
CQIQC/Toronto Quantum Information Seminars
QUINF 200910
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.

Talks are held Fridays at 11 am unless otherwise indicated
PAST TALKS

Friday,
June 18
Stewart Library, Fields Institute
11am 
Hong Guo (Peking University, Beijing, China)
Two Key Techniques for the Security of Practical Quantum
Key Distribution System: Truly Random Number Generator and
Passive Scheme of Source Monitoring
Two key issues for the security of practical quantum key distribution
(QKD) system, i.e., truly random number generator (TRNG) and
monitoring of the QKD source, are addressed. For TRNG, two
schemes based, respectively, on the detection of photon number
statistics of diode laser, and on the continuous beat signal
detection of a VCSEL, is reported, which can produce truly
random numbers at 20 Mbit/s rate for any long time and the
true randomness of which is primarily confirmed by 3 sigmacriteria
up to 14 Gbit. In the security analysis of some QKD protocols,
the photonnumber distribution (PND) of QKD source is assumed
to be fixed and known to Alice and Bob, while Eve cannot control
or change it. In reallife experiment, the PND may deviate
from this assumption (the source is untrusted) and so a monitoring
for the source is needed. Previously, the active scheme for
the monitoring is proposed but did not work well in the experiment.
For monitoring the photon statistics of QKD source, we propose
a passive scheme with a beam splitter and a PD detector and
the experiment is realized in a reallife QKD system.

Thursday,
June 3
Stewart Library, Fields Institute
2pm 
Jiangbin Gong (National University of Singapore)
Preserving Known or Unknown Entangled States by Uhrig's
Dynamical Decoupling Sequence
Decoherence effects can completely kill quantum entanglement
within a very short time. Hence it will be of vast importance
if we can actively preserve quantum entanglement with a high
efficiency and with a universal scenario that does not require
our knowledge of the bath or of the systembath coupling.
We show that this is indeed possible, by successfully extending
Uhrig's dynamical decoupling sequence from onequbit cases
to general twoqubit systems. In particular, we explicitly
construct control sequences to lock a known but arbitrary
twoqubit state to the Nth order of time, using only N control
pulses. We then show that three layers of Uhrig's dynamical
decoupling sequence in a special order will be able to preserve
an unknown twoqubit entangled state to the Nth order of time,
using N^3 control pulses. The results may be also extended
to general multilevel quantum systems.

**PLEASE
NOTE DATE AND TIME**
Wednesday, May 26
Location Room MP606 
Alex Hayat (Department of Electrical Engineering,
Technion, Haifa, Israel)
Semiconductor Quantum Photonics
Miniaturizing quantum photonics is a rapidly growing field.
We introduced a concept of microcavity standingwave nonlinear
optics theoretically and experimentally, where the phasematching
requirement is translated into a nonlinear mode overlap.
We demonstrated experimentally the first observation of twophoton
emission in semiconductors  a process, in which electron
transition between energy levels occurs by the emission of
a photon pair. We proposed this phenomenon as an electricallydriven
roomtemperature source of energyentangled photons, much
more efficient than the downconversion schemes. We also proposed
twophoton absorption interferometry for characterization
of energy qubits.
First observations of electricallyinduced twophoton transparency
and twophoton gain in semiconductors are demonstrated experimentally,
and a scheme for a femtosecondscale g(4) measurement is implemented.

Mon.,
April 26
11:10 am
Room MP 606, 60 St. George Street 
Prof.
Tilman Pfau (Universität Stuttgart, Germany)
Lecture 3: Ultracold Rydberg chemistry and how to excite
Rydberg atoms in a microscopic glass box
I will discuss how quantum chemistry in the ultracold world
allows for novel binding mechanisms. The recently observed of
ultralongrange Rydberg molecules (dimers and trimers) are based
on quantum scattering of Rydberg electrons from polarizable
ground state atoms. Furthermore, we show calculations that reproduce
the observed binding energies remarkably well and reveal that
some of the excited states are purely bound by quantum reflection
at a shape resonance for pwave scattering. Finally as an outlook
on how long range interactions between neutral atoms could actually
lead to practical quantum devices like single photon sources
I report on our effort to observe the Rydberg blockade in micron
sized thermal vapor cells.

