
Toronto Quantum Information Seminars QUINF 200607
held at the Fields Institute
The Toronto Quantum Information Seminar  QUINF  is held
roughly every two weeks to discuss ongoing work and ideas
about quantum computation, cryptography, teleportation, et
cetera. We hope to bring together interested parties from
a variety of different backgrounds, including math, computer
science, physics, chemistry, and engineering, to share ideas
as well as open questions.

Talks are held
Fridays at 11 am unless otherwise indicated
29Jun2007 
Kiyoshi Tamaki
CANCELLED

22Jun2007 
Abraham Ungar, North Dakota State University
On the Bloch Vector of Quantum Information and Computation
A qubit is a two state quantum system, completely described
by the qubit density matrix $\rho(\mathbf{v})$ parametrized
by the Bloch vector $\mathbf{v}$ varying in the unit ball
of the Euclidean 3space. The only wellknown structure of
the space of all qubit density matrices is the convex structure.
Qubit density matrices give rise to the trace distance and
Bures fidelity between two qubit density matrices. Much to
their chagrin, Nielsen and Chuang admit:
"Unfortunately, no similarly [alluding to the trace
distance and its Euclidean geometric interpretation] clear
geometric interpretation is known for the [Bures] fidelity
between two states of a qubit". Nielsen and Chuang [1,
p. 410], 2000.
Surprisingly, Bures fidelity does have a novel rich geometric
and algebraic structure, but it lies in the hyperbolic geometry
of Bolyai and Lobachevsky rather than in Euclidean geometry
[2].
Following [3, 4, 5] I will introduce a novel "gyrovector
space" approach to the classical hyperbolic geometry
of Bolyai and Lobachevsky which, unexpectedly, turns out to
be fully analogous to the common vector space approach to
Euclidean geometry. I will then demonstrate that
(i) Bloch vector is not a vector but, rather, a gyrovector
(that is, a hyperbolic vector); and that
(ii) the space of all qubit density matrices possesses the
same novel, rich, nonassociative algebraic structure that
regulates (a) hyperbolic geometry and (b) Einstein's special
relativity theory.
In particular, I will show that Bures fidelity has a clear
hyperbolic geometric interpretation, and indicate further
applications of hyperbolic geometry in quantum information
and computation.
References
[1] Michael A. Nielsen and Isaac L. Chuang. Quantum computation
and quantum information. Cambridge University Press, Cambridge,
2000.
[2] J.L. Chen, L. Fu, A. A. Ungar, and X.G. Zhao, "Geometric
observation of Bures fidelity between two states of a qubit,"
Phys. Rev. A (3), vol. 65, no. 2, pp. 024303/13, 2002.
[3] Abraham A. Ungar. Beyond the Einstein addition law and
its gyroscopic Thomas precession: The theory of gyrogroups
and gyrovector spaces, volume 117 of Fundamental Theories
of Physics. Kluwer Academic Publishers Group, Dordrecht, 2001.
[4] Scott Walter. Book Review: Beyond the Einstein Addition
Law and its Gyroscopic Thomas Precession: The Theory of Gyrogroups
and Gyrovector Spaces, by Abraham A. Ungar. Found. Phys.,
32(2):327330, 2002.
[5] Abraham A. Ungar, Analytic Hyperbolic Geometry: Mathematical
Foundations and Applications. Singapore: World Scientific,
2005.

18May2007
11:00 a.m. 
Lucien Hardy Perimeter Institute for Theoretical Physics
Interacting qubits in the causaloid formalism
Pairwise interacting (qu)bits can be used to perform universal
(quantum) computation. The causaloid formalism was originally
developed as a tentative step in the direction of constructing
a theory of quantum gravity. This framework treats space and
time on an equal footing. Further, it is timesymmetric. It
is possible to put the theory of interacting bits (in the
classical case) and of interacting qubits (in the quantum
case) into this framework. It is hoped that, by placing classical
and quantum theory in this framework, we can gain some insight
into the nature of information processing in the two theories.

