processes even if built with currently available technology.
Tuesday, 22 April 2008
Dr Lampros Nikolopoulos, Centre for Theoretical Atomic, Molecular and Optical
Physics, Queen's University Belfast
Atoms in intense fields
The interaction of strong EM fields with atomic and molecular systems,
in the long wavelength limit has been associated with multiphoton
phenomena such as high order harmonic generation (HOHG), non-perturbative
above-threshold ionization (ATI) as well as with multielectron ejection.
At the other extreme, the short wavelength range, single-photon atomic
double ionization and/or triple excitation are fairly well understood, in
the weak-field limit, both experimentally and theoretically. Nowadays,
sources of strong radiation of UV/soft XUV regime, have developed either
with HOHG or with Free Electron Laser (FEL) techniques. On the basis that
phenomena involving the strong interaction between such radiation and AMO
systems are experimentally accessible, in addition to their fundamental
significance, I discuss the present theoretical and experimental status
as well as I present the kind of problems that we face when theory needs
to meet the experiment or the other way around. Ab-initio results,
including experimental results, on the generic systems of helium, a
two-electron system, are presented.
Tuesday, 18 March 2008
Dr Ulrik L Andersen, Department of Physics, Technical University of Denmark,
Lyngby
Cleaning continuous variable quantum states
Revolutionary new techniques are being developed worldwide for the way
information is prepared, distributed, processed and stored. The basic
idea is to take advantage of the laws of quantum mechanics that allow
efficient parallel processing and unconditional secure communication of
information encoded in quantum systems.
The ability to transmit, store and manipulate quantum information
without errors is prerequisite to the realization of most of these
quantum information protocols. As errors are inherent to any realistic
implementation, the future of quantum information processing strongly
relies on our ability to detect and correct these errors.
In this talk I summarize different cleaning protocols that purify,
distill or correct errors in continuous variable quantum systems.
Tuesday, 4 March 2008
Dr Madalina Boca, Department of Physics, University of Bucharest
A relativistic generalization of the Kramers-Hennberger transformation
A relativistic generalization of the Kramers-Hennberger transformation
is presented. The low momentum regime approximation, valid for light
atoms interacting with arbitrarily intense plane wave laser pulses is
defined. It is shown that in this regime the transformed Dirac equation
reduces to two uncoupled Pauli type equations, one for the electron,
the other for positron.
Using a further unitary transformation the evolution equation for the
electron is brought into a form which allows the separation of
relativistic effects from the retardation ones.
Qualitative predictions on the behaviour of the atom in the superintense
laser pulse are presented and finally a numerical example is discussed.
Tuesday, 26 February 2008
Prof Gary J Ferland, Institute of Astronomy, Cambridge University
Quasars - the birth of massive galaxies
The high redshift quasars are the most distant objects we can observe
spectroscopically. Found at distances exceeding 10 billion light years,
the emission we observe was produced when the Universe had an age of
roughly one gigayear. This youth limits and simplifies the amount of
stellar nucleosynthesis that can have occurred. I will discuss the
implications of the observed heavy-element emission lines in even the
highest redshift objects. These lines form in a highly chaotic environment,
in which atomic physics selection effects determine much of what we observe.
The spectrum shows that the interstellar medium of the host galaxy is
enriched in the heavy elements, which means that the quasar phenomenon
must be associated with, but takes place after, the rapid formation and
evolution of massive stars. The FeII spectrum provides an independent
chronometer that measures when the universe passed through an age of
1 gigayear. All of this work underscores the importance of atomic
physics in understanding the young universe.
Tuesday, 19 February 2008
Dr Wonmin Son, Quantum Information Science Group,
School of Physics & Astronomy, University of Leeds
Macroscopic entanglement in many particle systems
In the presentation, I sketch simple approaches how to identify
entanglement in many particle systems. Entanglement witness gives
a bound whose violation of an operator expectation values identifies
entanglement in the system. Based on a given spin chain model, we show
how thermodynamic properties can be link with entanglement witness.
It is related with critical properties of ground state so that quantum
phase transition can also be detected through the entanglement witness.
