This term the seminars will be held
on
Tuesdays at 3.00 pm in Room
0G.007, David Bates Building
(location), followed by tea/coffee and biscuits in
room 01.040 from 4.00 pm.
You are very welcome!
Seminar coordinator: Dr G Gribakin -
g.gribakin@qub.ac.uk
(Past
Seminars in 2000-2011)
Seminars
2012-2013:
Wednesday, 15 May 2013, 3:00 pm, 01.006, David Bates Building
Dr Arnau Riera, Physics Department, Freie Universitaet Berlin
Thermal equilibrium in quantum theory
Closed quantum systems follow a unitary time evolution and therefore
never equilibrate. Thermodynamics and quantum theory are then
apparently incompatible. In this talk, I will review the recent
progress in the project of deriving thermodynamics from quantum
theory. This approach not only makes quantum theory and thermodynamics
compatible, but also explains the statistical behavior of
thermodynamics as a consequence of objective quantum uncertainties due
to entanglement, making unnecessary the randomness introduced by hand
in the standard derivation based on classical mechanics.
Tuesday, 16 April 2013, 4:00 pm, 0G.007,
David Bates Building
Dr Neil Drummond, Physics Department, Lancaster University, Lancaster
Positrons in Homogeneous Electron Gases
Quantum impurity problems, in which a single particle of one species is
immersed in a fluid of particles of another species, arise in a number of
contexts in condensed matter physics. One particularly important case is
that of a positron immersed in an electron gas, because this situation is
relevant to positron annihilation spectroscopy studies of defects in metals
and semiconductors. However, providing a suitably accurate first-principles
description of a positron in an electron gas is a significant challenge for
theory due to the strong correlation between the positron and the electrons.
In this talk I will consider the behaviour of a single positron immersed in
a homogeneous electron gas and show that suitably modified density functional
theory and quantum Monte Carlo methods can give an accurate description of
the key properties of this system, such as the relaxation energy,
annihilation rate and the momentum distribution of the annihilation radiation.
Tuesday, 19 March 2013, 3:00 pm, 0G.007, David Bates Building
Dr Silvia Bergamini, Department of Physical Sciences, The Open University,
Milton Keynes
Cold Rydberg atoms for quantum computation
Cold Rydberg atoms display strong, long-range interactions that may be used
to produce entanglement between atoms at distances. A phenomenon known as
Rydberg blockade that forbids multi-atom excited states can generate robust
entanglement of atomic ensembles. In this talk I will present schemes to
achieve gate operation between ensemble of atoms using the Rydberg blockade
and a protocol for quantum computation based on mixed states in atomic
ensembles.
Tuesday, 12 March 2013, 3:00 pm, 0G.007, David Bates Building
Dr Mariona Moreno, Universitat Autonoma de Barcelona
Ultracold atoms in optical lattices: detecting strongly correlated phases
at finite temperature
Dilute gases trapped and cooled down to quantum degeneracy constitute an
exceptional toolbox to investigate quantum many-body phenomena. In particular,
optical lattices (that is, standing light waves formed by
two counterpropagating light beams that can act on the atoms as an external
periodic potential) allow simulating in a highly controllable and tunable
way many-body physics in the strongly correlated regime.
In the first part of my talk I will provide a very basic introduction on
optical lattices: how are they implemented in an experiment, and what is
the most well-known model (the Hubbard model) that one can simulate with such
systems. In the second part of the talk I will present an anomalous
macroscopic effect observed in a two-component fermionic mixture at finite
temperature: the atomic cloud volume increases when adiabatically increasing
the attractive interaction between particles from the two-components. Whereas
typical methods to detect strongly correlated phases are based on the
properties of the ground state or low-lying excitations, here, this
counterintuitive effect can be seen as a signature of such strongly
interacting phase at finite entropies. I will present a simple but
intuitive picture that can explain this effect and some calculations that
can reproduce (at least qualitatively) the experimental result found in [1].
[1] Anomalous expansion of attractively interacting fermionic atoms in an
optical lattice, L. Hackermüller et al., Science 327, 5973
(2010).
Tuesday, 5 March 2013, 3:00 pm, 0G.007, David Bates Building
Dr Dan Murtagh, RIKEN, Advances Sciences Institute, Atomic Physics Laboratory,
and ASACUSA Collaboration (CERN)
Low energy positron beams and their applications in scattering experiments
and antihydrogen production
In this presentation, measurements performed using a magnetically guided
positron beam of the excited state positronium formation cross-section from
He, Ar and Xe will be discussed. Further to this, the same apparatus was
used to measure the simultaneous ionization-excitation cross-section for
positrons impact on CO2 and N2 targets. It was found
that the cross-section for this process was a factor of 2 larger than in the
case of electron impact.
