
THE
FIELDS INSTITUTE FOR RESEARCH IN MATHEMATICAL SCIENCES 
Toronto
Quantum Information Seminars
201415
Fields
Institute, 222
College St.



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.
July 3, 2015
RM 210

Stephen Hughes (Queen's University)
How to "fix" Purcell's formula for leaky optical cavities
and plasmonic nanoresonators
Two of the most common and useful metrics for characterizing the
properties of optical cavities are the Q value (quality factor) and
the effective mode volume V. The Purcell effect is a beautiful example
of a situation in which a cavity with a large Q/V ratio enhances the
spontaneous emission rate of an atom or quantum dot. In Purcell's
original paper, a modest abstract published in the proceedings of
the American Physical Society meeting at Cambridge in 1946, Purcell
formulated the enhanced spontaneous emission factor in a very elegant
way as scaling with Q/V. It is no exaggeration to say that Purcell's
formula has been the workhorse for cavity physics for decades, but
it turns out to be wrong! At least it turns out to be wrong in general
with the way that the modes and effective mode volume are obtained
for open and dissipative resonators. In this talk, I will argue that
most, if not all, confusion about cavity modes can be removed by a
proper treatment within the framework of quasinormal modes (QNMs),
defined as the frequency domain solutions to the wave equation with
open boundary conditions. Using these QNMs, I will describe a newly
developed mode expansion technique that can be used to evaluate the
electric field from a dipole emitter at arbitrary positions outside
and within optical cavities and plasmonic resonators. I will then
introduce a rigorous definition of the Purcell factor and enhanced
spontaneous emission factor and point out why the usual expression
for effective mode volume is wrong. Several applications of the theory
for modelling hybrid plasmoniccoupled emitter systems will be exemplified,
including metaldimer single photon sources and plasmonmediated entanglement
between two quantum dots.

June 19, 2015
RM 210

Aharon Brodutch (Institute for Quantum Computing, University
of Waterloo)
"Should we care about quantum discord?"
Quantum bipartite systems can be correlated in various ways. For
pure states, the total correlation can be quantified using mutual
information while the entanglement entropy can be used to quantify
the 'quantum correlation'. For mixed states the situation is not as
clear cut. First, there is no unique measure of entanglement. Second,
there are some tasks involving bipartite separable state that have
no classical analogue. The 'quantum part' of the correlation can therefore
be quantified in different ways depending on the task, these can be
entanglement monotones or more general quantum correlation measures
that do not vanish for separable states, e.g. discord.
In this talk I will begin with a brief overview of the mathematical
properties of quantum discord and similar quantities. I will then
give some physical examples of where these quantities can be useful
and some intuition on where they are probably useless.

May 29, 2015
*ROOM 332Fields Institute*

Marc Dignam (Queen's University)
Nonlinear Dynamics of Photons in Lossy, Coupled Photonic Crystal
Cavities
There has been a great dealt of interest in recent years in developing
nanophotonic systems for use as singlephoton and entangled photon
sources. One promising class of such systems is semiconductor photonic
crystal slabs that contain line and/or point defects. In this talk
I will discuss the formalism that we have developed to model nonlinear
photon generation and propagation in coupleddefect cavities in photonic
crystal slabs. The key to our approach is the nonhermitian projection
of the photon dynamics onto a set of lossy and potentially nonorthogonal
cavity quasimodes. I will apply our approach to single quantum dots
coupled to multiple cavities and to photon pair generation due to
spontaneous fourwave mixing in coupled optical resonator waveguides.

May 1, 2015
RM 210

Paul Barclay (University of Calgary)
Diamond nanophotonic optomechanics: towards hybrid quantum systems
Nanophotonic optomechanical devices allow highly sensitive optical
coupling to nanomechanical resonances, providing opportunities for
probing their quantum properties, and for creating sensing and information
processing technologies. Owing its desirable optical and mechanical
properties, single crystal diamond is an attractive material for implementing
optomechanical devices. In addition, diamond hosts color centers whose
highly coherent electronic and nuclear spins are promising for quantum
information science. We have recently demonstrated a diamond chipbased
optomechanical system which allows sensitive readout of mechanical
resonances with ultrahigh mechanical quality factor. This waveguide
optomechanical device exhibits striking nonlinear nanomechanical behaviour,
can be optically cooled to mK temperatures, and may allow coupling
between nanomechanical resonances and single diamond spins. We have
also created high quality factor diamond optical microcavities, which
are promising devices for highfrequency optomechanics. This talk
will review this work, together with related optomechanics results
obtained in more conventional semiconductor materials such as silicon
and GaP.