15May2007
11:00 a.m. 
(PLEASE NOTE NONSTANDARD DATE)
Vadim Makarov, Dept. of Electronics and Telecommunications,
Norwegian University of Science and Technology
Practical attacks on quantum key distribution systems
Quantum cryptography allows guaranteely secure distribution
of a secret key over an open optical channel. The security
against eavesdropping is based on the known laws of physics
and is confirmed by rigorous theoretical proofs. However,
the model of legitimate users' equipment used in the proofs
has so far been limited. The proofs have assumed idealized
models of optical and electrooptical components in legitimate
users' setups and have omitted some component imperfections.
These omitted imperfections, as it has been shown, open possibilities
for successful attacks.
Most quantum cryptosystems today contain two or more single
photon detectors. In this talk, I will consider two nonidealities
of single photon detectors, and how Eve can exploit them.
The first nonideality is a dependence of relative efficiency
of '0' detector versus '1' detector on an external parameter
controllable by Eve (e.g., timing of the incoming pulses).
The second nonideality is a saturation behavior of a passivelyquenched
avalanche photodiode, where it becomes completely blinded
by a moderately strong light. I illustrate both imperfections
with experimental data, show how Eve can construct successful
attacks using them, and present some calculations on how strong
the nonideality should be to allow for a successful attack.
I also consider countermeasures legitimate users could devise.

27Apr2007
11:00 a.m. 
Marco Lucamarini,
Department of Physics, University of Camerino
Quantum cryptography with a twoway quantum channel
It is a common belief that twoway quantum channels can not
represent a practical tool for quantum cryptography because
of their high loss rate. I will debate this question by an explicit
example of a twoway quantum cryptosystem that provides key
distribution rates higher than its oneway counterpart on a
small and mediumscale distance. I will also present other
potentialities and a few experimental results pertaining to
this new protocol.

20Apr2007
11:00 a.m. 
Mohsen Razavi, Institute for Quantum Computing, University
of Waterloo
Quantum Key Distribution over Long Distances
Quantum key distribution (QKD) is believed to be the first
realizable application of quantum information science. In
fact, over short distances, such QKD systems are commercially
available. Over long distances, however, the scenario is much
different. If we do not have access to a network of trusted
nodes for key regeneration, the only known solution for longdistance
QKD relies on entanglement swapping or quantum repeater systems.
Such systems face their own implementation challenges, including
the need for quantum memory devices and highly efficient gates
and detectors. In this talk, I discuss a variety of physical
requirements for quantum repeaters, compare different architectures
for entanglement distribution, and address the prospect of
developing these systems in the near future.

12Mar2007
12:00 p.m. 
Hans Hübl, Walter SchottkyInstitut and PhysikDepartment
E25, TU München
Related Seminar of Interest  "News on an old donor:
Manipulation and detection of the spin states of phosphorus
in silicon"
One of the proposed solidstate realizations of quantum computing
is based on the electronic and nuclear spins of phosphorous
donors in silicon. The strong KohnLuttinger oscillations
of the donor wave function in the indirect bandgap semiconductor
Si, which complicate the exchange interaction of neighboring
31P donors, can be suppressed by using strained silicon layers.
Additionally, the strain will also affect the wave function
at the donor atom, which can be observed directly via the
hyperfine interaction between the donor electron and its nucleus
in electron spin resonance. In this talk, I will present the
results of detailed experimental and theoretical investigations
of the hyperfine interaction by electrically detected magnetic
resonance (EDMR), using the spin dependent 31PPb0 recombination
as a spintocharge transfer. Furthermore, experiments studying
the sensitivity limit of this detection mechanism will be
summarized showing that as few as 50 P donors can already
be resolved in nanostructured devices.
Beside the principal detection of phosphorus donors in silicon
using EDMR, I discuss the observation of Rabi oscillations
by investigating the current transient after the application
of a microwave pulse which allows to read out the spin state
of the electron. Pulsed EDMR experiments can be extended to
Hahn echo tomography which allows to determine the T2 time
of the specific spintocharge transfer system used.

