FIELDS INSTITUTE FOR RESEARCH IN MATHEMATICAL SCIENCES
Quantum Information Seminars
the Fields Institute, Fridays,
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.
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 non-linear 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 multi-particle
quantum walks for investigating areas from condensed matter science
to complexity theory.
Kurizki, Weizmann Institute of Science
Sara Hosseini, Australian National University
Experimental verification of quantum discord in continuous variable
states and operational significance of discord consumption
We introduce a simple and efficient technique to verify quantum discord
in unknown Gaussian states and certain class of non-Gaussian states.
We show that any separation in the peaks of the marginal distributions
of one subsystem conditioned on two different outcomes of homodyne
measurements performed on the other subsystem indicates correlation
between the corresponding quadratures and hence nonzero quantum discord.
We also demonstrate that under certain measurement constraints, discord
between bipartite systems can be consumed to encode information that
can only be accessed by coherent quantum interaction.
Savannah Garmon, Osaka Prefecture University
Bound states, scattering states and resonant states in PT-symmetric
open quantum systems
We study the point spectrum and transmission scattering spectrum
in extended optical lattice models incorporating balanced elements
of energy amplification and attenuation in a central scattering region.
These serve as prototype models to illustrate more general concepts
relevant to PT-symmetric physics in an open systems context. For a
given system geometry, we study two boundary conditions: purely outgoing
waves and scattering states. For the boundary condition consisting
of purely outgoing waves we obtain the discrete spectrum associated
with the scattering region. In this case we reveal that, unlike the
Hermitian case, PT-symmetric open quantum systems permit eigenstates
with complex-valued eigenvalues to appear in the first Riemann sheet
in the complex energy plane. We also demonstrate the presence of and
classify two different types of exceptional points appearing in the
discrete eigenvalue spectrum. We also demonstrate the presence of
what we term a resonance in continuum (RIC) for certain parameter
values. Finally, we consider the scattering wave boundary conditions,
under which we demonstrate that further imposing PT-symmetry on our
scattering state results in a perfect transition through the scattering
David McKay (James Franck Institute, University of Chicago)
Cavity QED with superconducting qubits - a multi-pole approach
Superconducting Josephson-junction qubits are an emerging technology
for quantum information processing. They offer a scalable, tunable,
and coherent platform to study quantum systems. These qubits can be
engineered with strong coupling to two or three-dimensional microwave
cavities which implements the cavity QED paradigm - coherent coupling
of a two-level system to a harmonic oscillator. Cavities enable high-fidelity
qubit readout and a common "bus" for qubit-qubit interactions.
In this talk, I will discuss our device at the University of Chicago
which couples two superconducting transmon-type qubits using a planar
multi-cavity (multi-pole) quantum filter. The multi-pole architecture
allows for high contrast two-qubit gates; on-resonance the qubit-qubit
interactions are strong, but off-resonance the interactions are exponentially
suppressed in the number of filter poles. I will outline our adiabatic
multi-pole (AMP) entangling gate protocol which we utilize to prepare
a Bell state with >90% fidelity in 100ns. The multi-pole architecture
is a promising approach towards scalable multi-qubit circuits, lattice
based quantum simulation, and photonic registers.
|May 16, 2014
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 speed-ups 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
May 9, 2014
Prof. Norbert Lütkenhaus (University of Waterloo)
Beating Classical Communication Resources by Quantum Communication
The area of quantum communication complexity searches for communication
tasks that can be solved more efficiently using quantum states as
carriers of information rather than their classical counterparts.
Some of these protocols show an exponential savings in communication
resources when operating in the quantum domain. Will it be possible
to realize this abstract advantage in practical quantum optical implementations?
The long-term goal would be to find protocols that convince a classical
optical communication engineer that using the quantum domain is preferable.