As the second example, I will discuss the entanglement properties of
the superposed dimmer array of arbitrary qudit systems. In this model,
we found condition of a criticality, which had been derived from the
geometric configuration of the subsystems and dimensionality of the
composed systems.
Tuesday, 5 February 2008
Dr Michael Lysaght, Centre for Theoretical Atomic, Molecular and Optical
Physics, Queen's University Belfast
Time-dependent study of multielectron atoms in intense few-cycle laser
fields
The study of atomic systems interacting with intense ultra-short light
pulses is currently attracting significant attention. These ultra-short
pulses have recently opened up the possibility of studying electronic
motion within atoms in the time domain. At present, the most advanced
theoretical approaches for the description of atoms irradiated by intense
few-cycle light fields are approaches dedicated to two-active electron
systems. However, very few theoretical approaches are currently available
to describe complex multielectron atoms, such as Ne and Ar, irradiated by
intense ultra-short light pulses. In order to describe the response of
such complex atoms to intense ultra-short light pulses, an approach is
needed which can accurately describe both the multielectron atomic
structure and the multielectron response to the few-cycle light field. At
Queen's we have recently developed an ab initio time-dependent approach
based on R-matrix theory that is capable of describing the multielectron
atomic response to intense ultra-short light pulses. To verify the
accuracy of our new approach, we have investigated multiphoton ionisation
of Ar and Ne irradiated by intense 390 nm light. Ionisation rates obtained
using the current time-dependent approach are in excellent agreement with
the most accurate rates available. We have also recently employed the new
approach to obtain momentum distributions of electrons emitted when Ne is
irradiated by a sequence of two ultra-short pulses. We have compared our
results with the results of a simple model which allows us to extract the
essential physics involved in these emission processes. In this talk I
will describe the new time-dependent approach and how we have applied it
to obtain the results mentioned above. I will also discuss our future
plans for the approach and code development.
Tuesday, 29 January 2008
Dr Gleb Gribakin, Department of Applied Mathematics and Theoretical Physics,
Queen's University Belfast
Progress in understanding positron annihilation on molecules
Electron-positron annihilation is a fundamental process involving matter
and antimatter. It is also at the heart of such applications as positron
emission tomography (PET), positron annihilation spectroscopy of solids,
and is our main source of knowledge about the positrons in the Universe,
in particular, in our galaxy. In this talk I will review the basic physics
of low-energy positron annihilation with atoms and molecules. I will
explain the origins of strong enhancement of positron annihilation
on polyatomic molecules, i.e., positron vibrational Feshbach resonances.
I will look in more detail at a number of molecules (methyl halides, methanol)
where theory has achived a very good agreement with recent experimental
results from Cliff Surko's group (Univerity of California, San Diego),
and point to remaining open questions.
Tuesday, 27 November 2007
Dr Charles
Clark, Joint Quantum Institute, National Institute of Standards
and Technology and University of Maryland Gaithersburg, MD
20899, USA
Condensed
matter physics at nanograms/cubic centimeter
Ultracold gases confined in optical lattices provide experimentally
accessible analogues of important condensed matter systems
- even though their densities are typically 100,000 times
less than that of air! Optical lattice systems can be produced
with a wide range of geometries, and with flexible control
of the energies for atomic hopping between lattice sites,
and of the on-site atomic interaction energies. Using such
pristine model systems, controlled independent-particle phenomena
like coherent Bloch oscillations and collective effects like
the superfluid-Mott insulator transition, have recently been
observed with much greater clarity than has ever been attained
in traditional condensed matter physics. This field has just
entered an era in which it seems possible to use optical lattices
to realize many of the iconic model systems of condensed matter
physics - such as the Hubbard model and interacting spin models
- and thus perhaps solve some of these computationally intractable
models through analogue quantum simulation. Moreover, there
are opportunities for synthesizing optical lattice "materials"
that manifest many of the exotica of conensed matter physics
- skyrmions, anyons, quantum hall states, topologial excitations,
Kagome lattices, etc. I will discuss recent studies of the
interplay between Anderson and Mott localization in optical
lattice systems with disorder, and the construction and application
of non-Abelian gauge potentials which yield rich physics beyond
the quantum Hall effect.