April 24, 2015
RM 210

Richard Haglund (Vanderbilt University)
Is Vanadium Dioxide credible for ultrafast photonic switching?
Ever since picosecond optical switching of the semiconductortometal
transition in vanadium dioxide (VO2) was discovered two decades ago,
there has been growing interest in the potential for photonic applications
of this deceptively simple oxide. But until recently, it appeared
that the bottleneck in switching times for the rutiletomonoclinic
structural transition  of order hundreds of picoseconds at the very
least  counseled a skeptical, if suspended, judgment. Recent demonstrations
of a shortlived metallic monoclinic state in VO2 suggest that the
potential indeed real, provided that highquality VO2 thin films can
integrated in silico. After motivating the idea of alloptical photonic
switching in silicon from recent results, I will describe direct and
indirect evidence for the monoclinic metallic state, and then consider
the materials issues that will have to be solved to realize hybrid
siliconVO2 photonics in practice.

April 10, 2015 POSTPONED
RM 210

Girish Agarwal (Oklahoma State University)
Dicke Superradiance, Entanglement and Quantum Interference
As the superposition principle is basic to quantum mechanics, the
interference effects occur very widely in quantum systems. Interferences
are especially important in quantum optics and one of the most remarkable
developments has been the possibility of interference of independent,
but indistinguishable particles. These interferences, besides being
very fundamental, have important applications in the context of quantum
entanglement and superradiance. An important component of superradiance
is the initial preparation of the system in Dicke states which can
be shown to be multiparticle entangled states. Thus the production
of Dicke states with higher number of excitations remains a challenge
and a new option is the repeated measurements of photons starting
from a fully excited system of quantum emitters. These ideas are quite
generic and applicable to a wide variety of quantum sources. Our studies
suggest new kind of enhancement in the efficiency of nonlinear optical
processes in systems prepared in entangled states.

March 20, 2015
11:00am
RM 210

Joel Yuen (Massachusetts Institute of Technology)
Spectroscopy and topological phases for organic excitons
The understanding and control of energy flow at the nanoscale via
exciton dynamics in organic materials is of fundamental chemical and
physical interest, but is also technologically relevant for the design
of novel photovoltaic materials. In the first part of my talk, I will
explain some of our work designing spectroscopic protocols to understand
exciton dynamics under coherent illumination via ultrafast Quantum
Process Tomography (QPT), a technique which retrieves the time evolution
of the quantum state of the excitons via nonlinear spectroscopy (1,2).
As an application, I will describe the first ultrafast QPT experiment
carried out with the Nelson and Bawendi groups at MIT on a nanotubular
Jaggregate system at room temperature.
In the second part, I will describe current work (3) designing topologically
nontrivial phases that robustly and selectively move excitons in particular
spatial directions of a molecular crystal, simulating solid state
"topologically protected" phenomena like the Quantum Hall
Effect, which are robust against material imperfections and static
disorder.
(1) J. YuenZhou, Jacob J. Krich, Masoud Mohseni, and A. AspuruGuzik,
Quantum state and process tomography of energy transfer systems via
ultrafast spectroscopy, Proc. Nat. Acad. Sci. USA. 108, 43, 17615
(2011).
(2) J. YuenZhou, D. Arias, D. Eisele, J. J. Krich, C. Steiner, K.
A. Nelson, and A. Aspuru. Guzik, Coherent exciton dynamics in supramolecular
lightharvesting nanotubes revealed by ultrafast quantum process tomography,
ACS Nano 8 (6) 5527 (2014).
(3) J. YuenZhou, S. Saikin, N. Yao, and A. AspuruGuzik, Topologically
protected excitons in porphyrin thin films, in press, Nature Materials
13, 1026 (2014).
Host: Paul Brumer (CQIQC) pbrumer@chem.utoronto.ca

