
Toronto Quantum Information Seminars QUINF 200708
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.
Organizing Committee:Daniel James, Aephraim Steinberg, Paul
Brumer or HoiKwong Lo. (Physics, University of Toronto)

Talks are held
Fridays at 11 am unless otherwise indicated
Fri.,30May2008
11:10am to 12:10pm

BeiLok Hu, University of Maryland and Perimeter Institute
for Theoretical Physicss
NonMarkovian Entanglement Dynamics of Two Qubits Interacting
Through a Quantum Field
Two necessary requirements on the physical conditions of a
system suitable for quantum information processing are the
sustenance of a sufficient degree of quantum coherence and
the preservation of quantum entanglement. Interaction of a
quantum system with its environment has a tendency to diminish
or destroy its quantum coherence and entanglement. Our research
has focused on these two issues. Working with simple systems
but with more probing analysis, we aim to provide results
in regimes physically relevant but often glossed over in textbook
treatments. In particular we focus on the nonMarkovian (processes
involving memory) regimes which correspond usually to short
time, low temperature conditions or for strongly coupled or
correlated systems. These are also the conditions more conducive
to quantum information processing. In this talk I present
results [1] from studies
of the nonequilibrium dynamics of a pair of qubits made of
twolevel atoms at a finite distance apart and interacting
with one common electromagnetic field but not directly with
each other. The case of two
qubits each interacting with its own field has been studied
by Yu and Eberly [2] earlier who reported the appearance of
'sudden death' of quantum entanglement in time. With two qubits
in the same field, where
the field mediates the qubits through induced interaction,
the behavior is much more complex [3]. This is also a more
commonly encountered situation such as in the construction
of quantum gates. We obtain
analytic expressions for the dependence of quantum entanglement
on time and on the spatial separation between the two qubits,
the latter is an effect which has never been obtained, or
even conjured, in entanglement
studies. Our investigation also brings out a new perspective
on some basic issues such as nonlocality in quantum entanglement
understood in the EPR sense. We assert that the quantum mechanical
interpretation of
entanglement is incomplete because the causal propagation
of the interceding field is left out completely in prior studies.
Inclusion of this key element from relativistic quantum field
theory considerations may paint alter our view and understanding
of these basic issues at the foundation of quantum mechanics.
[1] C. Anastopoulos, S. Shresta and B. L. Hu, Quantum Entanglement
under
NonMarkovian Dynamics of Two Qubits Interacting with a Common
Electromagnetic Field, under consideration by Phys. Rev. A
[quantph/0610007].
[2] T. Yu and J. H. Eberly, Phys. Rev. Lett. 93, 140404 (2004).
[3] Z. Ficek and R. Tanas, Phys. Rev. A 74, 024304 (2006).

Fri,
23May08
11:00am12:00noon 
Stephen Bartlett, University of Sydney
Identifying phases of matter that are universal for quantum
computation
A recent breakthrough in quantum computing has been the realization
that quantum computation can proceed solely through singlequbit
measurements on an appropriate quantum state  for example,
the ground state of an interacting manybody system. It would
be unfortunate, however, if the usefulness of a ground state
for quantum computation was critically dependent on the details
of the system's Hamiltonian; a much more powerful result would
be the existence of a robust ordered phase which is characterized
by the ability to perform measurementbased quantum computation
(MBQC). To identify such phases, we propose to use nonlocal
correlation functions that quantify the fidelity of quantum
gates performed between distant qubits. We investigate a simple
spinlattice system based on the clusterstate model for MBQC,
and demonstrate that it possesses a zero temperature phase
transition between a disordered phase and an ordered "cluster
phase" in which it is possible to perform a universal
set of quantum gates.