12Mar2007
11:00 a.m.

Prem Kumar, Northwestern University
Fiberoptic Quantum Communications
Keeping in mind the ubiquitous standard optical fiber for
longdistance transmission and the widespread availability
of efficient active and passive fiber devices, we have been
developing telecomband resources for practical quantum communications
and information processing in wavedivisionmultiplexed (WDM)
fiber optical networks. In this talk, I will present our recent
results on telecomband infiber entanglement generation,
characterization, storage, and longdistance distribution
for various quantum information processing applications.
Joint Quantum Optics/AMO Seminar and QUINF Seminar

02Mar2007
11:00 a.m. 
Andrei Klimov, University
of Guadalajara, Mexico
Discrete PhaseSpace Structure and Mutually Unbiased Basis
Operators
Several construction methods for mutually unbiased bases have
been proposed in the literature. Typically they involve either
direct construction of the basis vectors, or sets of operators
are derived where each set's (simultaneous) eigenstates are
mutually unbiased with respect to every other set's. It is subsequently
common to map the states onto lines in the corresponding discrete
phase space. We show how to derive mutually unbiased bases from
the reverse mapping. We start by considering the most general
phasespace structures compatible with the concept of mutually
unbiased bases, namely bundles of discrete space curves intersecting
only at the origin and satisfying certain properties and develop
a new method based on the analysis of geometrical structures
in the finite phasespace for construction of Mutually Unbiased
Bases (MUB) operators. In the case when the Hilbert space dimension
is an integer power of a prime, there exist several classes
of curve bundles with different properties, lines being a special
case. We also consider transformations between different kinds
of curves, and show that in the twoqubit case, they all correspond
to local transformations, and more specifically they correspond
to rotations around the Blochsphere principal axes. Nevertheless,
in the case of more that two qubits several non isomorphic structures
appear, which can be naturally classified in terms of discrete
curve bundles.
The existence and a possibility of regular searching of such
non isomorphic MUBs allows us to introduce a concept of complexity
of tomographic scheme for state determination of multiqubit
systems.

23Feb2007
11:00 a.m. 
Ashwin Nayak, University of Waterloo, and Perimeter
Institute for Theoretical Physics
Search via quantum walk
Although the original problem may not be formulated in terms
of graph search, computational problems can often be recast
as the problem of searching for a special kind of vertex in
a graph. This turns out to be a particularly useful view to
take for designing efficient algorithmsquantum particles
exploring a graph may detect special (or ``marked'') vertices
quadratically faster than classical particles, as illustrated
by Ambainis (2004) and Szegedy (2004).
In this talk, we will see a quantum walk based algorithm
that may be defined for an arbitrary ergodic Markov Chain.
It combines the benefits of two previous approaches while
guaranteeing the better form of run time. The algorithm is
both conceptually simple and avoids several technical difficulties
in the analysis of earlier approaches. It thus seems to demystify
the role of quantum walks in search algorithms.
We will begin with example search problems and algorithms
based on random walk, describe amplitude amplification and
phase estimation (two useful building blocks for quantum algorithms),
and sketch how their confluence gives our search algorithm.
This is joint work with Fr'ed'eric Magniez (CNRSLRI), J'er'emie
Roland (UC Berkeley), and Miklos Santha (CNRSLRI).

29Jan2007
11:00 a.m. 
André Stefanov, Institut
für Experimentalphysik, Universität Wien
Implementation of Simple Quantum Algorithms using Optical
Cluster State
Quantum computers promise to be more efficient and powerful
than their classical counterparts. In the oneway quantum computer
model, a sequence of measurements processes qubits, which are
initially prepared in a highly entangled cluster state. We present
here an optical implementation of a 4qubit cluster state and
we show how different algorithms can be experimentally realized
by performing the corresponding sequence of measurements. We
demonstrate deterministic one and twoqubit gate operations
as well as Grover's quantum search algorithm. A major advantage
of optical quantum computation is the very short time for one
computational step achievable by using these ultrafast switches.
With present technology this feedforward step can be performed
in less than 150 nanoseconds.
We also present how cluster states can be used to realize
quantum circuits simulating simple quantum games. Finally
we present the experimental realization of decoherence free
subspace cluster computation where each logical qubit is encoded
into two physical ones, and hence protected against phase
noise.