Fri.,
April 23
11:10 am
Stewart Library 
Prof.
Tilman Pfau (Universität Stuttgart, Germany)
Lecture 2: Strongly interacting Rydberg atoms
I will introduce Rydberg atoms and their mutual interaction
which can be of van der Waals or dipolar character. This interaction
leads a blockade mechanism which is observed in a BEC. We will
see that this strong interaction will allow us to emulate the
ground state properties of spin Hamitonians as they are discussed
in condensed matter physics. Universal scaling behavior in the
quantum critical region of the underlying phase diagram is observed.
The laser excitation to the Rydberg state can be coherent despite
strong interactions. To prove this experimentally rotary echo
sequences are applied. 
Wed.,
April 21
11:10 am
Room MP 606, 60 St. George Street 
Prof.
Tilman Pfau (Universität Stuttgart, Germany)
Lecture 1: A purely dipolar quantum gas
I will report on the realization of a purely dipolar quantum
gas, where the longrange and anisotropic interaction between
magnetic chromium atoms is determining the physical properties.
We will discuss the stability diagram of a dipolar gas and the
dipolar collapse dynamics. In the outlook I will show how spin
orbit coupling in dipolar gases can give rise to a quantum version
of the Einstein de Haas effect. The same coupling can also be
used for demagnetization cooling, an idea that dates back to
the first laser cooling proposal by Alfred Kastler in 1950.

**PLEASE
NOTE DATE AND TIME**
Monday,
April 19
11:10 am
Stewart Library 
**CANCELED**
Nicolas Brunner (University of Bristol)
Why is quantum nonlocality limited?
Quantum mechanics is a nonlocal theory, however not a maximally
nonlocal one according to relativity. More precisely, there
exist alternative theories containing more nonlocality than
quantum mechanics that still respect the nosignaling principle.
Why these theories are unlikely to exist in nature, and what
physical principle limits quantum nonlocality is still not
known today, despite an intensive research effort. After briefly
reviewing general nonsignaling theories, in particular focusing
on nonlocal boxes, I will present recent work which aims
at recovering quantum correlations from informationtheoretic
principles, such as communication complexity and information
causality.

Friday, Apr.16
11:10 am
FIELDS ROOM 210

Barry Sanders (University of Calgary)
Machine Learning for Precise Quantum Measurement
Quantum measurement schemes aim to surpass the standard quantum
limit (essentially partition noise) and strive to reach the
quantum limit (precision inversely proportional to number
of injected particles). One particularly promising category
of quantum measurement schemes employs a feedback mechanism:
leading particles are detected with the resultant information
used to control the instrument in order to extract progressively
more information during passage of the pulse.
Clever quantum feedback schemes have been devised but are
restricted to ideal conditions. In general quantum feedback
schemes are challenging to design so we decided to adapt machine
learning theory to quantum information inputs and employ our
theory to devise adaptivefeedback quantum measurement schemes.
In particular our approach replaces guesswork in quantum measurement
by a logical, fullyautomatic, programmable routine. We show
that our method yields schemes that outperform the best known
adaptive scheme for interferometric phase estimation. Furthermore
our approach can be adapted to the realworld case where the
instrument would learn
through trial and error an effective quantum feedback routine.

Friday, April 9
11:10 am
Stewart Library

Steve Flammia (Perimeter Institute for Theoretical
Physics)
Ultra Fast Quantum State Tomography
Everybody hates tomography. And with good reason! Experimentalists
hate it because it is inefficient and difficult. Theorists
hate it because it isn't very "quantum." But because
of our current lack of mesoscale quantum computers capable
of convincingly performing nonclassical calculations, tomography
seems like a necessary evil. In this talk, I will attempt
to banish quantum state tomography to the Hell of Lost Paradigms
where it belongs. I hope to achieve this by introducing several
methods for learning quantum states more efficiently, in some
cases exponentially so. The first method runs in polynomial
time and outputs a polynomialsized classical approximation
of the state (in matrix product state form), together with
a rigorous bound on the fidelity. The second result takes
advantage of the fact that most interesting states are close
to pure states to get a quadratic speedup using ideas from
compressed sensing. I'll also show simulations of this second
method that demonstrate how well it works in practical situations.
Both of these results are heralded, and require no a priori
assumptions about the state.
This is joint work with S. Bartlett, D. Gross, R. Somma (first
result), and D. Gross, Y.K. Liu, S. Becker, J. Eisert, (second
result; arXiv:0909:3304).

