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

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, low-energy mechanical oscillators could allow to reveal the effect of a modified commutator. Here we analyze the free evolution of high quality factor micro- and nano-oscillators, 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

Device-independent measurement and its applications


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 “device-independent”. 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 measurement-device-independent quantum key distribution scheme. Meanwhile, the device-independent measurement also finds its applications in other areas, such as entanglement witness. With a measurement-device-independent 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


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 error-disturbance relation. In contrast, we have presented and proven a Heisenberg-type 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 trade-off relation for errors in qubit measurements, using the same concepts as we did in the position-momentum case. We show that the combined errors in an approximate joint measurement of a pair of 1-valued 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 trade-off 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


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 quantum-classical 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 measurement-device-independent techniques and quantum repeaters.


22 January 2014 - CSE/082

Matteo G. A. Paris

University of Milan

An invitation to quantum estimation theory


Several quantities of interest in physics are non-linear 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


The standard standing-wave description of optical cavities can be used for example to calculate the total photon scattering rate of experiments with resonant and near-resonant laser driving. However, it cannot be used to calculate the photon scattering rates through the different sides of a two-sided optical cavity. To overcome this problem, we introduce a traveling-wave 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 two-sided optical cavity, arXiv:1307.3545 (2013). [2] E. S. Kyoseva, A. Beige, and L. C. Kwek, New J. Phys. 14, 023023 (2012).


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, Max-Planck-Institut 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


I will review ideas to approach the Graph Isomorphism Problem with tools linked to Quantum Information.


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., zero-error 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


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

Continuous-Variable Quantum Cryptography with Entanglement in the Middle


We analyze the performance of continuous-variable 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


Teleportation is what we usually associate with the fuzzy disappearance and re-appearance 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


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, problem-specific 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 quantum-enhanced 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


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


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


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 device-independent quantum cryptography


Device-independent 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 device-independent schemes have been proposed.

In this talk, I will explain a previously overlooked weakness of existing device-independent 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 device-independent 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)