The operation of positron traps will be discussed within the context of the
ASACUSA-cusp trap experiment which aims to produce a beam of antihydrogen
amenable to spectroscopic investigation. In this case, the principle
objective is to produce a cold dense cloud of positrons with which to form
antihydrogen via 3-body recombination with antiprotons.
Thursday, 14 February 2013, 4:00 pm, 0G.007,
David Bates Building
Dr Aurelien Dantan, Department of Physics and Astronomy, Aarhus University,
Denmark
Cold ions in optical cavities
Owing to their very good localization and coherence properties, trapped and
laser-cooled ions are an excellent medium for cavity quantum electrodynamics
studies. I will first present recent experiments exploiting the coupling of
large 40Ca+ ion crystals to the field of an optical
resonator for quantum optics and quantum information processing, and leading
to, e.g., the observation of cavity electromagnetically induced transparency
and all-optical switching at low-light levels. I will also discuss experiments
using few- and many-ion crystals for cavity optomechanics investigations.
Tuesday, 29 January 2013, 3:00 pm, 0G.007,
David Bates Building
Stephen M. Barnett, Department of Physics, University of Strathclyde,
Glasgow G4 0NG, UK
The Enigma of Optical Momentum
It is more than 100 years since the battle began to determine the correct
form of the momentum of light inside a material medium. The two principal
contenders for the momentum density are those attributed to Abraham,
gAbr=E×H/c2
and to Minkowski, gMin=D×B.
The work of Minkowski suggests that the momentum of each photon exceeds
its value in free space by a factor of the refractive index. Abraham's
approach, however, requires that the momentum is less than that in free
space by the same factor.
Compelling arguments have been proposed for each of the two candidate
momenta. A simple application of Newton's first law of motion to the
combined system of a medium and a single photon, for example, leads
unambiguously to the Abraham form. An equally simple analysis of single or
double slit diffraction, however, leads equally powerfully to the Minkowski
form. The problem has not been resolved by experiment, moreover, with
experiments supporting both candidate momenta being reported.
The resolution, of course is that both are correct...
Tuesday, 4 December 2012, 3:00 pm, 0G.007,
David Bates Building
Hendrik Ulbricht, School of Physics and Astronomy, University of Southampton,
SO17 1BJ, h.ulbricht@soton.ac.uk
Centre of mass motion interferometry of molecules and nanoparticles
The motivation for de Broglie interference of heavy particles is manifold,
for example, to address the foundations of physics by invesitgating the
quantum to classical transition, the use of interferometric techniques for
applications such as: molecule metrology, molecule sorting, molecule quantum
interference lithography and investigations of van der Walls/Casimir-Polder
interactions, and to study the coherent manipulation of complex partciles
for instance to reconstruct the Wigner function of the motional quantum
state of the diffracted molecules. The centre of mass interferometry is
not affected by internal excitation of the molecules as impressively
demonstrated by our experiments. If however internal state dynamics is
coupled to the centre of mass motion by electric or optical fields, the
interference pattern is changed. I will explain our experiment on mapping
the dynamics of the change of conformation of hot molecules onto its centre
of mass motion while measuring the interference pattern. I will further
emphasise the status of the development of the Southampton molecule
interferometer, where we recently achieved 27% of quantum contrast.
Manipulation and cooling the motion of complex partciles to increase beam
coherences and beam brightness as well as decreasing speeds is essential
for interference experiments. Therefore we are interested in optical
techniques to effect the motion of molecules. I will report on our recent
experimental results on focusing a fullerene beam by a train of ultrashort
laser pulses. These results are confimed by Monte-Carlo simulations as
well as by adjusting an analytical model to the experimental parameter.
In the last part of my talk I shall illustrate our ideas for de Broglie
interference of polystyrene or glass spheres (beads) of up to 100 nm in
diameter. Our approach considers the use of optical tweezing and position
stabilisation as a launch pad for Talbot-Lau interferometry. We have very
recently started the first experiments.
