*Designing Spin
Lattice Hamiltonians
with Polar Molecules in Optical Lattices by Gavin Brennen (with A.M.
Micheli,
P. Zoller), Universität Innsbruck*

Motivated by theoretical
discoveries of emergent exotic phases from quasi-local spin
interactions
we show how to simulate a broad class of lattice spin models in one
or
more dimensions using trapped polar molecules in an optical lattice. The
spin
corresponds to electronic degrees of freedom of a molecule in its ground
rotational state. Spin dependent couplings between molecules are
obtained
using a microwave field to mix ground states with dipole-dipole coupled
rotationally excited states. The interaction strengths are fast
relative to decoherence times and any permutation symmetric two spin
coupling
is designable using several microwave fields. We illustrate how to
build
two models: one on a 2D square lattice with an energy gap providing
for
protected quantum memory, and another on stacked triangular lattices
leading to topological quantum computing.

*Classical Approaches
to
Quantum Decoherence by Paul Brumer, University of Toronto*

We demonstrate that the
"classical Wigner approach", wherein an initial state is
described
quantum mechanically, but where the subsequent mechanics is computed
classically, provides a useful tool for calculating rates of decoherence.
The
basis of this approach, as well as applications to several systems, will
be
described. A comparison of computed rates with simplified Master equation
models demonstrates the failure of these models and the need for realistic
computations.

*Tackling the
challenges of
quantum computing with trapped Cd+ ions by Louis Deslaurier, University of
Michigan*

Trapped atomic ions have
a
number of desirable features that make them well suited for quantum
information
applications. Pairs of hyperfine ground states can behave as an ideal
quantum
bit, and entanglement between multiple ions can proceed through Coulomb
interaction mediated by appropriate laser fields. Many of the critical
components of a trapped ion quantum computer have now been demonstrated
here at
Michigan as well as by a few other groups around the world. I will
review
the Michigan effort which consists of three main thrusts: Ion-ion
entanglement,
ion-photon entanglement, and scalable trap development.

*Coherent and incoherent evolution
in semiconductor
systems by Andrew Fisher, University College London*

We report treatments of the coherent and incoherent parts of the evolution
of
localized spin and charge qubits in semiconductor systems.

Our calculations are
based on
the Time-Convolutionless Projector (TCL) technique, as well as more
standard
Born-Markov methods. For quantum-dot charge qubits we will argue that
recent
experiments [1-3] are close to the intrinsic material limits, and show how
these limits scale with the size of the system down to the dimensions of
atomic
defects. For spin qubits, we show how coupling them through
localised
excitations with well-defined level structures [4] can provide
particularly
effective ways of minimising decoherence while producing entangled spin
states.

[1] Coherent manipulation of electronic
states
in a double quantum dot

Author(s): Hayashi T, Fujisawa T, Cheong HD, Jeong YH, Hirayama Y

Source: PHYSICAL REVIEW LETTERS 91 (22): Art. No. 226804 NOV 28 2003

[2] Charge-qubit operation of an isolated double quantum dot

Author(s): Gorman J, Hasko DG, Williams DA

Source: PHYSICAL REVIEW LETTERS 95 (9): Art. No. 090502 AUG 26 2005

[3] Manipulation of a single charge in a double quantum dot

Author(s): Petta JR, Johnson AC, Marcus CM, Hanson MP, Gossard AC

Source: PHYSICAL REVIEW LETTERS 93 (18): Art. No. 186802 OCT 29 2004

[4] Optically driven silicon-based quantum gates with potential for

high-temperature operation

Author(s): Stoneham AM, Fisher AJ, Greenland PT

Source: JOURNAL OF PHYSICS-CONDENSED MATTER 15 (27): L447-L451 JUL 16 2003

[5] Avoiding entanglement loss when
two-qubit
quantum gates are

controlled by electronic excitation

Author(s): Rodriquez R, Fisher AJ, Greenland PT, Stoneham AM

Source: JOURNAL OF PHYSICS-CONDENSED MATTER 16 (16): 2757-2772 APR 28
2004

*Decoherence and
Nonadiabatic Rate Processes by Raymond Kapral, University of
Toronto*

