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CQIQC/Toronto Quantum Information Seminars
QUINF 2010-11
held at the Fields Institute
The CQIQC/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.
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Talks are held Fridays at 10 am unless otherwise indicated (Starting
in September start time is 11 am)
| UPCOMING
TALKS |
Friday 10-June-2011
10:10 a.m.
Stewart Library |
Thomas
Jennewein, University of Waterloo
High transmission-loss and classical-quantum multiplexing
QKD enabled with short wavelength photons
We will present recent demonstrations of QKD at short optical
wavelengths (532nm, 810 nm), and how it offers unique applications
such as operation with ultra-high channel losses of 60dB, or
entanglement based QKD within existing - and fully active -
IT networks.
Short-wavelength QKD has been around ever since experiments
have been performed, and is therefore widely studied. One
important benefit for systems operating at that wavelength
is the use Si-avalanche-photo-diodes for the photon detection.
These offer high detection efficiency, very low dark counts
and free-running operation.
The visible wavelength range is ideally suited for free-space
applications, because the transmission through atmosphere
is very good and the beam diffraction is still small. The
use of such systems is to exchange quantum keys or entangled
photons between mobile sites, and in the future even satellites.
We implemented a system using the best available Si-detectors,
an advanced and high-precision timing analysis system, and
an ultrafast and modulated faint-laser source to achieve operation
at ultra-high losses of 60dB. Our system shows a viable approach
for operating under difficult situations such as the uplink
of photons to a satellite, as well high-background, and these
possible applications will be briefly reviewed.
In terms of fiber optic based transmissions, the current
systems usually favor telecom wavelength over the short wavelengths,
because existing IT infrastructure is single mode for telecom
signals. This comes at the cost of needing sophisticated,
and noisier photon detectors based on InGaAs. However, it
has been established that also short wavelengths can be utilized
over telecom optical fibers. The use of spatial and temporal
filters must be implemented to suppress higher order modes
in the optical fibers. We will show our recent experimental
demonstration of entanglement based QKD, where the photon
pairs created from a simple continuous wave operated entangled
photon source can be distributed symmetrically over telecom
optical fiber, and generated high quality secure keys. In
a further experiment we show that this system directly allows
the parallel transmission of QKD and classical telecom signals
over the same optical fibers.
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| PAST TALKS
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Friday 03-June-2011
10:10 a.m.
Stewart Library |
Claude
Fabre, University Pierre et Marie Curie-Paris 6 and Ecole
Normale Superieure
Quantum frequency combs : generation and use
Frequency combs, made of the coherent superposition of many
longitudinal modes, are good examples of highly multimode quantum
objects, that can be of interest in massively parallel quantum
information processing and quantum metrology. We have shown
theoretically that frequency combs generated by Synchronously
Pumped Parametric Oscillators (SPOPO) exhibit nonclassical features
such as multimode squeezing and multipartite entanglement, that
can be tailored at will by adjusting the shape of the pump pulses.
I will report on the first achievements of our SPOPO experiment
and show how these "quantum frequency combs" can be
used in the future to improve time measurements beyond the shot
noise limit.
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Friday 13-May-2011
2:00 PM to 3:00 PM,
Stewart Library
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Evgeny
Shapiro, University of British Columbia
Coherent nonlinear spectroscopy with noisy
broadband laser pulses
As a laser pulse is applied to an opaque scattering sample -
such as biological tissue, paint, suspension, or plastic - its
structure breaks down. In space, a coherent beam breaks into
a multitude of speckles. In the spectral domain, the pulse is
strongly modified due to the random transmission of the sample.
Both effects are deleterious for one's ability to use coherent
techniques for the spectral analysis of the sample.
I will review our ongoing work aimed at implementing nonlinear
spectroscopy with coherent broadband laser pulses that have
passed through opaque samples. Our goal is to use the quasi-random
spectrum of light for extracting spectral information [1,2].
At the same time, we use two-dimensional spatial light modulators
to correct for the spatial and temporal distortions due to
the multiple scattering in opaque samples.
[1] E.A. Shapiro, S.O. Konorov, V. Milner, "Interference
spectroscopy with coherent anti-Stokes Raman scattering of
noisy broadband pulses", arXiv: 1104.1164.
[2] X.G. Xu, S.O. Konorov, J.W. Hepburn, V. Milner, "Noise
autocorrelation spectroscopy with coherent Raman scattering",
Nature Physics 4, 125 (2008).