Tuesday, 27 November 2012, 3:00 pm, 0G.007,
David Bates Building
Professor Yu. V. Popov, Skobeltsyn Nuclear Physics Institute, Lomonosov
Moscow State University
Proton-helium transfer processes at high projectile energies: new trends
and results
New experimental and theoretical results will be presented obtained in
collaboration with the experimentalists from Frankfurt University
(A. Dorn et al.). Those are: charge transfer reaction leaving the final ion
He+ in its ground state, transfer excitation reaction leaving the
final ion in excited states, transfer ionization reaction, in which the
reaction plane is fixed, and 2D (kx, kz) and
(kperp, kz) distributions of the escaped electron
components are presented. FBA, SBA and DWBA calculations are presented also.
It is shown that for transfer excitation and transfer ionization cases the
angular distributions strongly depend on the ee-correlations in the helium
ground state. Also the role of higher Born terms (distortions) is shown.
The "new" mechanism of Voitkiv, and the 9D calculations of Madison are also
discussed in the report.
Wednesday, 14 November 2012, 4:00 pm, 02.005,
David Bates Building
Dr Davide Rossini, Scuola Normale Superiore (Pisa)
Interacting bosons in 1D lattices: statics and dynamics of topological
insulating phases
We accurately study the ground-state phase diagram of the
one-dimensional extended Bose Hubbard model with on-site and
nearest-neighbor interactions, at integer filling. Besides
conventional superfluid, Mott insulating and density-wave phases,
the presence of extended interactions is able to stabilize a
peculiar gapped phase, also named Haldane phase, which displays
long-range string ordering. In the second part of the talk we
discuss the possibility to perform adiabatic topological pumping
through such phase. By opening a gap between the Mott insulator
and the Haldane phase, we can encircle the quantum phase transition
point adiabatically and show that this enables dissipationless
transport of exactly one boson across the ring, as originally
predicted by Thouless. Our results have been obtained by means
of the density matrix renormalization group technique.
Tuesday, 6 November 2012, 3:00 pm, 0G.007,
David Bates Building
Dr Amelle Zair, Imperial College London, Department of Physics, Blackett
Laboratory Laser Consortium South Kensington Campus SW7 2AZ London UK
Control of High Harmonic generation by synthesised laser field
The High order Harmonic Generation (HHG) is a non-linear process where a
strong femtosecond laser field interacts with matter (atoms or molecules)
to up convert the fundamental frequency of the laser into its higher orders.
This up conversion occurs through a process at the level of the atom or the
molecule that consists in a sequence of steps described as followed:
ionisation of an electronic wave packet in the continuum, free propagation
of this wave packet in the continuum subject to the laser field that drives
it back to the core and a recombination of the wave packet to the core that
conserves the energy by emitting a burst of XUV coherent light, the high
order harmonic generation in hundreds of attosecond
(1 asec=10-18 sec). This ionisation, propagation and
recombination process can be seen classically as an electron trajectory
and as Feynman paths in the quantum version. Depending on the emission time
of the electron wave packet in the continuum, there exists different type
of trajectories that are called 'quantum paths'. Two of these trajectories
contribute significantly to the harmonic emission in the plateau region,
referred to as 'short' and 'long' trajectories. The harmonic emission for
these trajectories encodes any dynamical processes that occurred in the
cation during the time scale covered by these trajectories. Hence measuring
harmonic spectra, phase and polarisation gives information on the dynamical
processes. These dynamical processes can be categorised as followed:
intra-cation nuclear motion that includes structural changes (conformation);
intra-cation electrons dynamics that included field coupling of electronic
states.
To control these trajectories and enhance one type of trajectories over
others will allow us to study dynamical processes in cation for various
time scales. Our approach is to obtain this control by using a synthesised
laser field composed of two-colour or more complicated synthesis . I will
present an overview of what we are capable to do in terms of trajectories
control at Imperial College London and how these control open new degrees
of freedom for the investigation ultra-fast dynamical processes in
molecules.