Feb 25, 2015
12:00pm
RM 210

Guoxing Miao (University of Waterloo)
Spin Manipulation through Tunable Magnetic Semiconductors
Magnetic semiconductor materials are best known for their spinfiltering
properties which can effectively create highly polarized spin currents
from nonmagnetic electrodes. One can readily generate and analyze
spin information through quantum tunneling across these materials.
Magnetic semiconductors have another less explored property: interfacial
exchange fields onto neighboring electronic systems. Through indirect
exchange interaction between the localized magnetic moments and the
adjacent free electrons, the electron spins experience an effective
Zeeman field on the order of tens of Tesla. This effect is especially
pronounced on lowdimensional systems such as 2DEG and topological
materials. We first probe its strength on a cluster of Al nano dots
under Coulomb confinement, then proceed to show a number of material
and device concepts for spintronic and quantum information applications
originating from such spin filtering and interfacial exchange effects.

Feb 13, 2015
11:00am
RM 210

Vlad Pribiag (University of Minnesota)
Superconducting EdgeMode Transport in InAs/GaSb Double Quantum Wells
Topological insulators are characterized by boundary modes with very
strong spinmomentum coupling. In proximity to a conventional superconductor,
these modes are predicted to host topological superconductivity, an
exotic state of matter that supports Majorana zeromodes [1]. Localized
Majorana modes obey nonAbelian exchange statistics, making them interesting
building blocks for topological quantum computing. In this talk, I
will describe our current efforts to realize topological superconductivity
in nanostructures based on InAs/GaSb quantum wells, a twodimensional
topological insulator (2D TI). By electrostaticallygating the devices
we observe superconducting transport in all three regimes of the 2D
TI: bulk electrons, edge modes and bulk holes. We use superconducting
quantum interference measurements as a sensitive method for determining
the spatial distribution of the supercurrent in each regime. These
measurements reveal a clear transition from bulk to edgedominated
supercurrent under conditions of high bulk resistivity, which we associate
with the 2D topological phase. These experiments establish InAs/GaSb
as a promising platform for observing Majoranas modes and probing
their exchange statistics. [1] /J. Alicea,//Rep. Prog. Phys./ *75*
076501 (2012). [2] /V.S. Pribiag et al/., /arXiv/:1408.1701.

Jan 30, 2015
11:10am
RM 210

Edward Taylor (University of Toronto)
Decoherence immunity using Majorana fermions: state of play and possible
challenges
The central challenge to harnessing the power of quantum entanglement
to do useful tasks is figuring out how to maintain coherences for
long periods of time. As realized by Kitaev a little over a decade
ago, one way to do this is to encode quantum information in the wavefunction
of particles—socalled nonAbelian anyons—which obey neither
Bose nor Fermi statistics. Since then, there has been an enormous
amount of activity devoted to trying to find such particles in the
form of Majorana surface states (quasiparticles) of topological superconductors.
In this talk, I will review the theoretical and experimental state
of play before raising questions about the current theory orthodoxy,
which is based on the meanfield BCS theory of superconductivity and
excludes fluctuations—Goldstone modes—which can interact
with the Majoranas and modify their properties.

Jan 9, 2015
11:00am
RM 210

Ole Steuernagel (STRI, University of Hertfordshire)
Wigner flow reveals nonclassical features in quantum phase space
The behaviour of classical mechanical systems is characterised by
their phase portraits, the collections of their trajectories. Heisenberg's
uncertainty principle precludes the existence of sharply defined trajectories,
which is why traditionally only the time evolution of wave functions
is studied in quantum dynamics. These studies are quite insensitive
to the underlying structure of quantum phase space dynamics. We identify
the flow that is the quantum analog of classical particle flow along
phase portrait lines. It reveals hidden features of quantum dynamics
and extra complexity. Being constrained by conserved flow winding
numbers, it also introduces topological order into quantum dynamics.