**PLEASE NOTE DATE AND TIME**
Monday, March 15
11:10 am
Room MP 606, 60 St. George Street

Lev Vaidman, Tel Aviv University
Where is the Quantum Particle between two Measurements?
Wheeler Delayed Choice experiment, ElitzurVaidman Interactionfree
Measurement, and HostenKwiat Counterfactual Computation will
be discussed to answer Bohr's forbidden question: "Where
is a quantum particle while it is inside a MachZehnder Interferometer?"
I will argue that the naive Wheeler's approach fails to explain
a weak trace left by the particle and that the twostate vector
description is required.

Friday,
26 Feb
11:10am
Stewart Library

Arjendu
Pattanayak, Carleton College
Nonmonotonicity in the quantumclassical transition
The transition between a system behaving completely classically
and behaving completely quantummechanically is complicated.
The two kinds of behaviors can be very different particularly
when the system is nonlinear. We know the transition depends
on the size of the system, the temperature and environmental
effects, and on the nonlinear dynamics, so it is a multiparameter
landscape. But what is the shape of this landscape? In this
talk I present evidence for nonmonotonicity in the quantumclassical
transition in two different systems: A damped driven doublewell
oscillator, as well as the simple harmonic oscillator. I will
also discuss prospects for experimental verification of these
predictions. 
Tuesday,
23 Feb
2pm
Room MP 713, 60 St. George Street 
Morgan
Mitchell, ICFO, Barcelona
Quantum metrology with cold atoms: quantum nondemolition
measurements, spin squeezing, and nonlinear metrology
Quantum metrology studies the use of quantum states, interference,
and entanglement in precision measurement. Originally developed
for interferometric measurement of gravitational waves, in recent
years quantum metrology has expanded both theoretically and
experimentally to become a general technique with application
in several areas. I will describe experimental work using ensembles
of cold rubidium, an interesting quantum system for measurement
of magnetic fields. Using optical probes and paramagnetic Faraday
rotation, we demonstrate quantum nondemolition measurement
of spins at the projectionnoise limit. If time permits, I will
discuss the use of nonlinear Faraday rotation to make measurements
with scaling better than the "Heisenberg limit" of
linear measurements.

Tuesday,
23 Feb
11am
Davenport East Seminar Room, 80 St. George Street, Toronto 
Moshe Shapiro (Weizmann Institute of Science and University
of British Columbia)
Non destructive state reconstruction and the resolution
of the spectroscopic phase problem
We discuss the problem of non destructively reconstructing
unknown time evolving vibrational wave packets and show how
one can solve this problem and also reconstruct in a "pointbypoint"
manner the potential that governs the motion of such wave
packets, using as input only the power spectrum of the light
emitted from a small minority of replicas of the unknown states
and the potential to which the light emission occurs.

Friday,
Feb. 12, 2010
11:10 am
Stewart Library 
Ioannis Thanopoulos
Theoretical and Physical Chemistry Institute, National Hellenic
Research Foundation
Quantum dynamics of large molecules and control of multichannel
processes
We show that the quantum dynamics of a system comprised of
a subspace Q coupled to a much larger subspace P can be recast
as a reduced set of ordinary differential equations with constant
coefficients. These equations can be solved by a single diagonalization
of a general complex matrix. The efficiency of the method
is demonstrated via computations on large molecular systems,
as the radiationless transitions in pyrazine. We also present
a solution to the "MultiChannel Quantum Control"
problem, where selective and complete population transfer
from an initial bound state to M energetically degenerate
continuum channels is achieved. The control is affected by
coherently controlled Adiabatic Passage proceeding via N bound
intermediate states. We illustrate the viability of the method
by computationally controlling the multichannel photodissociation
of methyl iodide.

Friday,
Jan. 8, 2010
11:10 am
Stewart Library 
*** CANCELLED ***
Vladimir Korepin
Physics Department, State University of New York at Stony
Brook
Entanglement and Correlation Functions: Bethe Ansatz vs
Valence Bond Solid States
I will consider entanglement entropy and correlation functions
in two different set states [wave functions]: Bethe Ansatz
and Valence Bond Solid states [discovered by Affleck, Kennedy,
Lieb and Tasaki]. I will argue that these states are essentially
different. As measure of entanglement we will use von Neumann
entropy or Renyi entropy. Main results obtained analytically
by means of FisherHarwig formula, desribing Toeplitz determinants.
Information about the speaker:
http://en.wikipedia.org/wiki/Vladimir_Korepin
http://insti.physics.sunysb.edu/~korepin

Friday, Dec. 11, 2009
11:10 am
Stewart Library

Joe Altepeter, Northwestern University
Entangled Photon Polarimetry
The Polarimeter is a useful tool in nearly every branch of
modern optics, providing a complete, realtime, graphical
measurement of the polarization of an optical field. From
the perspective of quantum information processing, a polarimeter
actively monitors the complete quantum state of a single qubit
system. Here we present advances in twoqubit state visualization
and quantum state tomography which have made it possible to
realize a complete, realtime, graphical measurement of a
twoqubit system. In addition, we present work on the fiberbased,
telecomband entanglement sources for which this type of tool
will be immediately useful.