Tuesday, 6 November 2007
Prof
S. Chaturvedi, Institute for Mathematical Sciences, Imperial
College London and School of Physics, University of Hyderabad,
Hyderabad (India)
Classifying
Generalised Wigner distributions in odd dimensions
The question
of classifying Generalised Wigner Distributions as defined by
Gibbons et al is examined taking eigenvalues of the phase point
operators as the basis for categorising them. It is suggested
the the relevant group in this context is not the Clifford group,
as naively expected, but rather the extended clifford group--
clifford group augmented by complex conjugation operation.
A scheme for mapping phase point operators as elements of a
vector space over finite fields is developed and associated
representation theoretic aspects are briefly discussed.
[Work done
in collaboration with D. M Appleby and Ingemar Bengtsson]
Tuesday, 30 October 2007
Dr Pieter
Kok, Department of Physics and Astronomy, University of Sheffield
Optical
quantum computing with photonic and matter qubits
In this
talk I will give an overview of optical quantum computing with
photons and matter qubits. Whereas in principle it is possible
to do scalable quantum computing with only photons, linear optics,
and photo-detection, there are huge benefits to using matter
qubits with optical transitions. This leads naturally to the
use of the one-way model of quantum computing. Recent experiments
in Michigan and Paris indicate that this is a feasible route
to quantum computing.
Tuesday, 18 September 2007
Alisher
Kadyrov, Curtin University of Technology, Perth, Western Australia
Electron-Impact
Ionization of Atoms and Problems of Formal Scattering Theory
It is well known that conventional scattering theory is formally
valid only when interactions are short-ranged. In the two-body
case the theory can be made to include the long-range Coulomb
interaction using so-called renormalization methods. However,
no renormalization method exists for three charged particles.
Attempts to bypass the shortcoming of the general theory in
electron-impact ionization lead to introduction of trial quantities
like the Peterkop-Rudge integral [M. R. H. Rudge, Rev. Mod.
Phys. 40, 564 (1968)]. Formulations based on this integral
suffer from a number of problems including an ambiguous and
phase-divergent definition of the ionization amplitude. As
a first step to a better theory of ionization a formulation
of scattering theory for two charged particles without renormalization
will be presented. Then an alternative formulation of the
ionization theory free of ambiguity and divergence problems
will be given.
Tuesday, 11 September 2007
Teodora
Baeva, Institute for Theoretical Physics, University of Duesseldorf
High
harmonic generation from gases and plasma: between symmetry
and universality
The process of high harmonic generation due to the interaction
of laser with gasses and plasmas is discussed on the basis of
analytical theory and numerical simulations. The properties
of the high harmonic radiation in both cases are compared. It
is observed that, while the gas harmonics are highly symmetry
dependent, the spectrum of the plasma harmonics shows universal
features. The origin of this universality is explained within
the framework of the theory of relativistic spikes [1].
[1] T.
Baeva et. al., Phys. Rev. E 74, 046404 (2006).
Tuesday,
21 August 2007
Isao Shimamura, RIKEN, Japan
Time-Delay Matrix Analysis of Resonance Processes
Resonances
have long been known to occur quite often in atomic, molecular
and nuclear processes due to the formation of quasi-bound states
as intermediate states. They may greatly enhance or suppress
particular processes. Much attention has been paid to them recently
in relation to the resonance control of Bose-Einstein condensation,
resonant damage to bio-molecules, resonance control of processes
in a strong field, etc.
An
isolated resonance in an N-channel process may be described
by the Breit-Wigner formula for the scattering (S) matrix, which
consists of the resonance part involving the resonance energy
and the N partial widths, and the background part varying slowly
with the scattering energy. When two or more resonances overlap
each other, the S matrix elements and the cross sections take
very complicated forms, and the resonance analysis becomes extremely
difficult. This is because the interactions between these resonances
can affect different channels in various different ways.