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Wednesday,
Apr. 27
Stewart Library
Fields Institute
10am |
Gregor
Weihs (University of Innsbruck)
Quantum Optics With Exciton-Polaritons in Semiconductor Microcavities
Since their discovery in 1992 by Weisbuch and others, exciton-polaritons
have opened the world of semiconductor quantum optics. An exciton-polariton
is a quasiparticle formed as a superposition of a bound electron-hole
state (exciton) and a photon. Owing to the light effective mass
of a microcavity photon, exciton-polaritons exhibit peculiar
dispersion characteristics that have enabled a variety of applications.
Exciton-polaritons can interact via their exciton component
and they have shown parametric scattering, polariton lasing,
Bose condensation at temperatures of a few Kelvin and many of
the effect associated with superfluid behavior.
In our group we study the use of polariton parametric scattering
for the generation of entangled photon pairs. To this end
we have constructed a versatile experiment based on spatial
light modulators, so that we can explore various momentum-conservation,
i.e. phase-matching schemes. I will present results on the
parametric scattering of polaritons and ideas on how to suppress
bacground scattering mechanisms for generating clean entanglement.
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Friday,
Apr. 15
Stewart Library
Fields Institute
10am |
Robert
Hadfield, Heriot-Watt University, Edinburgh, UK
Quantum photonics with superconducting single-photon detectors
Advanced optical quantum information processing applications
place stringent demands on the performance of key components
such as single-photon detectors [1]. A new class of single-photon
detectors based on superconducting nanowires has recently emerged,
offering telecom-wavelength sensitivity, combined with low dark
counts, short recovery times and low timing jitter. I will describe
the basic operating principle of this type of device, the current
state-of-the-art and prospects for improvements. I will also
discuss implementations of these devices in quantum information
processing applications such as quantum key distribution [2],
characterization of quantum emitters and operation of quantum
waveguide circuits [3].
[1] Hadfield R. H. Nature Photonics 3 696 (2009)
[2] Takesue H. et al Nature Photonics 1 343 (2007)
[3] Natarajan C. M. et al Applied Physics Letters 96 211101
(2010)
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Friday,
Apr. 8
Stewart Library
Fields Institute
10am |
Moshe
Shapiro (University of British Columbia and Weizmann Institute
of Science)
Coherent Control of Entanglement and Spin Alignment
Coherent Control using two (H and V) perpendicular polarizations
is shown to give rise to a direct way of writing arbitrary "cluster
states" of entangled atoms or atom-ion pairs. The method
relies on our proven ability to control the directionality of
motion of (valence) electrons in dissociation processes, giving
rise to either the "forward" |f> or "backward"
|b> products. By irradiating a one dimensional array of molecules
(Na2) or molecular-ions (Ca2+) in an optical lattice with a
combination of two perpendicular (H and V) polarizations and
another source of light at half their frequency, one can entangle
light with matter AND craft clusters of entangled states composed
of arbitrary superpositions of sequences of atom (or atom-ion)
pairs, such as |H>|f>|b>...+|V>|b>|f>...
We then present a similar method to create a beam of spin
aligned atoms (Iodine or Na) or molecules.
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Friday,
Apr. 1
Stewart Library
Fields Institute
10am |
David Tannor (Weizmann Institute of Science)
Designing Laser Pulses to Control Photochemical Reactions:
Two New Pieces of the Puzzle
Since the invention of the laser, chemists have dreamed of
designing specially tailored laser pulses to control the outcome
of chemical reactions. This talk will start with a brief overview
of the field, introducing the theoretical concepts and the
current experimental status. Then two recent developments
in our group will be described. The first is a method based
for complete reconstruction of the excited state wave packet.
Significantly, the method applies to polyatomics as well as
diatomics, and has the potential to provide systematic mapping
of excited state potentials. This information is critical
to the ab initio design of pulses to control chemical reactions.
The second development is the use of the von Neumann time-frequency
representation to represent phase and amplitude shaped, ultrashort
laser pulses. The von Neumann basis provides an efficient
and intuitive representation that we show has significant
practical advantages for distilling pulse mechanisms, mapping
quantum control landscapes and for designing efficient closed-loop
laser control based on mechanistic building blocks. The talk
will conclude with some speculation about the prognosis for
laser control of chemical bond-breaking in the next 5-10 years.