Tuesday, 23 October 2012
Dr Brendan M McLaughlin, Centre for Theoretical Atomic, Molecular and
Optical Physics (CTAMOP) School of Mathematics and Physics, Queen's
University of Belfast, Belfast, BT7 1NN, UK
Photoionization of trans-Fe elements relevant to astrophysical applications
Photoionization of atomic elements is an important process in determining
the ionization balance and hence the abundances of elements in photoionized
astrophysical nebulae. It has recently become possible to detect neutron
n-capture elements (atomic number Z>30) in a large number of ionized
nebulae [1,2]. These elements are produced by slow or rapid n-capture
nucleosynthesis (the s-process and r-process, respectively). Measuring the
abundances of these elements helps to reveal their dominant production sites
in the Universe, as well as details of stellar structure, mixing and
nucleosynthesis. These astrophysical observations provide an impetus to
determine the photoionization and recombination properties of n-capture
elements. Planetary nebulae (PNe) progenitor stars may experience s-process
nucleosynthesis, in which case their nebulae will exhibit enhanced
abundances of trans-iron elements. The level of s-process enrichment for
individual elements is strongly sensitive to the physical conditions in
the stellar interior. Accurate assessment of elemental abundances in
astrophysical nebulae can be made from the direct comparison of the
observed spectra with synthetic non-local thermodynamic equilibrium (NLTE)
spectra, if the atomic data for electron and photon interaction processes
are known with sufficient accuracy. Experiments on heavy trans-Fe atomic
ions at third generation synchrotron radiation source, such the Advanced
Light Source (ALS) in Berkeley, California, USA, SOLEIL in Saint-Aubin,
France, ASTRID in Aarhus, Denmark and PETRA, in Hamburg, Germany, have
all highlighted the need for high quality theoretical work to fully
interpret experimental results. A recently developed theoretical code for
parallel computing architectures [3-4] (incorporating the necessary
relativistic effects within a Dirac-equation formulation) has been used
to perform detailed photoionization cross section calculations on a
variety of atomic ion species, e.g.; Fe [3], Se [4], Kr [5,6], Ar [8],
Xe [5] and W [7], in their low stages of ionization. Where possible we
compare our results with ongoing experiments being performed at the third
generation synchrotron light sources. These comparisons are necessary and
serve as the ultimate benchmark for our work in order to have confidence
in the atomic data to be incorporated into standard astrophysical
modeling codes such as CLOUDY, XSTAR and ATOMDB.
[1] N. C. Sterling, H. L. Dinerstein, and T. R. Kallman, Astrophys.
J. Suppl. Ser. 169, 37 (2007)
[2] N. C. Sterling and H. L. Dinerstein, Astrophys. J. Suppl. Ser. 174,
158 (2008)
[3] V. Fivet, M. A. Bautista and C. P. Ballance, J. Phys. B: At. Mol.
Opt. Phys. 45, 035201 (2012)
[4] B. M. McLaughlin and C. P. Ballance, J. Phys. B: At. Mol. Opt. Phys. 45,
095202 (2012)
[5] B. M. McLaughlin and C. P. Ballance, J. Phys. B: At. Mol. Opt. Phys. 45,
085701 (2012)
[6] G. Hinojosa, A. M. Covington, G. A. Alna'Washi, M. Lu, and
R. A. Phaneuf, M. M. Sant'Anna, C. Cisneros and I. Álvarez, A. Aguilar,
A. L. D. Kilcoyne, and A. S. Schlachter, C. P. Ballance, B. M. McLaughlin,
Phys. Rev A, in press (2012)
[7] A. Muüller, S. Schippers, J. Hellhund, A. L. D. Kilcoyne,
R. A. Phaneuf, C. P. Ballance, and B. M. McLaughlin, J. Phys. B:
At. Mol. Opt. Phys., in press (2012)
[8] A. M. Covington, A. Aguilar, I. R. Covington, G. Hinojosa,
C. A. Shirley, R. A. Phaneuf, I. Álvarez, C. Cisneros,
I. Dominguez-Lopez, M. M. Sant'Anna, A. S. Schlachter, C. P. Ballance,
and B. M. McLaughlin, Phys. Rev. A 84, 013413 (2011)
Tuesday, 9 October 2012
Dr Brian Gerardot, Institute of Photonics and Quantum Sciences, SUPA,
Heriot-Watt University, Edinburgh EH14 4AS, UK
Quantum coherence in semiconductors
Strong quantum confinement has led to the observation of discrete,
atom-like energy levels in solid-state quantum dots (QDs). However, for a
typical self-assembled QD the confinement potential of a particle spans
more than 104 atoms. A natural expectation for such a mesoscopic
system is that many-body interactions will dominate and lead to unwanted
decoherence. In this talk I will present a series of experiments which use
a resonant laser to probe the quantum states and their coherence in QDs,
both for two-level transitions and three-level systems. I will focus in
particular on a so-called spin-λ system. Indistinguishable
spin-λ systems form the basis for projective-measurement based
protocols to achieve scalability. I will discuss our investigations into
a spin-λ system based on hole spins which allows a single spin to
be initialized, coherently manipulated, and read-out using photons.
I will also address the prospect of fine-tuning the quantum states using
an externally applied uniaxial stress for desired applications. Finally,
I will present new insights into a common obstacle for indistinguishability
in solid-state systems: inhomogeneous broadening of transitions.
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