The
time delay in single-channel scattering, or the collision time
spent additionally (compared with the free passage) by the projectile
because of the interaction with the target, may be related to
the phase shift, and hence to the S matrix. It can be generalised
into "time-delay matrix" (F. T. Smith, 1960) for multichannel
scattering and may be related to the multichannel S matrix.
This time-delay matrix is found to be very useful, especially
for analysing overlapping resonances in a visually transparent
way, without missing resonances that can be easily missed by
the conventional resonance analysis method. The theory will
be outlined and some numerical examples will be shown.
Tuesday, 3 July 2007
Prof B�la Sulik, Institute of Nuclear Research (ATOMKI), Debrecen,
Hungary
Guiding of few keV ions in insulator nanocapillaries
Recently,
studying the interaction of slow, highly charged ions with the
walls of insulator nanocapillaries, Stolterfoht and co-workers
[1-3] discovered a phenomenon known as "capillary guiding".
They observed that a significant amount of few-keV highly charged
ions traveled along the direction of the axis of even strongly
tilted nanochannels without significant collisions with the
capillary walls. This phenomenon has been found for 10 μm
long and 100 nm diameter capillaries created in polyethylene
terephtalate (PET) membranes. Even for a deliberate misalignment
between the incident ion direction and the nanochannels axis,
a significant fraction of keV Ne7+ ions passed through,
and left the nanochannels following their axis with a narrow
angular distribution. This guiding effect has been explained
by a self-organizing charge-up process [1-4]. According to this
picture, the ions, which enter the capillary will charge up
parts of the cap-illary surfaces in such a way that finally
some of them will be guided to the exit of the capillary. In
equilib-rium, only a fraction of the incoming ions will pass,
while the rest provide the electrostatic field necessary for
the guiding. More recently, similar guiding effect has been
reported for SiO2 capillaries [5]. In the pre-sent
talk, the first results on capillaries formed in anodized alumina
(Al2O3) [6] will be reported. Experiments
recently performed in ATOMKI, Debrecen, indicate that the guiding
effect is probably universal for good insulators.
-
Stolterfoht N, et al, 2002 Phys. Rev. Lett. 88 133201
-
Stolterfoht N, et al, 2004 Vacuum 73 31
-
Stolterfoht N, et al, 2004 Nucl. Instr. Meth. B 225
169
-
Schiessl K, et al, 2005 Phys. Rev. A 72 062902
-
Sahana M B, et al, 2006 Phys. Rev. A 73 040901
-
M�t�fi-Tempfli S, et al, 2006 Nanotechnology 17 1
Tuesday, 5 June 2007
Prof Jonathan Sherratt, Department of Mathematics Heriot-Watt
University, Edinburgh
Mathematical modelling of cell adhesion in developmental biology
and cancer
Many
processes in the early development of embryos are regulated
by the extent to which cells stick to one another. This "cell
adhesion" is also critical in cancer biology, with cancer cells
typically adhering less to one another than their normal counterparts,
enabling them to break away from a solid tumour and enter the
blood stream. Cell adhesion is one of the more difficult aspects
of cell biology to model mathematically because it is intrinsically
non-local. I will describe a new integrodifferential equation
model for the process. I will discuss some basic mathematical
properties of the model, and will describe its application to
simple laboratory experiments on cell sorting. I will then present
two applications of the model: the process of somite formation
in early development, and the invasive phase of cancer metastasis.
The work I will discuss is a collaboration with Nicola Armstrong
and Kevin Painter (Heriot-Watt University).
