
THE
FIELDS INSTITUTE FOR RESEARCH IN MATHEMATICAL SCIENCES 
Toronto
Quantum Information Seminars
201314
at
the Fields Institute, Fridays,
11:10 a.m.



OVERVIEW
The Toronto Quantum Information Seminar 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.
Upcoming
Seminars 
April 14
Stewart Library

Dr. Ben Fortescue, (Southern Illinois University)
Tolerating Qubit Loss in Quantum Error Correction
Standard faulttolerant quantum error correction (QEC) schemes can
protect quantum information against arbitrary errors on individual
qubits, but with the implicit assumption that the underlying physical
systems remain within the qubit subspace. Faulttolerance may therefore
be lost under "leakage" errors (such as a photonic qubit
being absorbed) which violate this assumption, and techniques for
modifying QEC to cope with generic leakage can be very costly in terms
of the extra operations required. I will discuss a more specific "loss"
model of leakage, applicable to many implementations (photonic ones
especially), and show that it allows for faulttolerance to be analysed
and achieved at much less cost in common CalderbankShorSteane (CSS)
codes.

April 25
Room 210 
Prof. Marko
Loncar, Harvard University

May 2,2014
11:00 a.m. 
Dr. Raul Garcia
(Max Plank Institute) 
May 7, 2014
Room 210 
Prof. Sven
Höfling (University of St Andrews)

May 9, 2014
Room 210

Prof. Norbert
Lütkenhaus (University of Waterloo)

May 16, 2014
Room 210 
Prof. Mile Gu (Tsinghua University)
Discord as a Consumable Resource
Correlations lie at the heart of our capacity to manipulate information.
The fewer the constraints on the correlations we can exploit, the
greater our capacity to manipulate information in ways we desire.
The rapid development of quantum information science is a testament
to this observation. Quantum systems may be so correlated that they
are `entangled', such that each of its subsystems possesses no local
reality. Exploitation of such uniquely quantum correlations has led
to many remarkable protocols that would otherwise be either impossible
or in feasible. However, the absence of entanglement does not eliminate
all signatures of quantum behaviour. Coherent quantum interactions
between separable systems that result in negligible entanglement could
still lead to exponential speedups in computation. The potential
presence of discord within such protocols motivated speculation that
discord might prove a better quantifier of the `quantum resource'
that coherent interactions exploit to deliver a `quantum advantage'.
In this presentation, I will give a brief tutorial of quantum discord.
I then introduce and demonstrate an operational method to use discord
as a physical resource. I show that under certain measurement constraints,
discord between bipartite systems can be consumed to encode information
that can only be accessed by coherent quantum interactions. The inability
to access this information by any other means allows us to use discord
to directly quantify this `quantum advantage'. I will outline recent
experiments done at the Australian National University and the University
of Queensland, where we experimentally encoded information within
the discordant correlations separable states. The amount of extra
information recovered by coherent interaction is quantified and directly
linked with the discord consumed during encoding. I survey the potential
applications of this phenomena, in both certification of entangling
operations, and protecting the benefits of entanglement in entanglement
breaking noise.

July 25
Room 210 
Prof. Gershon
Kurizki, Weizmann Institute of Science

Past Seminars 
April 4
Room 210

Angela Sestito, Università della Calabria, Italy
Understanding Quantum Mechanics: a formal analysis of nonlocality
theorems
Per se, quantum theory entails no violation of the locality principle;
conflicts between them arise if further conditions, which do not
belong to the genuine set of quantum postulates, are required to hold.
In this talk we explore consequences of adopting the criterion of
reality (R) stated by Einstein, Podolsky and Rosen in 1935. In
particular, we will analyze two possible interpretations of (R) and
implications that follow if we adopt them in connection with the nonlocality
theorem of Hardy: the contradiction  arising if a "wide"
interpretation is adopted does not arise in connection with a "strict"
interpretation of (R). We conclude that if the strict interpretation
of (R) is adopted the theorem fails in proving the inconsistency between
quantum mechanics and locality. Finally, we propose an ideal experiment
enabling the simultaneous assignment of the objective values of two
incompatible properties of a system made up of two separated noninteracting
spin particles when the strict interpretation of (R) is adopted.

Friday
March21
11:10 a.m.
Room 210 
Jianming Cai, Ulm
University
Nanoscale quantum sensing with color centers in diamond
Color centers are atomic defects in diamond that possess electronic
and nuclear spins. The rapid progress of experiments with color centers
in diamond indicates that they are promising systems for quantum information
processing, and more important for quantum sensing (imaging) under
ambient conditions.
We have devised strategies to achieve highly sensitive measurement
of weak signals, such as magnetic field, electric field, pressure,
and temperature in the presence of ambient noise, while achieving
nanometer spatial resolution. We are exploring various applications
of these strategies ranging from quantum information processing, fundamental
physics, to material science as well as biology including the structure
of proteins and the possible role of quantum effects in biological
functions.