Friday,
4Dec.2009
11:10am
Stewart Library

Dmitry Gavinsky NEC Labs,
Princeton, NJ, U.S.A.
Predictive Quantum Learning
We give the first unconditional separation of quantum and
classical learning. We demonstrate a relational concept class
that is efficiently learnable in a quantum predictive analogue
of PAC, while in any reasonable classical model exponential
amount of training data would be required. We show that our
separation is the best possible in several ways, in particular
there is no analogous result for a functional class, as well
as for several weaker versions of quantum learning.

Nov. 23, 2009
MP 606,
60 St. George St. 
Joint Quantum Optics CQIQC Seminar
Pablo Londero (Cornell University)
Nonlinear Optics in RubidiumFilled HollowCore Photonic
BandGap Fibers
Much of the success in optical quantum computing and quantum
key distribution has come from linear techniques, in part
due to the difficulty of developing systems which exhibit
optical nonlinearities at low photon numbers. Hollowcore
photonic bandgap fibers, when filled with Rb vapor, provide
a system with strong coupling to resonant light. This opens
the door to a variety of interesting quantumoptical experiments
where lowphoton number states can induce measurable nonlinearities,
and where moderate amounts of light can produce unusually
strong nonlinear effects. I will present recent experiments
in our Rbloaded fibers on electromagneticallyinducedtransparency
in the presence of buffer gas, fourwave mixing with gain
>100 and bandwidths >300 MHz at microwatt power levels,
and alloptical switching with a few thousand photons, as
well as some thoughts on future directions.

Nov. 9, 2009
MP 606,
60 St. George St.

Joint Quantum Optics CQIQC Seminar
Markus Buttiker (Universite de Geneve)
Traversal time for tunneling
The tunneling of particles through classically forbidden
regions is a basic quantum phenomenon. We review discussions
which investigate the dynamics of this process and in particular
attempt to answer the question: “What is the time a particle
needs to traverse a classically forbidden region”. Some
approaches lead to superluminal velocities. Similar to Brillouin
and Sommerfeld we are interested in approaches which yield
causal subluminal velocities. In particular we discuss in
some detail the propagation of monochromatic fronts of Stevens
and Buttiker and Thomas [1]. The use of such fronts to find
a sharp traversal time is not possible, however, using frequency
band limited sources or shorttime Fourier analysis allows
to determine a traversal time with the accuracy of the time
itself [2].
[1] Markus Buttiker and Harry Thomas, Ann. Phys. (Leipzig)
7, 602 (1988).
[2] Gonzalo Muga and Markus Buttiker, Phys. Rev. A 62, 023808
(2000).

October 30, 2009
Stewart Library
Fields Institute

TALK POSTPONED
Steve Flammia, Perimeter Institute for Theoretical
Physics
Ultra Fast Quantum State Tomography
Everybody hates tomography. And with good reason! Experimentalists
hate it because it is inefficient and difficult. Theorists
hate it because it isn't very "quantum." But because
of our current lack of mesoscale quantum computers capable
of convincingly performing nonclassical calculations, tomography
seems like a necessary evil.
In this talk, I will attempt to banish quantum state tomography
to the Hell of Lost Paradigms where it belongs. I hope to
achieve this by introducing several methods for learning quantum
states more efficiently, in some cases exponentially so. The
first method runs in polynomial time and outputs a polynomialsized
classical approximation of the state (in matrix product state
form), together with a rigorous bound on the fidelity. The
second result takes advantage of the fact that most interesting
states are close to pure states to get a quadratic speedup
using ideas from compressed sensing. I'll also show simulations
of this second method that demonstrate how well it works in
practical situations. Both of these results are heralded,
and require no a priori assumptions about the state.
This is joint work with S. Bartlett, D. Gross, R. Somma (first
result), and D. Gross, Y.K. Liu, S. Becker, J.Eisert, (second
result; arXiv:0909:3304).