The computation of the
rates
of quantum mechanical reactions in condensed phase environments entails
sampling from suitable initial states and quantum evolution of operators
that
characterize metastable sets. The talk will focus on the simulation of
such
rates using quantum-classical Liouville dynamics that describes the
evolution
of a quantum subsystem coupled to a classical bath. Simulations of
nonadiabatic
quantum-classical dynamics can be carried out in terms of ensembles of
surface-hopping trajectories that involve evolution on single adiabatic
surfaces, interspersed with quantum transitions to coherently coupled
adiabatic
surfaces where the trajectory contribution carries a phase, followed by
transitions to single adiabatic surfaces that destroy the coherence. The
presence of these coherent segments and the destruction of the coherence
by the
environment play an important role in determining the rates and mechanisms
of
quantum reactions in the condensed phase. The role of decoherence on
quantum
rate processes will be illustrated by computations on proton transfer
reactions
in solution and on model two-level systems.

*High fidelity
Josephson
phase qubits . winning the war on decoherence by Nadav Katz, Physics
Department
and California NanoSystems Institute, UCSB*

Superconducting qubits
are
considered one of the leading technologies for scalable quantum
computation.
The Josephson phase qubit can be thought of as a .tunable atom. where the
energy levels used as the qubit are tuned via external controls.
Decoherence
due to coupling to various material defects has been identified as a major
source of decoherence. Recently we have dramatically improved the
coherence of
our qubits by quantifying various decoherence pathways, and removing them
via
optimized fabrication. I will present some recent results, including
quantum
state tomography of the qubit state, analysis of qubit evolution due to
partial
measurement, and the generation of various Bell states in coupled
qubits.

*Measuring and
Reducing the
1/f Noise in Josephson Junctions for Potential use as qubits by Jan Kycia,
Department of Physics and The Institute for Quantum Computing, University
of Waterloo*

Critical current fluctuations can be
a
major source of intrinsic decoherence of qubits based on Josephson
junctions.
We have measured the 1/f noise due to critical current fluctuations in
macroscopic Josephson junctions. We directly measure changes in the
critical
current Ic of a voltage biased junction and find the critical current to
fluctuate by about 10E-5 at a frequency of 1 Hz. A second way in which we
determine 1/f noise due to critical current fluctuations is by measuring
the
noise of either dc or rf SQUIDs. In order to not exceed the critical
current of the Josephson junction, we operate the rf SQUID in the
dispersive
mode. By using the same device as dc or rf SQUID, we can compare the 1/f
noise
of voltage biased and non-voltage biased Josephson junctions. In
this
talk, I will describe how we make these measurements and present our
progress
in understanding and reducing this noise.

*Stretching the
electron as
far as it will go by Gordon Semenoff, UBC Vancouver*

It is argued that zero
modes
of majorana fermions can mediate a teleportation-like process with actual
transfer of electronic material between well separated points. The
process is
illustrated using a quasi-realistic, exactly solvable model of a quantum
wire
embedded in a p-wave superconductor.

*Restoring Coherence
Lost
to a Mesoscopic Bath by LuJ. Sham (with Wang Yao and Ren-Bao Liu),
UCSD*

We present a quantum
solution
to the electron spin decoherence without stochastic assumptions. The
many-body
problem of a mesoscopic system of N interacting nuclear spins in the
presence
of a single electron spin is solved by a nuclear pair-correlation
approximation, shown to be valid when N is bounded. Computation results on
free-induction decay of the single electron spin coherence and the
restoration
of coherence will be presented. The question of the applicability of our
theory
to a wider class of physical systems will be presented for
discussion.

*Many-body EPR breakup
and
fragments' decoherence by Moshe Shapiro, UBC Vancouver*

We explore EPR correlations in the breakup of polyatomic
molecules into
two mutually entangled fragments. We discuss how this
entanglement is
related to "coherent control" and give a derivation based on
the
properties of the dissociated wave function that no
information is
transferred, not even at a speed smaller than the speed of
light, from one
entangled partner to the other concerning its measurement or
lack thereof.
We also show how one can attain a variable degree of
entanglement, using
which, we explain some experimental results as due to the
gradual
switching off of entanglement.