19Jan2007
11:00 a.m.

Robert Raussendorf, Perimeter
Institute for Theoretical Physics
Faulttolerant quantum computation with high threshold in
two dimensions
We present a scheme of faulttolerant quantum computation for
a local architecture in two spatial dimensions. The error threshold
is 0.59 percent for each source in an error model with preparation,
gate, storage and measurement errors.
Joint work with Jim Harrington. See quantph/0610082.

12Jan2007
11:00 a.m. 
Patrick Hayden, School of Computer Science,
McGill University
Quantum Information Theory: Four Lessons from the Land of
Large n
What should your average quantum information scientist know
about quantum Shannon theory? Over the past few years, the asymptotic
theory of quantum information, known as quantum Shannon theory,
has advanced tremendously. However, while many of the field's
most important insights are simple to explain, they remain largely
unknown to all but a small group of afficionados. In this talk,
I'll present four lessons from this "land of large n",
ranging from the surprising to the useful and the amusing to
the painful.
If you've ever:
* assumed that correlation can be decomposed into quantum
and classical parts
* wondered why so many otherwise welladjusted people are
obsessed by some mathematical problem called the "additivity
conjecture"
* thought that being "more than certain" is paradoxical
then this talk is for you.

5Jan2007
11:00 a.m. 
Aaron J. Pearlman
Ultrafast NbN superconducting singlephoton optical detector
for quantum communications
We evaluate the NbN singlephoton detector (SSPD) for the
purpose of integration into a fiberbased quantum communication
system, namely the DARPA quantum key distribution (QKD) network.
We first review freespace system measurements to characterize
the SSPD in terms of counting rate and timing jitter and then
demonstrate its utility in two fiberbased systems. The first
utilizes fibercoupled SSPDs placed in a cryogenfree refrigerator
capable of reaching mK temperatures, and the SSPDs are evaluated
in terms of system quantum efficiency (SQE) and dark counts
over a broad temperature range. The second system, utilizes
fibercoupled SSPDs assembled on an insert placed in a standard
helium dewar with each fiber permanently glued to a device.
The SSPDs, evaluated in terms of SQE, dark counts, and timing
resolution, show that the system provides relatively high
fiberdetector coupling efficiency, good timing resolution,
and can integrate easily into the DARPA network.
We also investigate the SSPD’s limitations by analyzing
a model which takes into account the SSPD detection mechanism
and device inductance to predict its response time. We then
optimize the SSPD meander geometry in designing devices with
high SQE and counting rate in terms of area, stripe width,
fill factor, and thickness using detailed inductance simulations.
We will also present a novel low inductance SSPD design and
model its photoresponse.
With these designs and measurement results, we show that
the SSPD outperforms its superconducting and semiconducting
counterparts for quantum cryptography systems with high clock
rates. Thus, the SSPD, with its combination of high QE, and
low timing jitter at telecommunications wavelengths, as well
as low dark counts, make it a natural choice for the DARPA
network and quantum cryptography systems in general.