Mar. 7
Stewart Library 
Prof. Gerd Leuchs, Max Planck Institute for the Science of Light,
Erlangen, Germany
Distributing entanglement via separable states
Whether or not entanglement is necessary prerequisite for quantum
information protocols had been debated ever since the first experiments
on NMR quantum computing, which were performed successfully with separable
systems. There were several hints that entanglement is sufficient
in such applications but that it is not absolutely necessary. The
first was that the entanglement in a Werner state [1] vanishes discontinuously
as the state is gradually tuned towards more mixedness. Yu and Eberly
[2] found another example of a discontinuous disappearance of entanglement.
In a seminal paper Ollivier and Zurek [3] introduced quantum discord
as a measure for quantum correlations, which does not show such discontinuities.
Quantum discord can be seen as a measure for the entanglement, which
can be extracted from a separable system in a mixed state. Along this
line, three experiments recently demonstrated distributing entanglement
with separable states [4,5,6].
References:
1. R.F. Werner, Phys. Rev. A 40, 4277 (1989)
2. H. Ollivier, W.H. Zurek, Phys. Rev. Lett. 88, 017901 (2002) 3.
T. Yu and J.H. Eberly, Phys. Rev. Lett. 93, 140404 (2004) 4. A. Fedrizzi
et al., Phys. Rev. Lett. 111, 230504 (2013) 5. C.E. Vollmer et al.,
Phys. Rev. Lett. 111, 230505 (2013) 6. Ch. Peuntinger et al., Phys.
Rev. Lett. 111, 230506 (2013)

Feb. 28
Room 230 
Prof. Guifre Vidal, Perimeter Institute
Tensor Networks for Quantum Manybody Systems
Quantum manybody systems are hard to study because the associated
Hilbert space, containing all possible manybody states, is huge:
its dimension grows exponentially in the system size. In recent years,
however, progress in our understanding of quantum entanglement has
revealed that a large class of manybody states of interest are highly
atypical and such that we can actually efficiently represent them
with a mathematical structure called tensor network. As a result,
it is now possible to accurately simulate, say, a quantum spin chain
made of thousands of interacting spins. In this Colloquium I will
(i) review the computational challenge posed by quantum manybody
systems; (ii) summarize our current understanding of manybody entanglement;
and (iii) give a gentle introduction to tensor networks as an efficient
description of manybody states.

Jan. 31
Room 210 
Prof. Arjendu Pattanayak, Carleton College
Surprises in the quantumclassical transition for computed Lyapunov
exponents
Completely classical behavior is very different from completely quantummechanical
behavior, particularly for nonlinear or chaotic systems, even though
the transition between the two happens as a function of controllable
parameters, such as the size of the system or environmental effects.
I report on recent work exploring this multiparameter transition.
The first set of results is on the behavior of calculated quantum
Lyapunov exponents Lambda for a Duffing oscillator system as a function
of effective action beta as well as the system damping parameter Gamma.
In general Lambdas decrease as beta increases (chaos decreases as
the system becomes more quantal, as expected). However, we identify
anomalous regions where Lambdas
increase with beta, including going from negative to positive with
increasing beta; and also regions where the quantum results do not
tend smoothly to the classical results. All anomalous results correspond
to windows of regularity embedded in a larger chaotic parameter regime,
which inverts the usual paradigm: The classically regular behavior
is the most challenging for quantumclassical correspondence. I also
report on progress on various other projects including (a) studying
how classical control algorithms work in controlling quantum systems
across this parameter regime and (b) on the control of the quantum
chaos.

Nov. 22
Room 210

Dr. Pascale Senellart, CNRS
Cavity quantum electrodynamics with semiconductor quantum dots
A semiconductor quantum dot is a promising system to develop a solidstate
quantum network. Like real atoms, quantum dots can emit single photons,
polarization entangled photon pairs, indistinguishable photons... Moreover,
the spin of a carrier trapped in a quantum dot can present long coherence
times and be used as a stationary quantum bit that one can optically
manipulate and measure. However, the scalability of a quantum dot based
quantum network requires implementing a highly efficient single photonsingle
quantum dot interface so as to collect every photon emitted by a quantum
dot and symmetrically, to ensure that every photon sent onto the device
interacts with the quantum dot. Controlling the spontaneous emission
of a quantum dot in a cavity is an efficient way to build such an interface.
In this talk, we will present our recent results along this research
line.
We have developed an insitu lithography technique to deterministically
insert a single quantum dot into a pillar optical microcavity. In the
lightmatter weak coupling regime, we obtain ultrabright sources of
quantum light. We demonstrate sources of indistinguishable single photons
with brightness as large as 79 % collected photon per pulse. With coupled
pillar cavities, we also fabricate bright sources of polarization entangled
photon pairs. The potential of these sources for quantum information
processing is demonstrated by implementing an entangling controlledNOT
gate. In the light matter strong coupling regime, we demonstrate optical
nonlinearities for only 8 incident photons per pulse. Finally, we present
a novel photonic structure and a technology allowing the electrical
control of the devices, a critical step for the scalability of a quantum
network based on semiconductor quantum dots.

Nov 04, 12:00 PM
60 St. George Street, MP 606 
Robin Côté,
University of Connecticut
Quantum information with ultracold Rydberg atoms and molecules

Oct. 4
Room 210

Dr. Nicolas Menicucci (The University of Sydney)
A gigantic, 10,000mode, continuousvariable cluster state
Cluster states are an entangled resource state that enable quantum
computing using adaptive measurements alone. This is surprising when
one considers what this means: one can quantum compute simply by *looking*
at a quantum systems in a particular way! The continuousvariable
incarnations of these states are simple to make using lasers and can
be scaled up with ease. In this talk, I will describe the theoretical
underpinnings of measurementbased quantum computation using continuousvariable
systems, and I will report on their experimental realization, including
the recent demonstration of a 10,000mode (!) cluster state. This
is the largest entangled state ever created to date in which every
constituent quantum system is individually addressable. Issues related
to error correction and fault tolerance  many of which remain open
problems  will also be discussed.



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