* *

*Quantum
superconductor-metal transition be Boris Spivak, University of
Washington*

I will discuss a theory
of
quantum superconductor-metal transition. To do so I will consider a
system of
superconducting grains embedded in a normal metal. At zero temperature
this
system exhibits a quantum superconductor-normal metal phase transition as
a
function of grain size and grain concentration. In the framework of this
model
the transition can take place at arbitrarily large conductance of the
normal
metal. As a result, it is possible to construct an asymptotic theory of
the
phenomenon. I will concentrate on properties of the exotic metallic phase
and
will discuss a relation of the theory to the theory of 2D
localization.

*Measuring and
manipulating
coherence in photonic and atomic systems by Aephraim Steinberg, University
of
Toronto*

The manipulation and
"complete" characterisation of quantum states have become major
topics in experimental and theoretical physics, with crucial roles to play
in
quantum information & control. I will discuss experimental
progress
in these areas drawing from two of our experiments, one on the production
of
highly entangled n-photon states (where n>2), and the other on the
manipulation of the quantum vibrational states of atoms trapped in an
optical
lattice. In the first, we have realized that in typical single-mode
systems (for instance, polarisation-entangled photons in optical fibre),
one
must be careful to differentiate between loss of coherence (between
different quantum
states) and loss of indistinguishability (of "different"
photons). The standard tomographic approach breaks down when photons
become partially distinguishable due to degrees of freedom not directly
probed
experimentally. We extend the ideas of tomography to develop a
theory of
quasi-complete state characterisation for such systems, and demonstrate it
on a
two-photon system. In the second, we have developed techniques for
state
control & characterisation for an ensemble of atoms trapped in an
optical
lattice, and measure the decoherence, due to a combination of
inhomogeneities and
inter-well tunneling. We present progress on "pulse echo"
or
"dynamical decoupling" techniques for restoring coherence, and
on
using superoperator extraction to optimize such pulse
sequences.

*Role of Conduction
Electrons in the Decoherence of Impurity-Bound Electrons in a
Semiconductor by
Kuljit S. Virk (with J.E. Sipe), University of Toronto*

We study the dynamics of
impurity bound electrons (qubits) interacting with a bath of conduction
band
electrons in a semiconductor. Only the exchange interaction is
considered.
This is applicable to the specific system of a silicone lattice at nonzero
temperatures doped with a density n_{D} of phosphorus atoms. Each
P
atom donates an electron, which either becomes a conduction electron or is
captured by another ionized P atom forming an .atom. with hydrogen-like
properties. The captured electrons are usually in s-states with a .Bohr
radius. of about 25 Å and a binding energy of about 44 meV. The
conduction
electrons form a gas of approximately free particles, the density of which
builds up (from zero at T = 0) as temperature rises and more donors are
ionized. This gas forms a gas with which the qubits interact by
scattering
conduction electrons, and consequently undergo decoherence.

We derive master
equations
for the density matrices of single and two-qubit systems under the usual
Born
and Markov approximations. The bath mediated RKKY interaction in the
two-qubit
case arises naturally. It leads to an energy shift significant only when
the
ratio (R_{T}) of the inter-qubit distance to the thermal deBroglie
wavelength of the bath electrons is small. This bath mediated interaction
is
shown to be an important factor in determining decoherence times; the
effect
decreases monotonically with R_{T}. We also discuss the effect of
this
on the purity and fidelity of the Bell states in the case of two-qubit
systems.

*Quantum computation
in the
chaotic regime by Joshua Wilke, SFU*

Even in the absence of
external
influences the operability of a quantum computer can fail as a result of
internal imperfections. We examine the dynamics of a CNOT gate performed
on two
qubits which interact through residual interactions with the rest of a
quantum
computer which has one-- and two--body flaws. Contrary to expectation we
observe less decoherence in the chaotic regime where two--body flaws are
strongest. In addition, we find that poor fidelity is primarily caused by
a
coherent shift rather than decoherence or dissipation. Our results suggest
that
greater attention should be given to internal sources of error and
associated
correction schemes.