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Monday,
Mar.14
BA6183, 40 St. George Street
4:10pm |
Man-Duen Choi (Mathematics Department, University
of Toronto)
Special Seminar in Quantum Information - What are quantum
channels through my old dream?
In 1970, I started off my adventure in the mathematical wonderland
of completely positive linear maps (alias, quantum channels).
Now, I am awaken to the new era of quantum computers, with
all sorts of information processes in the setting of non-commutative
analysis. As the time runs backwards in an alternating world
through the looking glass, I have to come back to the same
old scene to release myself from quantum entanglements. In
particular, I shall give a MODERN report on my 1975 paper
(Completely positive linear maps on complex matrices, Linear
Algebra Appl. 10 (1975), pp. 285-290) which has been cited
in more than 500 recent research articles (as shown in Google
Scholars of March 2011).
This is an expository talk in the simple language of mathematics;
no background knowledge of information theory will be assumed
for this talk.
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Friday,
Feb. 18
Stewart Library
Fields Institute
10am |
Darin
Ulness (Concordia)
Noisy Light Spectroscopy: Putting Noise to Good Use
Noisy light spectroscopy is an alternative approach, distinct
from short-pulse and continuous wave methodologies, for probing
ultra-fast dynamics via nonlinear optical processes. Although
not a mainstream technique, noisy light does offer a new perspective
on interesting physical phenomena. This talk will provide a
overview of both the theoretical and experimental aspects of
noisy light spectroscopy. Applications of noisy light to hydrogen
and halogen bonding will be presented as well.
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Tuesday,
Feb. 1
Stewart Library
Fields Institute
2pm |
Vadim
Makarov (Norwegian University of Science and Technology)
Cracking commercial quantum cryptography
Quantum cryptography, unbreakable in principle, can currently
be hacked through implementation loopholes. I present a loophole
recently explored by researchers from the Norwegian University
of Science and Technology (Trondheim, Norway) and Max Planck
Institute for the Science of Light (Erlangen, Germany).
Most of today's quantum cryptography systems use single-photon
detectors based on avalanche photodiodes. These detectors
operate part-time in the linear regime, in which they respond
deterministically to a short bright-light pulse producing
a click when the pulse peak power exceeds a certain threshold.
Furthermore, these detectors can be blinded to single photons
by bright-light illumination, through several different mechanisms
connected to detector electronic and thermal properties. We
show how this killer superposition of loopholes can be used
to launch a perfect attack against a quantum key distribution
system, eavesdropping the complete secret key without alerting
the legitimate users. We have shown experimentally that this
vulnerability is fully present in commercial quantum cryptosystems,
Clavis2 from ID Quantique and QPN 5505 from MagiQ Technologies.
We propose how to build a plug-and-play eavesdropper for both
cryptosystems, using off-the-shelf components. In a separate
experiment on an entanglement-based research cryptosystem,
we have built a full eavesdropper and actually demonstrated
100% eavesdropping of the 'secret' key. This class of loopholes
should be patchable, but how to do it in practice remains
an
open question.
The talk will include an equipment demonstration of full
detector control by an eavesdropper.
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Friday,
Jan. 21
Stewart Library
Fields Institute
10am |
Guillaume
Gervais (McGill)
Non-Abelian quantum statistics and the Moore-Read Pfaffian
state
In 1937, italian physicist Ettore Marojana modified the Dirac
equation so that it would admit only real solutions (as opposed
to complex-valued solutions). Such solutions are known as Majorana
fermions, a class of particles that are interestingly their
own anti-particles. Recently, Majorana fermions have migrated
into condensed matter physics, as they have been predicted to
occur as elementary excitations of systems containing many interacting
fermions. In particular, they are predicted to exist in chiral
p-wave superconductors, in superfluid 3He, and for the so-called
Moore-Read Pfaffian state thought to be realized for some fractional
quantum Hall states. The interest for the Majoranafermions is
largely driven by that they obey to non-abelian braiding quantum
statistics, a necessary property for the construction of a topological
quantum computer that would, in principle, be immune to local
perturbations.
In this talk, I will briefly review the recent progress in
the field of non-abelian quantum statistics with a strong
emphasis on the physics of the "5/2" fractional
quantum Hall state where such statistics are thought to occur.