But there is no bonus point for saying 'quantum' ... we need to measure
success in terms of resources as counted by the classical optical
In this talk we will present a specific quantum communication protocol
that can be implemented using laser pulses and linear optics and which
beats classical communication. The measure is usage of Hilbert space
dimensions, which translates to a significant reduction in required
optical power levels and leakage of information. While this alone
might not convince a classical optical communication engineer (yet?),
we will show how our findings change the current view of communication
complexity as a purely theoretical field without practical impact.
|May 7, 2014
Prof. Sven Höfling (University of St Andrews)
Integrated Quantum Photonics
Quantum information processing is an emerging field which promises
secure communication or computational speed-ups for certain important
computational problems if they are tackled with quantum computers. This
has stimulated intense research on a variety of quantum bit (qubit)
carriers and quantum technological platforms. Single photons are a prime
qubit for the propagation and processing of quantum information, as
they can be transmitted over long distances with low loss and manipulated
by linear optical elements. However, the production, processing and
detection of single photons is still mostly realized using bulky free-space
or fiber-optic devices, posing severe challenges if more complex quantum
circuits with high functionality going beyond a few photonic qubits
are considered. Waveguide integrated quantum photonic circuits provide
a route to overcome such limitations , where we target in this work
the full integration of active and passive quantum devices on a single
For the development of a quantum integrated photonics platform on GaAs,
we develop a waveguide platform for the integration of single photon
sources based on InGaAs quantum dots (QDs), superconducting single photon
detectors, electro-optic tuners, directional couplers and splitters.
Single-photon sources coupled to waveguides are realized by embedding
QDs in photonic crystal cavities . In order to tune individual QDs
spectrally for indistinguishable photon emission, on-chip electrical
control is established. Waveguide single-photon detectors are demonstrated
by patterning superconducting NbN nanowires on top of ridge waveguides,
resulting in high efficiency and low jitter . They can also be arranged
to implement on-chip photon auto-correlation measurements  or photon
number resolved detection . The presented building blocks are a key
for integrated quantum photonics based quantum information processing
and results towards this goal and key features of this platform will
In conclusion, we have demonstrated the key building blocks of a scalable
quantum photonic integrated circuit. They are based on the same GaAs/AlGaAs
material basis and therefore can be integrated on the same chip. This
should open the way to solid-state quantum processing with several tens
 A. Politi, J. C. F. Matthews, and J. L. O'Brien, Science 325, 1221
 T. B. Hoang, J. Beetz, L. Midolo, M. Skacel, M. Lermer, M. Kamp,
S. Höfling, L. Balet, N. Chauvin, and A. Fiore, Appl. Phys. Lett.
100, 061122 (2012).
 J. P. Sprengers, A. Gaggero, D. Sahin, S. Jahanmirinejad, G. Frucci,
F. Mattioli, R. Leoni, J. Beetz, M. Lermer, M. Kamp, S. Höfling,
R. Sanjines, and A. Fiore, Appl. Phys. Lett. 99, 181110 (2011). 
D. Sahin, A. Gaggero, T. B. Hoang,1 G. Frucci, F. Mattioli, R. Leoni,
J. Beetz, M. Lermer, M. Kamp, S. Höfling, and A. Fiore, Opt. Express
21, 11162 (2013).
 D. Sahin, A. Gaggero, Z. Zhou, S. Jahanmirinejad, F. Mattioli, R.
Leoni, J. Beetz, M. Lermer, M. Kamp, S. Höfling, and A. Fiore,
Appl. Phys. Lett. 103, 111116 (2013).
|Dr. Raul Garcia (Max
Ultimate communication capacity of quantum optical channels
Optical channels, such as fibers or free-space links, are ubiquitous
in today's telecommunication networks. A complete physical model of
these channels must necessarily take quantum effects into account
in order to determine their ultimate performances. Specifically, Gaussian
bosonic quantum channels have been extensively studied over the past
decades given their importance for practical purposes. In spite of
this, a longstanding conjecture on the optimality of Gaussian encodings
has yet prevented finding their communication capacity. In this talk
we will present a recent result that solves this conjecture and establishes
the ultimate achievable bit rate under an energy constraint. We will
conclude discussing further implications of our result.
Room BA 1210, Bahen Centre, 40 St. George Street, Toronto
Prof. Marko Loncar, Harvard University
Quantum Nanophotonics and Nanomechanics with Diamond
Diamond possesses remarkable physical and chemical properties, and
in many ways is the ultimate engineering material - the engineers
best friend! For example, it has high mechanical hardness and
large Youngs modulus, and is one of the best thermal conductors.
Optically, diamond is transparent from the ultra-violet to infra-red,
has a high refractive index (n = 2.4), strong optical nonlinearity
and a wide variety of light-emitting defects. Finally, it is biocompatible
and chemically inert, suitable for operation in harsh environment.