Dec 5, 2014
11:00am
RM 210

Hamed Majedi (University of Waterloo)
Photon Detection and Generation by Superconductor and Semiconductor
Nanostructures
The generation, manipulation, control and detection of quantum states
of lights such as single and entangled photons are at the heart of
quantum photonics. The integration and combination of single photon
sources, passive optical circuits and single photon detectors are
key enabling technology for two main reasons; first, it provides a
feasible route toward scalable quantum photonic processors that are
genuinely useful in practical applications and form "quantumopticslabonachip"
and second, it enables building more complicated devices such as quantum
amplifiers, repeaters and transceivers that are necessary for some
applications such as quantum communication networking.
After a brief introduction to various technologies for single photon
detectors and sources, I will focus on our research work on two keyelements
of integrated quantum photonics, namely Superconducting Nanowire Single
Photon Detector (SNSPD) and IIIV NanoWire Quantum Dot (NWQD) single
and entangled photon sources.
In SNSPD part, I will focus on quantum tomographic characterization
of SNSPD and introduce our original contribution on how gated SNSPD
increase the detection speed by an order of magnitude. In NWQD part,
I will present the results for the first demonstration of polarizationentangled
photon generation from a single InAsP quantum dot embedded in an InP
nanowire in collaboration with Philip Poole's group at NRC and Gregor
Weihs's group at University of Innsbruck. At the end, I will introduce
the challenges and our ongoing attempts to integrate these two devices
on a single chip.

Nov 21, 2014
11:00am
RM 210

Yaoyun Shi (University of Michigan)
How to generate the first secret, then as many as you like
Secrecy is randomness. A perfect secret is one for which all the
alternatives are equally likely to the adversary. For secrecy to be
possible, we have to assume that the world is not deterministic. Here
we show how this necessary assumption, together with the validity
of quantum mechanics and relativity, will allow us to generate the
first and almost perfect secret, and then to expand it to be arbitrarily
long. Unlike all existing solutions, the security of our construction
is provable, unconditional (as opposed to computational), and verifiable.
Our method can also be used for the important task of distributing
cryptographic keys.
Technically speaking, we formulate a precise model of extracting
randomness from quantum devices whose innerworkings may be imperfect
or even malicious. We then construct such a "physical extractor"
that needs only a single and arbitrarily weak classical source, and
the output randomness can be arbitrarily long and almost optimally
close to uniform. This is impossible to achieve for classical randomness
extractors, which cannot increase entropy and requires two or more
*independent* sources.
Our construction also differs from quantumbased random number generators
in the market, as they all require that the users trust their quantum
innerworkings. Such a trust threatens security when the devices are
defective or were procured from an untrusted vendor. Several features
of our construction, such as maximum noise tolerance and unit quantum
memory requirement, have fundamentally lowered the implementation
requirements.

Nov 14, 2014
11:00am
RM 210

Nancy Makri (University of Illinois)
QuantumClassical Path Integral
The path integral formulation of timedependent quantum mechanics
provides the ideal framework for rigorous quantumclassical or quantumsemiclassical
treatments, as the spatially localized, trajectorylike nature of
the quantum paths circumvents the need for meanfieldtype assumptions.
However, the number of system paths grows exponentially with the number
of propagation steps. In addition, each path of the quantum system
generally gives rise to a distinct classical solvent trajectory. This
exponential proliferation of trajectories with propagation time is
the quantumclassical manifestation of time nonlocality, familiar
from influence functional approaches.
A quantumclassical path integral (QCPI) methodology has been developed.
The starting point is the identification of two components in the
effects induced on a quantum system by a polyatomic environment. The
first, “classical decoherence mechanism” dominates completely
at high temperature/lowfrequency solvents and/or when the systemenvironment
interaction is weak. Within the QCPI framework, the memory associated
with classical decoherence is removable. A second, nonlocal in time,
“quantum decoherence process” is also operative at low temperatures,
although the contribution of the classical decoherence mechanism continues
to play the most prominent role. The classical decoherence is analogous
to the treatment of light absorption via an oscillating dipole, while
quantum decoherence is primarily associated with spontaneous emission,
whose description requires quantization of the radiation field. The
QCPI methodology takes advantage of the memoryfree nature of systemindependent
solvent trajectories to account for all classical decoherence effects
on the dynamics of the quantum system in an inexpensive fashion. Inclusion
of the residual quantum decoherence is accomplished via phase factors
in the path integral expression, which is amenable to large time steps
and iterative decompositions. Preliminary tests on dissipative twolevel
systems and fully atomistic simulations of charge transfer in solution
suggest that the QCPI methodology is realistically applicable to many
processes of chemical and biological interest.