15Dec2006
11:00 a.m. 
Jean Christian Boileau, Institute
for Quantum Computing, University of Waterloo
Obtaining the DevetakWinter bound for Quantum Key Distribution
in Terms of Entanglement Distillation
Bennett and Brassard proposed the first QKD protocol in 1984,
but it was not until the work of Dominic Mayer in 1996 that
it was proven to be unconditionally secure. One of the noticeable
advances in the security proof technique was accomplished by
Lo and Chau, followed by Shor and Preskill, when they related
BB84 to entanglement distillation. Subsequently, techniques
for unconditional security proofs have greatly evolved. One
of the most complete security proofs for QKD protocols using
single photon encoding and oneway communication has been proposed
by Renner, Gisin and Kraus.
In standard entanglement distillation proofs and in the original
paper by Renner, Kraus and Gisin, the quantum state prior
to the raw key measurement is or could be diagonalized in
the Bellbasis. Excluding preprocessing, the secret key generation
rate obtained in that manner is asymptotically close to 1H(p_{uv})
where H(p_{uv}) is the Shannon entropy of the bit and the
phase error rate of the system representing the key. However,
for some QKD protocols, there are other symmetrizations that
give a better lower bound for secret key generation rates
derived using only oneway communication.
An improved secure rate for some QKD protocols involving
measurements of nonorthogonal states can be calculated by
symmetrizing the state "earlier" in the protocol,
as was acknowledged recently by Kraus et al. for the case
of SARG04. As we show for the case of spherical code and the
Singapore protocol where the QKD protocol follows some symmetries
(i.e. the effective channel is dephasing), the symmetrization
proposed by Renner, Gisin and Kraus can be done before a socalled
filtering operation. In the absence of such symmetry, the
quantum de Finetti theorem as described in Renner's thesis
can be used instead to obtain a state that is close to separable.
Applying the DevetakWinter lower bound to such state, we
obtain a secret key rate that can be higher than 1H(p_{uv})
(i.e. the Devetak and Winter bound is given by I(X:B)I(X:E),
where I(X:B) or I(X:E) is the mutual information between Alice
and Bob, or Alice and Eve, supposing that Alice, Bob and Eve
share by a cqq state).
One of our contributions is to derive this improved bound
from the perspective of entanglement distillation. To do so,
it is necessary to introduce the concept of a shield, which
is a system that Eve cannot access and that does not contain
the key. We show that for the prepareandmeasured QKD protocol,
the state of the shield can be written approximately as \sigma^
v where v describes the phase error pattern, and that the
secret key generation rate is given by 1H(p_{u})H(p_{v})+\chi(\sigma_v,
p_v), where \chi is the Holevo information. We also show that
this rate is equivalent to the Devetak and Winter bound.
Joint work with J.M. Renes.

30Oct2006
11:00 a.m. 
Thomas Coudreau, Université
de Paris VII  Denis Diderot
Quantum memories with trapped ions: theoretical results and
ongoing experiments
The great enemy of quantum information is decoherence, through
which a quantum system quickly becomes classical. Trapped ions
form an ideal medium for quantum information as they can be
well controlled using laser beams while being relatively well
isolated from external noise sources. I will show how ensembles
of cold ions can be used as quantum memories either to store
the quadratures describing intense light beams (continuous variable
regime) or for very long lived qubits, based on the principles
of topological protection.
Joint Quantum Optics/AMO Seminar and QUINF Seminar

24Oct2006
2:00 p.m. 
Thomas Coudreau, Université
de Paris VII  Denis Diderot
Quantum properties of selfphase locked Parametric Oscillators
Optical Parametric Oscillators (OPOs) consist of nonlinear (chi2)
media inserted inside a cavity. When operated above the oscillation
threshold, these devices generate intense, phase coherent optical
beams. Triply resonant OPOs have been known for a long time
to generate very large intensity quantum correlations. I will
show that, when phase locking is introduced between the output
beams, one can generate a record amount of entangled light with
unique properties. I will also describe the principles of a
novel device which can emit polarization entangled light.
Joint Quantum Optics/AMO Seminar and QUINF Seminar

20Oct2006
11:00 a.m.

Scott Aaronson,
Institute for Quantum Computing, University of Waterloo
The Learnability of Quantum States
Traditional quantum state tomography requires a number of measurements
that grows exponentially with the number of qubits n. But using
ideas from computational learning theory, I'll show that "for
most practical purposes" one can learn a quantum state
using a number of measurements that grows only linearly with
n. Besides possible implications for experimental physics, this
learning theorem has two applications to quantum computing:
first, a new simulation of quantum protocols, and second, the
use of trusted classical advice to verify untrusted quantum
advice.