In particular, I will discuss the conundrum of the spin polarization
for that state, as well as discussing the hypothetical adiabatic
cooling of non-abelian states which could be used as an experimental
proof of "non-abelianness".
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Friday,
Dec. 17
Stewart Library
Fields Institute
10am |
Christoph
Simon (University of Calgary)
Towards efficient photon-photon gates
The implementation of efficient quantum gates between individual
photons is an important goal in quantum information processing.
Achieving this goal poses two complementary challenges. On
the one hand, it requires strong interactions between individual
photons. On the other hand, the gate operations have to be
realized in such a way that they preserve the single-mode
character of the photons despite those strong interactions.
I will give a brief overview over different approaches that
have been proposed, and discuss where they stand with respect
to these two challenges. In particular I will explain the
limitations due to transverse multi-mode effects, and show
how they could be overcome in a scheme based on dipole-dipole
interactions between Rydberg state polaritons [1]. Finally
I will present a recent proposal for photon-photon gates in
Bose-Einstein condensates [2], which exploits the long storage
times and collisional interactions available in this system,
where the interactions are enhanced by the adiabatic compression
of the condensate and the use of a Feshbach resonance. I will
conclude by arguing that we may have reached a critical mass
of technology and theoretical understanding to allow successful
experiments on efficient photon-photon gates in the foreseeable
future.
References:
[1] B. He, A. MacRae, Y. Han, A.I. Lvovsky, and C. Simon,
Transverse
multi-mode effects on the performance of photon-photon gates,
arXiv:1006.3584
[2] A. Rispe, B. He, and C. Simon, Photon-photon gates in
Bose-Einstein condensates, arXiv:1010.0037s.
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Friday,
Dec. 10
Stewart Library
Fields Institute
10am |
Joseph
Emerson (Department of Applied Math and Institute for Quantum
Computing, University of Waterloo)
Robust benchmarking of quantum processes via randomization
I will describe an experimental method for efficiently benchmarking
quantum information processes. The method is based on characterizing
the survival probability under sequences of random quantum processes
drawn from a unitary 2-design. I will show how the protocol
provides a reliable estimate of the average error-rate for a
set of target operations (eg quantum gates) under a very general
noise model that allows for both time and gate-dependent errors.
I will discuss the conditions under which this estimate remains
valid and illustrate features of the protocol through numerical
examples.
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Friday,
Nov. 5
Stewart Library
Fields Institute
10am |
Mark M. Wilde (McGill University)
Entanglement boosts quantum turbo codes
One of the unexpected breakdowns in the existing theory of
quantum serial turbo coding is that a quantum convolutional
encoder cannot simultaneously be recursive and non-catastrophic.
These properties are essential for a quantum turbo code to
have an unbounded minimum distance and for its iterative decoding
algorithm to converge, respectively. Here, we show that the
entanglement-assisted paradigm gives a theoretical and practical
"turbo boost" to these codes, in the sense that
an entanglement-assisted quantum (EAQ) convolutional encoder
can possess both of the aforementioned desirable properties,
and simulation results indicate that entanglement-assisted
turbo codes can operate reliably in a noise regime 5.5 dB
beyond that of standard quantum turbo codes. Entanglement
is *the* resource that enables a convolutional encoder to
satisfy both properties because an encoder acting on only
information qubits, classical bits, gauge qubits, and ancilla
qubits cannot simultaneously satisfy them. We give several
examples of EAQ convolutional encoders that are both recursive
and non-catastrophic and detail their relevant parameters.
Finally, simulation results demonstrate that interleaved serial
concatenation of EAQ convolutional encoders leads to a powerful
code construction with excellent performance on a memoryless
depolarizing channel.
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Friday,
Oct. 22
Stewart Library
Fields Institute
10am |
Alireza Shabani (Princeton University)
Compressed Tomography of Quantum Dynamical Systems
A fundamental problem in characterization of complex quantum
systems is the exponential growth in the required physical
resources with the size of system. We develop an efficient
method for complete estimation of an unknown quantum process/Hamiltonian
with a polynomial number of experimental configurations via
employing techniques known as compressed sensing. We demonstrate
that by O(s \log d) random local preparations and measurements
settings one can fully identify a quantum process/Hamiltonian
for a d-dimensional system, if it is known to be nearly s-sparse
in a basis. We present the first experimental implementation
of this method for two- and four-photon quantum optical systems
with a significant reduction in physical resources compare
to known tomography techniques. Moreover, we demonstrate robustness
of this technique by performing efficient high-fidelity estimation
of two-qubit photonic phase gates under various decoherence
strengths.