Oct 17, 2014
11:00 a.m.
RM 210 
Josh Combes (Perimeter Institute for Theoretical Physics)
Perimeter Institute and Institute for Quantum Computing
In 1988 Yakir Aharonov, David Albert, and Lev Vaidman wrote a paper
provocatively titled "How the result of a measurement of a component
of the spin of a spin1/2 particle can turn out to be 100". In
this paper they defined a quantity, similar to the expectation value
of an operator, called the "weak value" of an operator.
The weak value of an operator has many weird properties which has
lead some researchers to: (1) think that quantum paradoxes are solved
by this defined quantity, and (2) suggest that the weak value can
be used to perform sensitive measurements. In this talk I will address
both points. First, I argue that the phenomenon of anomalous weak
values is not limited to quantum theory. In particular, I show that
the same features occur in a simple model of a coin subject to a form
of classical backaction with pre and postselection. Second, I will
explain how rigorous estimation and detection theory imply that weak
values do not aid quantum metrology. This is joint work with Chris
Ferrie of the University of New Mexico.

Oct 10, 2014
11:00 a.m.
RM 210 
Paola Cappellaro (Massachusetts Institute of Technology)
Quantum control strategies for imaging and spectroscopy
Quantum control techniques have proven effective to extend the coherence
of qubit sensors, thus allowing quantumenhanced sensitivity at the
nanoscale. The key challenge is to decouple the qubit sensors from
undesired sources of noise, while preserving the interaction with
the system or field that one wishes to measure. In addition, tailoring
the sensor dynamics can help reveal temporal and spatial information
about the target.
In this talk I will show how we can use coherent control of quantum
sensors to reconstruct the arbitrary profile of timevarying fields,
while correcting the effects of unwanted noise sources. These control
techniques can be further used to reveal information about classical
and quantum noise sources. For example, they can achieve high frequency
resolution, thus allowing precise spectroscopy and imaging of the
spatial configuration of a spin bath.
I will illustrate applications of these strategies in experimental
implementations based on the NitrogenVacancy center in diamond.

Oct 3, 2014
11:00 a.m.
RM 210 
ManDuen Choi (University of Toronto)
The Principle of Locality made simple
In physics, the Principle of Locality states that an object is influenced
directly only by its immediate surroundings. This could be transformed
to a simple mathematical statement of NO wisdom at all. Nevertheless,
with extravagent assumption (on the obvious truth) and fascinating
explanation (of the ultimate nonsense), the Principle may become a
big Law/Theory/Theorem or a tremendous Paradox to shake your heart/body.
This is an expository talk of my own adventure in the quantum wonderland
(concerning the structure problems of direct sums and tensor products).
No working knowledge of quantum information is required in this talk.

Sept 19, 2014
12:30 p.m
Fields RM 210

Boris Braverman (Massachusetts Institute of Technology)
Progress toward a spin squeezed optical atomic clock beyond the standard
quantum limit
State of the art optical lattice atomic clocks have reached
a relative inaccuracy level of $10^{18}$, making them the most stable
time references in existence. One limitation to the precision of these
clocks is the projection noise caused by the measurement of the atomic
state. This limit, known as the standard quantum limit (SQL), can be
overcome by entangling the atoms. By performing spin squeezing, it is
possible to robustly generate such entanglement and therefore surpass
the SQL of precision in optical atomic clocks. I will report on recent
experimental progress toward realizing spin squeezing in an ${}^{171}$Yb
optical lattice clock. A highfinesse micromirrorbased optical cavity
mediates the atomatom interaction necessary for generating the entanglement.
By exceeding the SQL in this state of the art system, we are aiming
to advance precision time metrology and expand the boundaries of quantum
control and measurement.

