Quantum Information Seminars
@ Computer Science (York)
16
September 2015  at 11.30 in RCH/042 Meeting Pod 1
David Vitali
University of Camerino
Probing deformed
commutators with macroscopic harmonic oscillators
Abstract
A minimal observable length is a common feature of theories that
aim to merge quantum physics and gravity. Quantum mechanically,
this concept is associated to a nonzero minimal uncertainty in
position measurements, which is encoded in deformed commutation
relations. In spite of increasing theoretical interest, the subject
suffers from the complete lack of dedicated experiments and bounds
to the deformation parameters are roughly extrapolated from indirect
measurements. As recently proposed, lowenergy mechanical oscillators
could allow to reveal the effect of a modified commutator. Here
we analyze the free evolution of high quality factor micro and
nanooscillators, spanning a wide range of masses around the Planck
mass, and compare it with a model of deformed dynamics. Previous
limits to the parameters quantifying the commutator deformation
are substantially lowered
M. Bawaj et al., Nature Communications 6, 7503 2015doi:10.1038/ncomms8503

20
February 2015  at 11.00 in CSE/202
Xiongfeng Ma
Tsinghua University
Deviceindependent
measurement and its applications
Abstract
In practice, imperfections in measurement devices may bring
false conclusions in quantum information tasks and sometimes lead
to disastrous consequences. This is also called detection loophole.
In quantum cryptography, imperfect measurement devices open up
security loopholes to hacking. This loophole can be solved by
making the measurement “deviceindependent”. That is, the correctness
of the experiment conclusion would be independent of physical
realizations of the measurement. In this talk, I will discuss
some of the recent developments in the experiment demonstrations
of the measurementdeviceindependent quantum key distribution
scheme. Meanwhile, the deviceindependent measurement also finds
its applications in other areas, such as entanglement witness.
With a measurementdeviceindependent scheme, we can show that
an entanglement witness can be realized without detection loopholes. 
4
June 2014  CSE/082
Paul Busch
University of York
The Heisenberg measurement
uncertainty controversy and its resolution
Abstract
Reports on experiments recently performed in Vienna [Erhard
et al, Nature Phys. 8, 185 (2012)] and Toronto [Rozema et al,
Phys. Rev. Lett. 109, 100404 (2012)] include claims of a violation
of Heisenberg’s errordisturbance relation. In contrast,
we have presented and proven a Heisenbergtype relation for joint
measurements of position and momentum [Phys. Rev. Lett. 111, 160405
(2013)]. To resolve the apparent conflict, we formulate here a
new general tradeoff relation for errors in qubit measurements,
using the same concepts as we did in the positionmomentum case.
We show that the combined errors in an approximate joint measurement
of a pair of 1valued observables are tightly bounded from below
by a quantity that measures their degree of incompatibility. The
claim of a violation of Heisenberg is shown to fail as it is based
on unsuitable measures of error and disturbance. These measures
are used in an inequality that was formulated by Ozawa as a correction
of a wrong inequality that is incorrectly attributed to Heisenberg.
We will see that Ozawa’s quantities overestimate the errors
and are found, ironically, to obey a tradeoff relation of the
Heisenberg form in the qubit case. Finally we show how the experiments
mentioned may directly be used to test our error inequality.
The talk is based on our recent paper “Heisenberg uncertainty
for qubit measurements”, available as arXiv:1311.0837, published
in Phys. Rev. A 89, 012129 (2014). 
26
Feb 2014  CSE/082
Mohsen Razavi
University of Leeds
Toward public quantum
communication networks
Abstract
Quantum communications is perhaps the most advanced
of the emerging technologies that rely on quantum information
science. It offers secure communications immune to any possible
computational advancement in the future. Having successfully demonstrated
over dark and commercial fibre, it is now the right time to step
up the efforts and plan for extending this technology to the public
level, where every home user can enjoy its benefits. In this talk,
I will describe some of the technological challenges that hybrid
quantumclassical networks are facing, and introduce some of the
possible solutions. In particular, we look at several possible
topologies for such networks, and discuss different generations
of such hybrid networks relying on measurementdeviceindependent
techniques and quantum repeaters. 
22
January 2014  CSE/082
Matteo G. A. Paris
University of Milan
An invitation to
quantum estimation theory
Abstract
Several quantities of interest in physics are
nonlinear functions of the density matrix and cannot, even in
principle, correspond to proper quantum observables. Any method
aimed to determine the value of these quantities should resort
to indirect measurements and thus corresponds to a parameter estimation
problem whose solution, i.e. the determination of the most precise
estimator, unavoidably involves an optimization procedure. In
this lecture I review local quantum estimation theory, which allows
to quantify quantum noise in the measurements of non observable
quantities and provides a tools for the characterization of signals
and devices, e.g. in quantum technology. Explicit formulas for