Tuesday, 29 May 2007
Prof. W. T. Coffey, Department of Electronic & Electrical
Engineering, School of Engineering, Trinity College, Dublin
Quantum Brownian Motion
Recent
progress in our understanding of quantum effects on the Brownian
motion in an external potential is reviewed. This problem is
ubiquitous in physics and chemistry particularly in the context
of decay of metastable states, for example, the reversal of
the magnetization of a single domain ferromagnetic particle,
kinetics of a superconducting tunnelling junction, etc. Emphasis
is laid on the establishment of master equations describing
the diffusion process in phase space analogous to the classical
Fokker-Planck equation. In particular, it is shown how Wigner's
[E. P. Wigner, Phys. Rev., 1932, 40, 749] method of obtaining
quantum corrections to the classical equilibrium Maxwell-Boltzmann
distribution may be extended to the dissipative non-equilibrium
dynamics governing the quantum Brownian motion in an external
potential yielding a master equation for the Wigner distribution
function in phase space. The explicit form of the master equation
so obtained contains quantum correction terms and in the classical
limit, reduces to the classical Klein-Kramers equation. For
a quantum oscillator, the method yields an evolution equation
coinciding in all respects with that of Agarwal [G. S. Agarwal,
Phys. Rev. A, 1971, 4, 739]. In the high dissipation
limit, the master equation reduces to a semiclassical Smoluchowski
equation describing noninertial quantum diffusion in configuration
space. The Wigner function formulation of quantum Brownian motion
is further illustrated by finding quantum corrections to the
Kramers escape rate, which in appropriate limits reduce to those
yielded via quantum generalizations of reaction rate theory.
Tuesday, 15 May 2007
Mag. Robert Prevedel, Faculty of Physics, University of Vienna
Experimental one-way quantum computing with linear optics
In
recent years, one-way quantum computing has become an exciting
alternative to existing proposals for quantum computers. In
this specific model, coherent quantum information processing
is accomplished via a sequence of single-qubit measurements
applied to an entangled resource known as cluster state. During
my talk, I will introduce, review and discuss the unique properties
of this model and present recent experiments that have realized
one-way quantum computational tasks.
Tuesday, 8 May 2007
Dr Jimena Gorfinkel, Department of Physics and Astronomy,
Open University
Recent theoretical studies of electron-molecule collisions
In
my talk I will briefly describe the R-matrix method as applied
to the study of electron-molecule collisions. I will discuss
its successes and limitations when dealing with processes of
applied relevance.
I
will then talk about some of my current work on the treatment
of near threshold ionisation and collisions with molecules of
biological relevance (and why theoretical work on electron molecule
collisions is relevant in this field). Finally, I will discuss
some initial attempts at devising a method to treat electron
collisions with molecular clusters and molecules immersed in
media.
Tuesday, 24 April 2007
Prof Nigel Badnell, Department of Physics, University of Strathclyde,
Glasgow
Calculations for electron-ion collisions and photoionization
processes for plasma modeling
We
review our studies of atomic collision processes relevant to
the spectroscopic diagnostic modelling of astrophysical and
fusion plasmas.
We
consider dielectronic recombination (DR) of the Fe M-shell,
its benchmarking [1] by ion storage ring experiments at Heidelberg,
and its importance for the modelling of absorption features
in active galactic nuclei [2]. We also comment on the effect
of our new L-shell DR data on coronal ionization balance for
all elements up to Zn [3], and on results from CLOUDY for photoionized
plasmas.
For
photoionization, we look at the revised opacities from the Opacity
Project (OP) which include inner-shell transitions and so extend
the validity of the OP data into the stellar interior [4]. We
note recent applications of it to the study of gravity mode
stellar pulsations which suggest a preference for OP data over
OPAL opacities for the `Z-bump' [5].
We
consider large-scale R-matrix electron impact excitation calculations
along isoelectronic sequences, up to Zn, which provide a new
level of `baseline data' for spectroscopic diagnostic modelling
codes such as CHIANTI and ADAS.
Finally,
we look to the future and the demands of ITER on theory for
describing collision processes involving heavy species such
as W, Xe.
-
Badnell, J. Phys. B, 39 4825 (2006)
-
Badnell, Ap. J. Lett., 651 L73 (2006)
-
Bryans et al, Ap. J. S., 167, 343 (2006)
-
Badnell et al, MNRAS, 360, 459 (2005)
-
Jeffery & Saio, MNRAS, 371, 659, (2006)
Tuesday, 17 April 2007
Dr Christina Cobbold, Department of Mathematics, University
of Glasgow
A mathematical model of insect development: Spatial and temporal
consequences
Host-parasitoid
systems are common in the insect world. Parasitoids are typically
flies or wasps while caterpillars are a typical example of a
host. A key aspect to modelling host-parasitoid interactions
is describing the timing of parasitism events. We present a
generic difference equation model and show that the timing of
parasitism can have a significant impact on the dynamics of
the model. In particular, we show how the period and amplitude
of temporal cycles are affected by paramtisim.