Sept 12, 2014
11:00 a.m.
RM 210 
Raphael Pooser (Oak Ridge National Labs)
Quantum Sensors: Data at the information frontier of physics
Quantum information processing has a host of applications, including
quantum key distribution and quantum computing as some of the most
prominent. In all of these applications, sensing and control are needed
in order to maintain the fidelity of quantum information. In quantum
sensors, information stored in quantum mechanical systems is extracted
and put to use, either in subsequent control signals, or in general
information processing applications. Some famous examples of quantum
sensors include atomic clocks, cold atom interferometers, or BoseEinstein
condensates used in gravitometers, accelerometers, etc. Some of the
original proposals for quantum sensors involved optical fields. In
particular, sensors that exploit continuously variable degrees of
freedom have been of interest since the discovery of quantum noise
reduction. One of the first examples proposed by Caves is the use
quantum noise reduction to achieve interferometric sensitivity in
the quantum regime. Advanced LIGO is an example of an upcoming application.
In addition to LIGO, in recent years continuous variables have seen
renewed interest. In this talk we will discuss quantum sensors and
their applications with a focus on the sensors developed at ORNL.
We use quantum noise reduction to produce subshotnoise limited sensing
devices, particularly in quantum plasmonic sensors and displacement
sensors using MEMS cantilevers. Some applications for these devices
include trace detection or quantum information applications, such
as removing bias from QRNGs through adaptive control. We will also
discuss other sensing types that use both discrete and continuous
variables, such as quantum compressive imaging, and single photon
detection applications.

Aug 29, 2014
11:00 a.m. RM 210 
Robert Boyd (University of Ottawa)
Menzel's Experiment: Violation of Complementarity?
In 2012, the group of Ralf Menzel in Potsdam, Germany published an
article in PNAS that appeared to violate the accepted quantum mechanical
notion of complementarity. Specifically, they observed interference
with good fringe visibility in a Young's twoslit experiment, even
though, through use of a quantum protocol, they were able to deduce
through which slit each photon had passed. Our group has recently
articulated an explanation for these unexpected results (Bolduc et
al., PNAS 2014). Our explanation is that the Potsdam group had inadvertently
violated a fairsampling assumption by means of the manner in which
they collected and analyzed their data.

Aug 8, 2014
11:00 a.m. 
Ioannis Thanapoulos (National Hellenic Research Foundation)
Quantum dynamics by the Effective Modes Differential Equations method
We show that the nonMarkovian quantum dynamics of a system comprised
of a subspace Q coupled to a much larger subspace P can be described
by a set of Effective Modes Differential Equations (EMDE). The computational
efficiency of the method is demonstrated by investigating the 24mode
decay dynamics and laser control of the radiationless transitions
from the second to the first singlet electronic excited state of the
pyrazine molecule.

Aug 7, 2014
11:00 a.m. 
Thomas Monz (University of Innsbruck)
Topological qubits
Arbitrarily long quantum computation requires techniques to overcome
errors accumulated during the operation. Here, different approaches
have been proposed, with topological quantum computation yielding
one of the highest thresholds against errors. In this talk I will
first provide a brief introduction into topological quantum computation,
in particular the color code. Subsequently I will show how, for the
first time, a qubit has been topologically encoded using an iontrap
based quantum computer. The presented experimental data illustrates
how we can detect all physical singlequbit errors, perform the entire
set of Clifford operations on this logical qubit and investigate its
coherence properties. The presentation is concluded by an outline
on upcoming milestones and their experimental as well as theoretical
challenges.