Fri,
9May08
11:00am12:00noon 
Katya Babourina,
University of Queensland
Quantum noise in a nano mechanical Duffing resonator
We determine the small signal gain and noise response of an
amplifier based on the nonlinear response of a quantum nanomechanical
resonator. The resonator is biased in the nonlinear regime by
a strong harmonic bias force and we determine the response to
a small additional driving signal detuned with respect to the
bias force. 
Fri,
25Apr08
11:00am12:00noon 
Jeremy O'Brien,
Centre for Quantum Photonics, University of Bristol
Quantum information science with photons on a chip
Quantum information science has shown that quantum mechanical
effects can dramatically improve performance for certain tasks
in communication, computation and measurement. Of the various
physical systems being pursued, single particles of light
– photons – have been widely used in quantum communication,
quantum metrology, and quantum lithography settings. Low noise
(or decoherence) also makes photons attractive quantum bits
(or qubits), and they have emerged as a leading approach to
quantum information processing [1,2].
In addition to single photon sources and detectors, photonic
quantum technologies require sophisticated optical circuits
involving highvisibility classical and quantum interference
with photons. While a number of photonic quantum circuits
have been realized for quantum metrology [3], quantum lithography,
and quantum logic gates [4]. These demonstrations have relied
on largescale (bulk) optical elements bolted to large optical
tables, thereby making them inherently unscalable.
Quantum technologies based on photons will likely require
an integrated optics architecture for improved performance,
miniaturization and scalability. We demonstrate highfidelity
silicaonsilicon integrated optical realizations of key quantum
photonic circuits, including twophoton quantum interference
with a visibility of 94.8(5)%; a controlledNOT gate with
an average logical basis fidelity of 94.3(2)%; and a path
entangled state of two photons, relevant to quantum metrology,
with fidelity >92% [5].
The monolithic nature of these devices means that the correct
phase can be stably realized in what would otherwise be an
unstable interferometer, greatly simplifying the task of implementing
sophisticated photonic quantum circuits. We fabricated 100's
of devices on a single wafer and find that performance across
the devices is robust, repeatable and well understood. We
have also demonstrated an all optical fibre CNOT gate [6].
These results show that it is possible to directly “write”
sophisticated photonic quantum circuits onto a silicon chip,
which will be of benefit to future quantum technologies based
on photons, as well as the fundamental science of quantum
optics.
[1] E Knill, R Laflamme, G J Milburn, Nature 409, 46 (2001)
[2] J L O’Brien, Science 318 1567 (2007)
[3] T Nagata, R Okamoto, J L O'Brien, K Sasaki, S Takeuchi
Science 316, 726 (2007)
[4] J L O'Brien, G J Pryde, A G White, T C Ralph, D Branning,
Nature 426, 264 (2003)
[5] A Politi, M J Cryan, J G Rarity, S Yu, J L O’Brien
Science to appear (2008) / arXiv:0802.0136
[6] A S Clark, J Fulconis, J G Rarity, W J Wadsworth, J L
O’Brien Nature Physics under review / arxiv/0802.1676

Fri,
4Apr08
11:00am12:00noon 
Asoka Biswas, Dept. of Chemistry and CQIQC, University
of Toronto
Overlapping resonance in the control of decoherence: N
spins coupled to a bosonic bath
Coherent control of quantum systems rely upon the presence
of coherence, loss of which ("decoherence") results
in marked decrease in controllability. This issue is of significant
interest also in the subject of quantum computation. There
are several techniques either to avoid or to eliminate decoherence.
Those techniques demand either certain symmetry in the system
Hamiltonian or severe technical challenges in implementing
them. In this talk, I will discuss a more general approach
which can combat the above two issues. This approach is quite
fundamental and relies on quantum interferences between overlapping
resonances.
We demonstrate this technique by considering a system comprising
spinhalf particles interacting with a bosonic thermal bath.
In presence of overlapping resonances, decoherence of the
spinsystem can be minimized by choosing in an optimal way
an initial superposition of the spin states. We show the results
for an available spinboson system, namely, Cooperpair qubits
interacting with a nanomechanical oscillator.

Fri,
28Mar08
11:00am12:00noon 
Qin Wang, KTH
Royal institute of technology, Sweden
Experimently demonstration on decoystate QKD with heralded
single photon source
We have experimentally demonstrated a decoystate quantum key
distribution scheme (QKD) with a heralded singlephoton source
based on parametric downconversion. We used a oneway BB84
protocol with a four states and onedetector phasecoding scheme,
which is immune to recently proposed timeshift attacks, photonnumber
splitting attacks, and can also be proven to be secure against
Trojan horse attacks and any other standard individual or coherent
attacks. In principle, the setup can tolerate the highest losses
or it can give the highest secure key generation rate under
fixed losses compared with other practical schemes. This makes
it a quite promising candidate for future quantum key distribution
systems. 
Fri,
14Mar08
11:00am12:00noon 
Nicolas Godbout, École
Polytechnique de Montréal
Optical fibre technology for quantum communication and
quantum information processing
An overview of relevant available optical fibre technologies
for quantum information is presented. Examples of applications
in quantum cryptography networks and quantum information processing
are given. A scheme for few qubit processing using the clusterstate
model of quantum processing is introduced. 
Friday,
7March2008
11:10am to 12:10pm
Place: BA3004, Bahen Centre
*** PLEASE NOTE THE LOCATION*** 
Shohini Ghose, Wilfrid Laurier
University
Studies of chaos, entanglement and decoherence in a quantum
kicked top using cold atoms
The quantum kicked top has become a standard paradigm for theoretical
studies of quantum chaos in spin systems. We describe the first
experimental realization of a quantum kicked top using cold
Cesium atoms interacting with laser and magnetic fields. The
kicked top Hamiltonian can be accurately implemented using the
nonlinear AC Stark shift and a pulsed magnetic field. Preparation
of arbitrary initial states from a fiducial state can be achieved
using Stark shifts and magnetic fields,
Measurement of the complete spin density matrix is performed
via Faraday rotation of a probe laser. A variety of interesting
phenomena can be observed such as dynamical tunneling, rapid
spreading of the wave function in the chaotic phase space, signatures
of chaos in the evolution of nuclearspin entanglement and robustness
of the dynamics in the presence of decoherence. These dynamics
can be understood by examining the Floquet eigenstates of the
system.