02Oct2006
11:00 a.m. 
Jim Franson, University of
Maryland, Baltimore County
Entangled Photon Holes
Parametric downconversion can be used to create pairs of photons
that are entangled in energy and time. Photons entangled in
this way are emitted at the same time, but with a coherent superposition
of such times, which can violate Bell’s inequality and
can be used in quantum key distribution, for example. We have
recently introduced the idea of entangled photon holes, in which
a twophoton absorbing medium absorbs pairs of photons from
two laser beams at the same time, with a coherent superposition
of those times. Entangled photon holes can also violate Bell’s
inequality and may have some advantages in quantum communications.
A recent experimental demonstration of entangled photon holes
will also be discussed.
Joint Quantum Optics Condensed Matter Physics Seminar and
QUINF seminar

29Sep2006
11:00 a.m. 
Carlos A. Perez, Institute
for Quantum Computing, University of Waterloo
Quantum Cellular Automata and Single Spin Measurement
We propose a method for single spin measurement. Our method
uses techniques from the theory of quantum cellular automata
to correlate a huge amount of ancillary spins to the one to
be measured. It has the distinct advantage of being very efficient,
and to a certain extent faulttolerant. Under ideal conditions,
it requires the application of only $O(\sqrt[3]{N})$ external
radio frequency pulses to create a system of $N$ correlated
spins. It is also fairly robust against pulse errors, imperfect
initial polarization of the ancilla spin system, and does not
rely on entanglement. We study the scalability of our scheme
through extensive numerical simulation.
This is joint work with Michele Mosca (UW), Paola Cappellaro
(MIT), and David G. Cory (MIT).

15Sep2006
11:00 a.m. 
Dmitry Gavinsky, Department
of Computer Science, University of Calgary
On the Role of Shared Entanglement
Despite the apparent similarity between shared randomness and
shared entanglement in the context of Communication Complexity,
our understanding of the latter is not as good as of the former.
In particular, there is no known ``entanglement analogue'' for
the famous theorem by Newman, saying that the number of shared
random bits required for solving any communication problem can
be at most logarithmic in the input length ( i.e., using more
than O(log(n)) shared random bits would not reduce the complexity
of an optimal solution).
We prove that the same is not true for entanglement. We establish
a wide range of tight (up to a logarithmic factor) entanglement
vs. communication tradeoffs for relational problems.
The "lowend" is: for any t>2, reducing shared
entanglement from log^t(n) to o(log^{t1}(n)) qubits can increase
the communication required for solving a problem almost exponentially,
from O(log^t(n)) to \omega(\sqrt n).
The "highend" is: for any \eps>0, reducing shared
entanglement from n^{1\eps}\log(n) to o(n^{1\eps}) can increase
the required communication from O(n^{1\eps}\log(n)) to \omega(n^{1\eps/2}).
The upper bounds are demonstrated via protocols which are
exact and work in the simultaneous message passing model,
while the lower bounds hold for boundederror protocols, even
in the more powerful model of 1way communication. Our protocols
use shared EPR pairs while the lower bounds apply to any sort
of prior entanglement.

08Sep2006
11:00 a.m. 
Gennady Berman, Theoretical Division, Los Alamos National
Laboratory
Survival of quantum effects after decoherence and relaxation
I will review our results on a mathematical dynamical theory
for observables for open quantum nonlinear bosonic systems
for a very general class of Hamiltonians. We argue that for
open quantum nonlinear systems in the “deep” quasiclassical
region, important quantum effects survive even after the decoherence
and relaxation processes take place. Estimates are derived
which demonstrate that for a wide class of nonlinear quantum
dynamical systems interacting with the environment, and which
are “close” to the corresponding classical systems,
quantum effects still remain important and can be observed,
for example, in the frequency Fourier spectrum of the dynamical
observables and in the corresponding spectral density of the
noise. These preliminary estimates are presented for BoseEinstein
condensates, low temperature mechanical resonators, and nonlinear
optical systems prepared in large amplitude coherent states.