* *

*Holonomic quantum
computation in decoherence-free subspaces by Lian-Ao Wu, University of
Toronto*

We show how to realize,
by
means of non-abelian quantum holonomies, a set of universal quantum gates
acting on decoherence-free subspaces and subsystems. In this manner we
bring
together the quantum coherence stabilization virtues of decoherence-free
subspaces and the fault-tolerance of all-geometric holonomic control. We
discuss the implementation of this scheme in the context of quantum
information
processing using trapped ions and quantum dots.

*Entanglement between
solid
state systems, by Tony Leggett , Univ of Illinois (Urbana-Champaign), and
PITP, Vancouver*

Examples of types of qubit where the electromagnetic-field and solid-state
ingredients are both important include flux-mode Josephson systems,
quantum-optical cavities (where the metallic walls are essential to the
confinement of the modes), and most spectacularly surface plasmons, where the
metallic degrees of freedom are essential for the very existence of the mode. It
has frequently been argued in the literature that in all these systems the
"solid-state" degrees of freedom are strongly entangled. I shall argue that with
the parameters typical of existing experiments, this is true only for the
flux-more SQUID case; in particular, in existing experiments involving surface
plasmons, the entanglement of the solid-state degrees of freedom is extremly
small.

*Dynamics of the
Nuclear
Spin bath in molecular magnets: a test for decoherence, by Andrea Morello,
UBC,
Vancouver*

Any
conceivable application of the quantum properties of large spin systems
requires a thorough understanding of their coupling with the environment,
in
particular the nuclear spin bath. Molecular nanomagnets, forming
crystalline
arrays of identical and well-characterized high-spin clusters, are
benchmark
systems to study this problem in detail and make accurate comparisons
between
experimental results and theoretical predictions. I shall review our
recent
experiments on the spin dynamics and thermodynamics in a variety of
nanomagnets
at ultra-low temperatures, where both the nuclear-driven electron spin
dynamics
and the electron-driven nuclear spin dynamics can be studied, and
illustrate
how these results challenge our current understanding of decoherence in
quantum
spins.

* *

*Coherence Windows in
solid-state qubit systems, by Philip Stamp, UBC and PITP,
Vancouver*

I discuss the
competition
between spin bath decoherence at low energy scales and oscillator
bath-mediated
decoherence at higher energy scales, and the .coherence window. that
exists at
intermediate energy scales. The implications for experiments on magnetic
and
superconducting qubits are described. Then in the second part of the talk,
I
discuss new results on 2 problems. The first is decoherence in a large
class of
lattice models with particles coupled to an Ohmic oscillator bath- these
models
include the .Schmid. model, and the dissipative Hofstadter model. Second,
I
discuss new results on decoherence in quantum walks- this is a problem of
quantum diffusion on some graph which represents paths in a computational
Hilbert space.

*
Frustration of Decoherence and Entanglement-sharing in the Spin-bath, by
Andrew Hines, PITP and PIMS, Vancouver*

The monogamous nature of entanglement has been illustrated by
the
derivation of entanglement-sharing inequalities-bounds on the
amount of
entanglement that can be shared among the various parts of a
multipartite system. Motivated by recent studies of
decoherence, we
demonstrate an interesting manifestation of this phenomena
that arises
in system-environment models where there exists interactions
between the
modes or subsystems of the environment. I will discuss this
phenomenon
in the spin-bath environment, constructing an
entanglement-sharing
inequality bounding the entanglement between a central spin
and the
environment in terms of the pairwise entanglement between
individual
bath spins. The relation of this result to decoherence will be
illustrated using simplified system-bath models of
decoherence. I will
also discuss possible extensions to oscillator-bath models.

* *

*Electron spin for
quantum
computation: what we've learned, and where we go from here, by Joshua
Folk, UBC
Vancouver*

No ABSTRACT
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