Aug 7, 2014
2:00 p.m. 
Prof. Charlie Ironside (Curtin University)
A surfacepatterned chip as a strong source of ultracold atoms for
quantum technologies

Aug 1, 2014
11:00 am

Prof. Lianao Wu (University of Basque Country)
One Component Dynamical Equation and a Universal Control Theory
We use a Feshbach PQ partitioning technique to derive a closed one
component integrodifferential equation. The resultant equation properly
traces the footprint of the target state in quantum control theory.
The physical significance of the derived dynamical equation is illustrated
by both general analysis and concrete examples. We show that control
can be realized by fastchanging external fields, even fast noises.
We illustrate the results by quantum memory and controlled adiabatic
paths.

July 25, 2014
Room 210 
Prof. Gershon Kurizki, Weizmann Institute of Science
A thermal bath: more friend than foe?
Traditionally, the interaction of quantum systems with a thermal
bath is viewed as detrimental to their quantumness. Yet this is not
always the case, as the bath may actually promote quantumness, particularly
when systembath interactions are subject to control. I will review
our recent results concerning different types of control capable of
generating or enhancing quantum processes via the bath:
1. Control by modulation: By periodically modulating the energy of
twolevel or multilevel systems we may purify the state of the systems
or the bath they couple to, upon tailoring the modulation to the bath
spectrum. An intriguing consequence of such purification is the possibility
to cool a bath consisting of coupled spins down to absolute zero,
in apparent violation of Nernst's third law of thermodynamics. The
thermal bath may also mediate the transfer of quantum information
between distant systems, at a rate and fidelity controllable by the
modulation.
2. Control by state preparation: The quantum state of an oscillator
coupled to a thermalized qubit determines the amount and efficiency
of work extractable from the thermal bath, thereby retaining its quantum
features over surprisingly long time scales. Remarkably, certain quantum
states yield higher efficiency than allowed by the Carnot bound, yet
in full compliance with the second law of thermodynamics. In Nlevel
systems, appropriate state preparation allows for Nfold enhancement
of work extractable from the bath at steady state.
3. Control by bath engineering: The ability to control the coupling
of quantum systems to appropriately designed, axiallyguided modes
of the bath, may drastically enhance the range of entanglement mediated
by the bath, or lead to giant enhancement of bathinduced dispersion
forces, colloquially known as van der Waals and Casimir forces.

July 11, 2014
Stewart Library

Matthew Broome, University of New South Wales
My Quantum Optics Show and Tell: Topology, complexity and biology
Progress in optical quantum computation has started to slow in recent
times due to the problems associated with probabilistic quantum gates,
lack of good single photon sources and poor nonlinear optical materials.
However, by looking at other applications besides a fully scalable
quantum computer, we see that linear optics alone (beam splitters
and phase shifters) is a powerful tool for simulation or emulation
of interesting physical systems. In this talk I will discuss some
recent results from the University of Queensland's Quantum Technology
Lab that employ purely linear optical schemes for this purpose. In
particular, I will focus the talk around single and multiparticle
quantum walks for investigating areas from condensed matter science
to complexity theory.

July 4, 2014
Room 210 
Katja Ried, Perimeter Institute, Waterloo
How drug trials are simpler if your subjects are quantum (and other
applications of quantum causal models)
A fundamental question in trying to understand the world  be it
classical or quantum  is why things happen. We seek a causal account
of events, and merely noting correlations between them does not provide
a satisfactory answer. In classical statistics, a better alternative
exists: the framework of causal models has proven useful for studying
causal relations in a range of disciplines. We try to adapt this formalism
to allow for quantum variables, and in the process discover a new
perspective on how causality is different in the quantum world. One
of the peculiarities that arise in this context can be harnessed to
solve a task of causal inference  inferring the causal relation
between variables based on observed statistics  that is impossible
for classical variables. I will report on a recent experimental realization
of this scheme.
Time permitting, I will also discuss a more realistic approach to
the problem of characterizing quantum processes in the presence of
initial
correlations with an environment, viz nonMarkovian dynamics. Another
application of quantum causal inference arises in the context of quantum
field theory: if one couples two detectors to a quantum field at different
points throughout spacetime, this may allow one of them to causally
influence the other, via the field. We explore how different variables
of the model, such as the acceleration of the detectors and the ultraviolet
cutoff of the field theory, are reflected in the strength and quality
of the causal influence.

Back to top