Habitat
structure has also been shown to effect the insect population
dynamics. Thus we also present an extension to the model whereby
integro-difference equations are used to describe the probability
of insect dispersal. In this case habitat patch size is found
be important in determining host-parasitoid persitence.
Tuesday, 27 March 2007
Dr Emma Sokell, School of Physics, University College Dublin
Resonant Photoelectron Spectroscopy in the Gas Phase
Photoelectron
spectroscopy is a well established technique for investigating
atoms and molecules. However, two-dimensional photoelectron
spectroscopy, in which the yield of photoelectrons is measured
as a function of both electron energy and incident photon energy,
is still able to reveal unexpected behaviour. The requisite
sources of tuneable photons are synchrotrons. Two-dimensional
spectra are ideally suited to exploring the decay of photoexcited
resonance states, as the encompassing nature of the technique
often reveals features that are overlooked or misinterpreted
if only a small number of spectra are recorded at selected photon
energies. After briefly introducing the technique of two-dimensional
photoelectron spectroscopy, the talk will focus on specific
cases where the study of the decay of photo-excited resonant
states has provided new and often unexpected information. These
examples will range from one of the simplest atoms, helium,
through simple molecular systems including H2 and
D2, to larger molecules such as HCl. The comprehensive
nature of the experimental measurements has meant that complete
theoretical models have often not been available to fully explain
all of the results.
Tuesday, 13 March 2007
Prof Vincenzo Aquilanti, Department of Chemistry, University
of Perugia, Italy
Few-body quantum and many-body classical hyperspherical dynamics
for elementary reactions and phase transitions of neutral
and ionic nanoaggregates
The
hyperspherical method is a successful approach for the quantum
treatment of elementary chemical processes. It has been mostly
applied to three-atomic systems and current progress is on the
basic theoretical framework for the extension to four-body bound
state and reactive scattering problems.
Most
applications only exploit the advantages of the hyperspherical
coordinate systems, but its full power is in the representations
explicitly involving quantum hyperangular momentum operators
as dynamical quantities and hyperspherical harmonics as basis
functions.
In
terms of discrete analogues of the harmonics one has a universal
representation for the kinetic energy and a diagonal representation
for the potential (hyperquantization algorithm).
Recently,
advances have been on classical dynamics, based on "classical"
definitions of the hyperangular momenta and related quantities.
After a sketch of progress on the general quantum approaches
for three- and four-body systems, specifically on the basis
set issue; I will present formulation and implementations to
molecular dynamics of simple nanoaggregates.
Tuesday, 13 February 2007
Prof Tania Monteiro, Department of Physics and Astronomy,
University College London
AC-driven cold atoms
I
will look at the dynamics of a range of quantum systems subjected
to time-periodic potentials. The unifying feature of these systems
is that their dynamics can be analysed by the structure of their
Floquet states, which for a time-periodic system play a similar
role to the eigenstates of the Hamiltonian of a conservative
system. I will review coherent matter-wave experiments done
at UCL and elsewhere with pulsed and sinusoidally driven standing
waves of light and show that they may be understood by analysing
the form of their Floquet states; I will look at possibilities
for improving quantum state transfer in spin chains sujected
to fields which localise the Floquet states. I will review recent
results showing that (within the theoretical framework of a
simple sinusoidally-driven Bose-Hubbard model and the Floquet
states) the phenomenon of 'coherent destruction of tunnelling'
may have applications for manipulating the Mott-Insulator transition.