23Aug2006
11:00 a.m. 
Peter Turner, Institute for
Quantum Information Science, University of Calgary
Quantal and semiclassical approaches to the degradation
of quantum reference frames
There has been much recent activity in the study of quantum
reference frames, from their role as resources in quantum information
theory to their role in the application of superselection rules.
These studies make it clear that we must carefully distinguish
cases in which we take a reference frame for granted from those
in which we include the physical reference frame in our dynamics.
In this talk I will describe the degradation of a quantum reference
frame as it is used to make repeated measurements, where the
entangling of the frame and the system under study results in
an increasingly mixed state for the frame which is decreasingly
useful as a reference. We show that the `longevity' of the frame
as a useful reference scales quadratically with the `strength'
of that frame. I will also describe a recent semiclassical
approach to the degradation of a directional reference frame
where it is modelled as a random walk on the sphere.

18Aug2006
11:00 a.m. 
Nikolai Kiesel, Department
of Physics, LMU Munich and MPQ Garching
Experimental Applications of a Linear Optical Controlled
Phase Gate
I will present the experimental implementation of a probabilistic
linear optical controlled phase gate. It is operating on the
polarization degree of freedom of photons and is based on the
second order interference at a ‘magic’ beam splitter.
We characterized the gate performance with a tomographic set
of measurements and, by fitting a model of the gate to the obtained
data, we extracted the corresponding experimental parameters.
In a next step, the controlled phase gate served to entangle
two EPRpairs resulting in a fourphoton entangled cluster
state with a fidelity of 74.4 ± 1.2 %. We studied the
state using entanglement witnesses, showed the violation of
a bell inequality and verified its high entanglement persistency
against photon loss.
Finally, we demonstrated a teleportation and an entanglement
swapping experiment with a complete bell state analysis that
was based on the controlled phase gate.

12Aug2006

An International Workshop
Frontiers of Quantum Decoherence
Decoherence is a fundamental physical phenomenon which occurs
in a variety of quantum systems. It has recently emerged as
a priority research topic in many fields of science and technology.
This Workshop is intended to bring together international
experts from various disciplines, to discuss foundational
issues and to explore new research horizons. Our focus will
be on decoherence mechanisms in solidstate, atom/molecular
and photonic systems, and on quantum algorithms for mitigating
decoherence effects in quantum information processing.

08May2006
11:00 a.m. 
KarlPeter Marzlin,
Institute for Quantum Information Science, University of Calgary
Quantum Information with Atoms and Photons
Atomic gases and single photons are among the most promising
candidates to implement quantum information technology because
they can be well isolated from their environment. Despite this
advantage it is challenging to design controllable interaction
between these particles and to store or manipulate quantum information
in a reliable way. We have explored how electromagnetically
induced transparency can be used to create a large nonlinear
interaction between singlephoton pulses, to transfer optical
states between different photon modes, and to create an unusual
interaction between light fields. Furthermore, we have found
new results on the physical limitations of decoherencefree
states. The nature of these limitations points towards new directions
in the search for decoherencefree subspaces.

21Apr2006
11:00 a.m. 
Elinor Irish, University of
Rochester, Department of Physics and Astronomy
The Theory of Quantum Electromechanics
"Quantum electromechanics" combines superconducting
qubits and nanofabricated mechanical devices into a system analogous
to the canonical atomcavity system of quantum optics. Many
fascinating quantumoptical effects should be realizable in
this solidstate system. The prospect of achieving very strong
coupling even at large detuning suggests the exploration of
parameter regimes in the spinboson problem that are inaccessible
in quantum optics experiments. I will talk about my work on
the theory of quantum electromechanical systems, motivated in
particular by the search for practical schemes to observe the
quantum behavior of nanoscale mechanical resonators.