the symmetric logarithmic derivative and the quantum Fisher information
of relevant families of quantum states are presented, and the
connection between the optmization procedure and the geometry
of quantum statistical models is discussed in some details. Finally,
few applications, ranging from quantum optics to critical systems
are illustrated. 
13
November 2013  CSE/082
Almut Beige
University of Leeds
Coherent cavity networks
with complete connectivity
Abstract
The standard standingwave description of optical
cavities can be used for example to calculate the total photon
scattering rate of experiments with resonant and nearresonant
laser driving. However, it cannot be used to calculate the photon
scattering rates through the different sides of a twosided optical
cavity. To overcome this problem, we introduce a travelingwave
cavity Hamiltonian. When modelling a situation which can be analysed
taking either a fully quantum optical or a fully classical approach,
this Hamiltonian is shown to yields the same predictions as Maxwell's
equations. But it also applies to quantum optical experiments
beyond the scope of Maxwell's equations. For example, it can be
used to model the scattering of single photons through the fiber
connections of coherent cavity networks. Here we use this approach
to design coherent cavity networks with complete connectivity
with potential applications in quantum computing and the simulation
of the complex interaction Hamiltonians of biological systems.
[1] T. M. Barlow and A. Beige, Scattering light
through a twosided optical cavity, arXiv:1307.3545 (2013). [2]
E. S. Kyoseva, A. Beige, and L. C. Kwek, New J. Phys. 14, 023023
(2012).
Biography:
Since October 2005: Quantum Information Group, School of Physics
& Astronomy, University of Leeds
October 2003  September 2004: Department of Applied Mathematics
and Theoretical Physics (DAMTP), University of Cambridge
October 2002  September 2005: Quantum Optics and Laser Science
Group (QOLS), Imperial College London
August 2000  September 2002: Laser physics group, MaxPlanckInstitut
für Quantenoptik in Garching
June 1998  July 2000: Theoretical Quantum Optics Group, Imperial
College London
March 1998  May 1998: Quantum Theory Group, University of Potsdam
December 1992  January 1998: Quantum Optics Group, Institute
for Theoretical Physics, University of Göttingen 
6
November 2013  CSE/082
Simone Severini
University College London
The Graph Isomorphism
Problem and Quantum Information
Abstract
I will review ideas to approach the Graph Isomorphism
Problem with tools linked to Quantum Information.
Biography:
Simone Severini is a Royal Society URF and a
member of the Department of Computer Science at UCL. He contributed
to create new directions at the interplay between quantum theory,
discrete mathematics, and complex systems (e.g., zeroerror quantum
information, background independent models of gravity, etc.) with
ca. 100 theoretical papers, editorial work, and the organization
of major conferences on quantum information theory (e.g., TQC,
AQC, etc.). 
20
March 2013  2.00pm in LMB/021
Gerardo Adesso
University of Nottingham
Quantum correlations
versus entanglement in composite systems
Abstract
The correlations of multipartite quantum states
can have nonclassical features other than entanglement. After
giving a brief overview of the subject, we focus on the issue
of establishing a hierarchy between measures of entanglement and
compatible measures of general quantum correlations. We analyze
a family of measures of general quantum correlations for composite
systems, defined in terms of the bipartite entanglement necessarily
created between systems and apparatuses during local measurements.
For every entanglement monotone $E$, this operational correspondence
provides a different measure $Q_E$ of quantum correlations. Examples
of such measures are the relative entropy of quantumness, the
quantum deficit, and the negativity of quantumness. In general,
we prove that any so defined quantum correlation measure is always
greater than (or equal to) the corresponding entanglement between
the subsystems, $Q_E \ge E$, for arbitrary states of composite
quantum systems. In this respect, quantum correlations truly go
beyond entanglement. We then discuss the extent up to which they
can exist without entanglement, namely whether there are upper
bounds on some measure of quantum correlations for separable states
of a given dimension. Addressing this largely open question can
be very relevant for those applications for which general quantum
correlations, and not entanglement, provide the essential resources. 
13
March 2013  2.00pm in CSE/082
Christian Weedbrook
University of Toronto
ContinuousVariable
Quantum Cryptography with Entanglement in the Middle
Abstract
We analyze the performance of continuousvariable quantum key
distribution protocols where the entangled source originates not
from one of the trusted parties, Alice or Bob, but from the malicious
eavesdropper in the middle. This is in contrast to the typical
simulations where Alice creates the entangled source and sends
it over an insecure quantum channel to Bob. By using previous
techniques and identifying certain error correction protocol equivalences,
we show that Alice and Bob do not need to trust their source,
and can still generate a positive key rate. Such a situation can
occur in a quantum network where the untrusted source originated
in between the two users.