Tuesday, 23 January 2007
Dr Elinor Irish, Department of Applied Mathematics and Theoretical
Physics, Queen's University Belfast
The Theory of Quantum Electromechanics
"Quantum
electromechanics" combines a superconducting qubit and a nanofabricated
mechanical resonator into a system similar to an atom in an
optical cavity. Many fascinating quantum optical effects should
be realizable in this solid-state system. Additionally, new
effects may appear due to the possibility of very strong coupling
even at large detunings. I will talk about my work on the theory
of quantum electromechanical systems, motivated in particular
by the search for ways to observe the quantum behavior of nanoscale
mechanical resonators.
Tuesday, 5 December 2006
Michael Lysaght, School of Mathematics and Physics, Queen's
University Belfast
Tin based laser-produced plasma source development for EUV
lithography
Laser
produced plasmas containing tin are currently being researched
as possible extreme ultraviolet (EUV) sources for next generation
nanolithography tools. The optimisation of conversion efficiency
(CE) (i.e. the ratio of EUV energy output to laser input energy)
is still currently one of the outstanding issues confronting
the EUV lithography (EUVL) community, where a minimum CE of
3% is required by industry. This requirement forces the developers
of EUV sources to obtain a better understanding of the theoretical
limits of plasma emission. One of the key aspects of any attempt
to accurately model the plasma is the need for reliable fundamental
atomic data on Sn ions, which has been considerably lacking
to date. This talk will focus on the recent efforts of the Atomic
and Molecular Physics Research Group at University College Dublin
to provide some of this core data to the EUVL source modelling
community.
Tuesday, 7 November 2006
Prof Massimo Palma, Dipartimento di Scienze Fisiche ed Astronomiche,
Universita degli Studi di Palermo
Entanglement controlled electron transport
We
will show the interplay which exists between entanglement and
single electron transport properties in a 1D quantum wire in
the presence of two scattering magnetic impurities or dots.
We consider a system consisting of single electrons moving along
a 1D wire in the presence of two magnetic impurities. Such system
shows strong analogies with a Fabry - Perot interferometer in
which the impurities play the role of two mirrors with a quantum
degree of freedom: the spin. We discuss how the electron transmittivity
of the wire is affected by the presence of entanglement between
the impurity spins. In particular we will show that for suitable
values of the electron momentum, there are two maximally entangled
state of the impurity spins the first of which makes the wire
transparent whatever the electron spin state while the other
strongly inhibits the electron transmittivity. We will also
consider the Aharonov-Bohm (AB) interference oscillations of
electron transmission through a mesoscopic ring in which two
non-interacting magnetic impurities are embedded. Finally we
will discuss how maximally entangled states of the impurity
spins can be generated via scattering with a conduction electron.
Tuesday, 10 October 2006
Prof Rachid Mohallem, Universidade Federal
de Minas Gerais, Brazil
Molecular Isotope Symmetry Breaking and
Dipole Moments
Wednesday, 4 October 2006
Prof. Wolfgang Lange, Department of Physics and Astronomy,
University of Sussex
Cavity-QED: entangling ions with light
In
a cavity-QED system, the quantum states of single atoms or ions
can be coupled deterministically to those of single photons,
as long as the coherent interaction dominates any decay processes.
In this way, quantum information may be reversibly transferred
between ions and photons. An important application is the realization
of a quantum network, linking distant ion trap quantum processors
through photonic channels.
We
are currently implementing an ion-photon interface, based on
the controlled single-photon emission from an ion in an optical
resonator. This provides a method to distribute entanglement
over long distances. But also local ion-entanglement may be
created, by coupling two ions to the same cavity mode. The successive
optical entanglement of ion pairs in a long chain generates
a cluster state and thus a resource for one-way quantum computation.
In
contrast to these deterministic schemes, we also investigate
the probabilistic entanglement of ions, conditioned on the detection
of photons emitted from the cavity. The technique exploits the
fact that it is impossible to distinguish two ions through their
interaction with the cavity mode. In the talk, I will discuss
the status of experiments and prospects for the realization
of theoretical schemes.
Tuesday, 13 June 2006
Prof. Ronald McCarroll, Universit� 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 Schr�dinger 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, Universit� 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 Schr�dinger 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
Schr�dinger 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. |
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