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Monday,
Oct. 18
Room MP606
(60 St. George St.)
Fields Institute
10am |
Alan
Migdall (NIST)
Single-photon detector, source, and metrology efforts at
NIST
NIST has ongoing efforts to advance single-photon and photon-number-resolving
detectors, single-photon and entangled-photon sources, and the
metrology of those devices for applications in quantum communications
and quantum information processing. The detector efforts include
improved circuitry for both Si and InGaAs single-photon avalanche
photodiodes to improve efficiency and count rate, while reducing
dark counts and afterpulsing, and to improve the compatibility
of these devices with gigahertz-rate single and correlated-photon
sources. We are also working on improved processing of the output
signal from photon-number-resolving detectors to maximize the
information extracted. Another scheme uses active optical switching
and an array of detectors to increase allowable detection rates
and reduce detection deadtime. Source efforts include bright
and efficient sources based on parametric downconversion and
fourwave mixing. Metrology efforts in the photon counting regime
include both conventional metrology tied to detector-based standards
and correlated-photon metrology using photon pair sources, with
our ultimate goal to make higher accuracy metrology available
to the photon-counting community at large.
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Monday, Sept. 27
Stewart Library,
Fields Institute
2pm
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Elanor Huntington (University of New South Wales)
Quantum Hindsight: Quantum Parameter Estimation Using Smoothing
Quantum parameter estimation has many applications, from
gravitational wave detection to quantum key distribution.
The most commonly used technique for this type of estimation
is quantum filtering, using only past observations. We present
the first experimental demonstration of quantum smoothing,
a time symmetric technique that uses past and future observations,
for quantum parameter estimation. We consider both adaptive
and nonadaptive quantum smoothing, and show that both are
better than their filtered counterparts. For the problem of
estimating a stochastically varying phase shift on a coherent
beam, our theory predicts that adaptive quantum smoothing
(the best scheme) gives an estimate with a mean-square error
up to 2-root-2 times smaller than nonadaptive filtering (the
standard quantum limit). The experimentally measured improvement
is 2.24.
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Friday,
Sept. 24
Stewart Library
Fields Institute
10am |
Ivan
Deutsch (University of New Mexico)
Extreme Spin Squeezing in Cold Trapped Atomic Ensembles
Squeezing of the collective spin of an ensemble of atoms
is intimately related to the creation of entanglement and
nonlinear evolution of the many-body system. We describe a
new approach to achieving a high degree of squeezing based
on coherent quantum feedback in a double-pass Faraday interaction
between an optical probe and an optically dense atomic sample.
A quantum eraser is used to remove residual spin-probe entanglement,
thereby realizing a single-axis twisting unitary map on the
collective spin. This interaction can be phase-matched, resulting
in *exponential* enhancement of squeezing as a function of
optical density for times short compared to the decoherence
time. In practice the scaling and peak squeezing depends on
decoherence, technical loss, and noise. Including these imperfections,
our model indicates that 10 dB of squeezing should be achievable
with laboratory parameters. With such squeezing as a base,
we can explore the atomic ensemble as a platform for continuous-variable
quantum information processing.
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Friday,
Sept. 17
Stewart Library, Fields Institute
11am |
Alessandro Fedrizzi (University of Queensland)
Discrete- and continuous-time quantum walks with single
photons
Quantum walks describe the evolution of quantum particles
on a graph. Due to their rich dynamics, they can emulate a
wide range of phenomena in real-world systems. In the first
part of my talk, I will present a stable and reasonably scalable
optical implementation of a discrete-time quantum walk on
a line. Single photons, encoded in polarisation, walk through
an interferometric network based on calcite beam displacers
and half-wave plates. We demonstrate full control of the decoherence
in the system and have access to all lattice sites at any
given time step. This allows us to investigate a host of scenarios,
such as the observation of signatures of topological phases
in artificial 1D systems. We implemented phase transitions
for topological phases of a certain symmetry class, distinguished
by their winding number. For transitions in phase space with
different topological invariants, we observe distinct bound
states which are not present otherwise. In the second part
of my talk, I present results on continuous-time quantum walks
with periodic boundary conditions in direct-write waveguides
with single photons and two-photon states.
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