Fri,
29Feb08
11:00am12:00noon
**revised location
Room 230 Fields. 
Robin Williams,
Institute for Microstructural Sciences, National Research Council
Scalable Routes to Entangled Photon Pair Sources –Gated
InAs/InP Quantum Dots in Photonic Crystal Microcavities
Entangled photon pairs (EPP) can be produced through the biexciton
(XX) – exciton (X) radiative decay cascade in semiconductor
quantum dots (QD) [13]. In existing devices, the requirement
to enforce degeneracy of the intermediate excitonic states,
whose degeneracy is lifted by the anisotropic exchange splitting
(AES) [25], has led to remedies that include the application
of large external magnetic fields [2], or the materials engineering
of individual dots [3, 4]. Such schemes are impractical if large
arrays of integrated EPP sources are to be constructed for quantum
information applications.
In the work presented here we propose a scheme for EPP generation
that does not require the removal of the AES. By application
of a lateral electric field to an individual, prepositioned
InAs quantum dot on a patterned InP substrate, we engineer
the quantum dot to introduce Hidden Symmetry within the sshell.
In such circumstances the biexciton binding energy vanishes,
“which path” information for the XXX radiative
cascade is not available through a photon energy measurement
and polarization entanglement is produced even if AES is still
present. Photoluminescence measurements as a function of applied
lateral electric field will be presented for individual InAs/InP
quantum dots emitting close to l=1300nm. The nucleation sites
of these dots can be controlled with nanometer precision using
an insitu, ‘nanotemplate deposition’ technique
[6], so that it is possible to build control structures, such
as electrostatic gates, around individual QDs. Such a capability
is a prerequisite if arrays of such dots are to be employed
for quantum information applications. Our measurements demonstrate
the removal of the biexciton binding energy, a reduction of
the AES, quenching of the neutral exciton emission and the
appearance of new, normally forbidden transitions involving
an sshell electron and pshell hole. Full ConfigurationInteraction
calculations will be presented that explain how the biexciton
binding energy can be removed through manipulation of the
electronhole Coulomb interaction and consequent introduction
of Hidden Symmetry.

Friday,
22Feb08
11:00am12:00noon 
Joseph Emerson,
Institute for Quantum Computing, University of Waterloo
Negativity and contextuality as criteria for classicallity
in discrete phasespace and other quasiprobability representations
of quantum theory
In recent years several quasiprobability representations of
finite dimensional quantum mechanics have been proposed as analogs
of the phase space representation of continuous quantum systems.
I will describe a formalism based on the theory of frames which
allows us to characterize the set of possible quasiprobability
representations for finite dimensional quantum systems that
satisfy two reasonable conditions. This formalism leads to a
direct proof that any such representation (that reproduces the
quantum statistics) is nonclassical in the sense that either
the states or the measurements must be modeled by negative valued
functions. This condition turns out to be equivalent to a proof
of contextuality. This formalism may lead to a new method for
assessing the degree of nonclassicality of a given quantum
information task or process. 
Friday,
15Feb08
11:00am12:00noon 
Yoritoshi Adachi,
Department of Materials Engineering Science, Osaka University
Efficient quantum key distribution with parametric downconversion
source
Quantum key distribution (QKD) allows two parties to share an
unconditional secret key. The first QKD protocol has been proposed
by Bennett and Brassard in 1984, which is called BB84. The practical
BB84 is vulnerable against photonnumber splitting (PNS) attacks,
however, it is shown that this problem can be solved by utilizing
information from a builtin decoy state. In order to prepare
a decoy state, it seems to need an additional complexity to
the experimental setup, such as the random amplitude modulation
for weak coherent pulse scheme. Here, we propose an efficient
QKD protocol based on photonpair generation from parametric
downconversion, which uses the only different postprocessing
of the classical data from the conventional protocol. Assuming
the use of practical detectors, we analyze the unconditional
security of the new scheme, and show that it improves the secure
key generation rate by several orders of magnitude at long distances.

Friday,
01Feb08
11:00am12:00noon
Note : The venue of Xingxing's talk has been changed to MP307
due to the closure of the Fields Institute.
It will start now, 11:10am!