13
February 2013  6.30pm in LMB/030&031 (as Public Lecture,
registration required)
Sam Braunstein
University of York (CS)
Teleporting to the Future
Abstract
Teleportation is what we usually associate with
the fuzzy disappearance and reappearance of space voyagers such
as Captain Kirk after the familiar command "beam me up Scottie".
Since its early use in science fiction, the term teleportation
has since been used to refer to the process by which objects are
transferred from one location to another, without actually making
the journey along the way. The "disembodied" nature
of teleportation raises some baffling questions. "What is
actually sent?" Is it the original
system that is reconstructed at the remote site or merely a copy?
So long as teleportation remains within the remit of science fiction,
these questions may seem since rather philosophical. But quantum
teleportation, unlike its science fiction inspiration, is a fact.
It has been achieved in laboratories the world over for the transfer
of single photons, atoms and even beams of light. How might this
new technology begin to affect our world and our lives? Is it
possible to scale teleportation up so that one day we could teleport
people? What might we learn from that, and to what use might teleportation
be put by generations to come?
In this lecture, Samuel Braunstein will explain what teleportation
is all about, and explore how the science of teleportation might
take us all to places where no man has gone before. 
11
February 2013  2.30pm in LMB/044
Marco Barbieri
University of Oxford
Experimental Boson
Sampling
Abstract
While universal quantum computers ideally solve problems such
as factoring integers exponentially more efficiently than classical
machines, the formidable challenges in building such devices motivate
the demonstration of simpler, problemspecific algorithms that
still promise a quantum speedup. We construct a quantum boson
sampling machine (QBSM) to sample the output distribution resulting
from the nonclassical interference of photons in an integrated
photonic circuit, a problem thought to be exponentially hard to
solve classically. Unlike universal quantum computation, boson
sampling merely requires indistinguishable photons, linear state
evolution, and detectors. We benchmark our QBSM with three and
four photons and analyze sources of sampling inaccuracy. Scaling
up to larger devices could offer the first definitive quantumenhanced
computation.

6
February 2013  2pm in CSE/082
Mauro Paternostro
Queen's University of Belfast
A route to quantumness
in mesoscopic systems:
Through the (quantum) looking glass, and what Alice found found
there
Abstract
After getting her Master in Theoretical Physics,
sleeping on the couch of her living room in a lazy summer afternoon,
Alice wonders what she can actually do with the optomechanical
cavities that her parents gave her as presents. She finds herself
amazed by the possibility to enforce nonclassical correlations
and create quantum states of optomechanical systems affected by
strong noise and decoherence. The girl thus starts wondering about
the possibility to build surreal optomechanical networks, challenging
the boring "quantum repeaters" paradigm, and thus building
up quantum interfaces between mechanical oscillators and other
systems (ultra cold atoms, BECs, massive molecules), but only
if you asks them gently!
In this seminar, we will try to find out what Alice discovered
when she woke up... 
30
January 2013  2pm in CSE/082
Viv Kendon
University of Leeds
Quantum walks and
algorithms
Abstract
Quantum walks are now a standard part of the quantum programmer's
toolbox. I will give an introduction to quantum walks and their
algorithmic uses, with some asides on how they can be used to
model physical phenomena and how they relate to current experiments.

20 November 2012  11.15am in
LMB/021
David Edward Bruschi
University of Leeds
Relativistic Quantum
Information
Abstract
The field of Relativistic Quantum Information
has enjoyed a rapid expansion in the past few years. This novel
and exciting field aims at exploring the overlap of relativity
with quantum information, in particular how relativistc effects
affect quantum information tasks. I will introduce the aims and
motivations behind the work done within this field and focus on
recent work that addresses localised systems which will be sued
to model realistic devices. I will discuss results and implications
and the open problems.

1 November 2012  10am in LMB/024
 CANCELLED (to be rescheduled) 
Roger Coldbeck
Institute for Theoretical Physics,
ETH Zurich and University of York (Maths)
Prisoners of their
own device: Trojan attacks on deviceindependent quantum cryptography
Abstract
Deviceindependent quantum cryptographic schemes
aim to guarantee security to users based only on the output statistics
of any components used, and without the need to verify their internal
functionality. Since this would protect users against untrustworthy
or incompetent manufacturers, sabotage or device degradation,
this idea has excited much interest, and many deviceindependent
schemes have been proposed.
In this talk, I will explain a previously overlooked weakness
of existing deviceindependent protocols, namely that naively
reusing untrusted devices can compromise previously generated
keys. Possible defences include securely destroying or isolating
used devices. However, these are costly and impractical.
I will discuss some more practical partial defences that aim to
achieve composable security of deviceindependent quantum key
distribution with device reuse in restricted scenarios.
This is based on http://arxiv.org/abs/1201.4407

Last modified: 19/10/2015 (Stefano
Pirandola)