These properties make diamond a highly desirable material for many
applications, including high-frequency micro- and nano-electromechanical
systems, nonlinear optics, magnetic and electric field sensing, biomedicine,
and oil discovery. One particularly exciting application of diamond
is in the field of
quantum information science and technology, which promises realization
of powerful quantum computers capable of tackling problems that cannot
be solved using classical approaches, as well as realization of secure
communication channels. At the heart of these applications are diamonds
luminescent defectscolor centersand the nitrogen-vacancy
(NV) color center in particular. This atomic system in the solid-state
possesses all the essential elements for quantum technology, including
storage, logic, and communication of quantum information.
I will review recent advances in nanotechnology that have enabled
fabrication of nanoscale optical devices and chip-scale systems in
diamond that can generate, manipulate, and store optical signals at
the single-photon level. Examples include a room temperature source
of single photons based on diamond nanowires 1 and plasmonic appertures
2, as well as single-photon generation and routing inside ring 3 and
photonic crystal resonators fabricated directly in diamond 4. In addition
to these quantum applications I will present our recent work on diamond
based on-chip frequency combs, as well as diamond
1. T.M. Babinec, B.M. Hausmann, M. Khan, Y. Zhang, J. Maze, P.R.
Hemmer, M. Lon?ar, "A bright single photon source based on a
nanowire," Nature Nanotechnology, 5, 195 (2010)
2. J.T. Choy, B.M. Hausmann, T.M. Babinec, I. Bulu, and M. Lon?ar,
"Enhanced Single Photon Emission by Diamond-Plasmon Nanostructures.,"
Nature Photonics, 5, 738 (2011)
3. B.J.M. Hausmann, et al, "Integrated Diamond Networks for
Nanophotonics", Nano Letters, 12, 1578 (2012)
4. M.J. Burek, et al, Free-standing mechanical and photonic
nanostructures in single-crystal diamond, Nano Letters, 12,
Dr. Ben Fortescue, (Southern Illinois University)
Tolerating Qubit Loss in Quantum Error Correction
Standard fault-tolerant 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. Fault-tolerance 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 fault-tolerance to be analysed
and achieved at much less cost in common Calderbank-Shor-Steane (CSS)
Angela Sestito, Università della Calabria, Italy
Understanding Quantum Mechanics: a formal analysis of non-locality
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 non-interacting
spin particles when the strict interpretation of (R) is adopted.
|Jianming Cai, Ulm
Nano-scale 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
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
Prof. Gerd Leuchs, Max Planck Institute for the Science of Light,
Distributing entanglement via separable states
Whether or not entanglement is necessary pre-requisite 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  vanishes discontinuously
as the state is gradually tuned towards more mixedness. Yu and Eberly
 found another example of a discontinuous disappearance of entanglement.
In a seminal paper Ollivier and Zurek  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].
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)
Prof. Guifre Vidal, Perimeter Institute
Tensor Networks for Quantum Many-body Systems
Quantum many-body systems are hard to study because the associated
Hilbert space, containing all possible many-body 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 many-body 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 many-body
systems; (ii) summarize our current understanding of many-body entanglement;
and (iii) give a gentle introduction to tensor networks as an efficient
description of many-body states.
Prof. Arjendu Pattanayak, Carleton College
Surprises in the quantum-classical transition for computed Lyapunov
Completely classical behavior is very different from completely quantum-mechanical
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 multi-parameter 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 quantum-classical 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
Dr. Pascale Senellart, CNRS
Cavity quantum electrodynamics with semiconductor quantum dots
A semiconductor quantum dot is a promising system to develop a solid-state
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 photon-single
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
We have developed an in-situ lithography technique to deterministically
insert a single quantum dot into a pillar optical microcavity. In the
light-matter 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 controlled-NOT
gate. In the light matter strong coupling regime, we demonstrate optical
non-linearities 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
Dr. Nicolas Menicucci (The University of Sydney)
A gigantic, 10,000-mode, continuous-variable 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 continuous-variable
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 measurement-based quantum computation using continuous-variable
systems, and I will report on their experimental realization, including
the recent demonstration of a 10,000-mode (!) 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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