Xingxing Xing, Dept. of Physics and CQIQC, University
of Toronto
Towards the atomphoton interface in Quantum information:
An ultrabright entangled photon source
Nonclassical light sources are important technologies for
quantum information because light is robust to decoherence
from the environment. The flip side to this advantage, however,
is the difficulty in controlling the quantum state of light.
The atomphoton interface serves as a promising solution for
making photons "talk to
each other". In this talk, I will discuss some ideas
in designing and implementing a suitable light source for
lightmatter interfaces and report progress made on a recent
research trip to ICFO in Barcelona where we built an ultrabright,
narrowband (~85000 pairs/mW) entangled photon source suitable
for exciting atomic (Rb) transitions.
Ultimately this system will allow us to use atoms to mediate
interactions between photons and provide the means to implement
quantum light state storage, controlled quantum gates, quantum
nondemolition measurements etc.
This project is in collaboration with Morgan Mitchell's group
in ICFOInstitut de Ciències Fotòniques, Spain,
and supported by the CIPI TEN programme and OCE.

Friday,
30Nov07
11:00am12:00noon 
Jonathan Oppenheim,
University of Cambridge
Intrinsic decoherence and the destruction of information 
Friday,
23Nov07
11:00am12:00noon 
Rolando Somma, Perimeter Institute for Theoretical
Physics
Quantum Computing the Physical World
If a large quantum computer (QC) existed today, what type
of physical problems could we simulate on it more efficiently
than conventional computer? In this talk, I argue that a QC
could solve some relevant physical "questions" more
efficiently than its classical counterpart. To show this,
I will use tools borrowed from quantum metrology and quantum
phase estimation and show how they can be implemented to obtain
quantum speedups. I will begin by focusing on the quantum
simulation of quantum systems and, time permitting, I will
also describe possible ways to simulate classical systems.

Friday,
09Nov07
11:00am12:00noon 
Kirill Shtengel, University of California, Riverside
NonAbelian Anyon Interferometry
Topologicallyordered states supporting excitations with
nonAbelian braiding statistics are expected to occur at several
observed fractional quantum Hall plateaux. I will begin by
presenting a proposal for interferometric experiments designed
to detect such nonAbelian quasiparticle statistics  one
of the hallmark characteristics of the MooreRead and ReadRezayi
states, which are likely candidates for the observed fractional
quantum Hall plateaux at nu=5/2 and 12/5 respectively. Aside
from their potential utility for experimental verification
of nonAbelian anyonic statistics, such interferometric experiments
appear to provide the most promising route for qubit read
out in a topological quantum computation. With these potential
applications in mind, I will also address interferometric
measurements of states having superpositions of anyonic charges
and discuss their measurement collapse behavior.

Friday,
26Oct07
11:00am12:00noon 
Frédéric Dupuis, University of Montreal
Quantum entropic security and approximate quantum encryption"
An approximate quantum encryption scheme uses a private classical
key to encrypt a quantum state while leaking only a very small
amount of information to the adversary. Previous work has
shown that while we need 2n bits of key to encrypt n qubits
exactly, we can get away with only n bits in the approximate
case, provided that we know that the state to be encrypted
is not entangled with something that the adversary already
has in his possession. In this talk, I will show a generalisation
of this result: approximate quantum encryption requires roughly
nt bits of key, where t is a lower bound on the quantum conditional
minentropy of the state to be encrypted given the
adversary's prior knowledge. I will show that this result
follows naturally from a quantum version of entropic security
and indistinguishability. This is joint work with SimonPierre
Desrosiers.

Friday,
7Sep2007
11:00am12:00noon 
Marcos Curty
Dept. of Electronic Engineering and Communications, University
of Zaragoza (Spain)
Oneway and Twoway Classical PostProcessing Quantum
Key Distribution
We investigate oneway and twoway quantum key distribution
(QKD) protocols. Our analysis is based on a simple precondition
for secure QKD in each case. In particular, the legitimate
users need to prove that there exists no quantum state having
a symmetric extension (in the case of oneway QKD), or that
there exists no separable state (twoway QKD) that is compatible
with the available measurements results. We show that both
criteria can be formulated as a convex optimization problem
known as a semidefinite program, which can be efficiently
solved. Moreover, we prove that the solution to the dual optimization
corresponds to the evaluation of an optimal witness operator
that belongs to the minimal verification set of them for the
given oneway (or twoway) QKD protocol. A positive expectation
value of this optimal witness operator states that no secret
key can be distilled from the available measurements results.
We apply such analysis to several wellknown QKD protocols
and obtain ultimate upper bounds on the maximal rate and distance
that can be achieved with these schemes.


