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Our seminars usually take place in the QuIC seminar room, campus Solbosh, building U, room UA3-219.

Upcoming seminars

In the past

General Linearized Theory of Quantum Fluctuations around Arbitrary Limit Cycles
Speaker: Carlos Navarrete-Benlloch, Max-Planck Institute for the Science of Light
Date: Thursday, April 19, 2018, 11:00.
Abstract: The theory of Gaussian quantum fluctuations around classical steady states in nonlinear quantum-optical systems (also known as standard linearization) is a cornerstone for the analysis of such systems. Its simplicity, together with its accuracy far from critical points or situations where the nonlinearity reaches the strong coupling regime, has turned it into a widespread technique, being the first method of choice in most works on the subject. However, such a technique finds strong practical and conceptual complications when one tries to apply it to situations in which the classical long-time solution is time dependent, a most prominent example being spontaneous limit-cycle formation. Here, we introduce a linearization scheme adapted to such situations, using the driven Van der Pol oscillator as a test bed for the method, which allows us to compare it with full numerical simulations. On a conceptual level, the scheme relies on the connection between the emergence of limit cycles and the spontaneous breaking of the symmetry under temporal translations. On the practical side, the method keeps the simplicity and linear scaling with the size of the problem (number of modes) characteristic of standard linearization, making it applicable to large (many-body) systems.

Based on: Phys. Rev. Lett. 119, 133601 (2017).

A gauge-invariant reversible cellular automata
Speaker: Nathanaël Eon, Université libre de Bruxelles
Date: Wednesday, March 21, 2018, 11:00.
Abstract: Gauge-invariance is a fundamental concept in physics—known to provide the mathematical justification for all four fundamental forces. We created discrete counterparts to the main gauge theoretical concepts, directly in terms of Cellular Automata. More precisely, we describe a step-by-step gauging procedure to enforce local symmetries upon a given Cellular Automaton. We apply it to a simple Reversible Cellular Automaton for concreteness. From a Computer Science perspective, discretized gauge theories may be applied to numerical analysis, quantum simulation, fault-tolerant (quantum) computation. From a mathematical perspective, discreteness provides a simple yet rigorous route straight to the core concepts.
Quantum Computation with Indefinite Causal Structures
Speaker: Fabio Costa, University of Vienna
Date: Wednesday, March 26, 2014, 15:00.
Abstract: Quantum computation is generally assumed to take place on a definite causal structure, in which gates are applied in a definite sequence within a circuit. The possible interplay between general relativity and quantum mechanics motivates the consideration of a more general model, where the order in which the gates are applied is controlled by a quantum system. This new model allows solving specific computational problems more efficiently than causally ordered circuits, providing a polynomial reduction in computational complexity. This model of quantum computation is also of practical interest, since it can be realized in table-top interferometric setups far from the relativisitic regime.
Renormalization as an inference problem
Speaker: Cédric Bény, University of Hannover
Date: Wednesday, October 23, 2013, 11:00.
Abstract: A large part of physics can be thought of as an attempt to infer the of a system’s state from imprecise coarse-grained measurements. This is an ill-posed problem because certain features of the state cannot be resolved by the measurements. However, an effective model can nonetheless be made of those features which can be observed. I will explain how those relevant and irrelevant parameters can be concretely characterized using quantum distinguishability metrics. Renormalization methods in condensed matter and in quantum field theory then emerge from the freedom that one has in modifying the irrelevant coordinates, given a reasonable model of the experimentalist's limitations. This unifying approach elucidates the role that information plays in the various formulations of the renormalization group, and provides an avenue for generalizations.
Transmission of classical information through Gaussian quantum channels: the review
Speaker: Oleg Pilyavets, QuIC, ULB
Date: Wednesday, October 2, 2013, 14:15.
Abstract: I will review recent results on classical information transmission through bosonic Gaussian quantum channels discussing both memoryless and memory channels. Specifically I will consider the classical capacity of these channels as well as information transmission rates achievable using homodyne and heterodyne measurements. I will also discuss different additivity problems for classical capacity arising in continuous variables quantum channels, which in turn lead to a notion of optimal channel memory. These results are mainly based on works: http://arxiv.org/abs/0907.1532, http://arxiv.org/abs/1303.4939, some newer conference proceedings, and a few works in preparation.
Device Independent Random Number Generation: quantifying randomness in a Bell scenario
Speaker: Olmo Nieto Silleras, LIQ, ULB
Date: Friday, July 7, 2013, 11:00.
Abstract: Randomness is an important resource for a number of applications, such as cryptography and computer simulations. Two types of randomness can be distinguished: apparent or pseudo-randomness, which is due to a lack of knowledge or to the high complexity involved in the description of the random process; and intrinsic, truly unpredictable randomness, which can only arise in quantum processes.
This genuine randomness is a direct consequence of two fundamental characteristics of quantum theory: the no-signalling principle, which forbids faster-than-light communication, and the non-local character of certain quantum correlations, in the sense that they can violate a Bell-type inequality. There thus seems to be a direct link between non-locality and randomness. This opens the way to secure device-independent applications, since the violation of a Bell inequality by the measurement outcomes suffices to certify the presence of genuine randomness, independently of the inner workings of the device.
Both from a fundamental point of view and in order to develop such applications it is thus of interest to be able to find a quantitative link between the amount of non-locality present in an experiment and the amount of genuine randomness generated. In this talk I will present a new method which provides tight bounds on the amount of randomness generated in a Bell experiment by exploiting all the statistics of the experiment. This improves over previous works, where tight bounds on the randomness were obtained by only considering the amount of Bell violation.
References: 1. S. Pironio et al., Random numbers certified by Bell’s theorem. Nature 464, 1021 (2010). 2. A. Acín, S. Massar & S. Pironio, Randomness versus Nonlocality and Entanglement. Phys. Rev. Lett. 108, 100402 (2012). 3. M. Navascués, S. Pironio & A. Acín, A convergent hierarchy of semidefinite programs characterizing the set of quantum correlations. New Journal of Physics 10, 073013 (2008).
Nonlocality of symmetric states and its applications in quantum information processing
Speaker: Zizhu Wang, QuIC, ULB
Date: Friday, June 21, 2013, 11:00.
Abstract: This talk is about the nonlocal properties of permutation symmetric states and the potential usefulness of such properties in quantum information processing. The nonlocality of almost all symmetric states, except Dicke states, is shown by constructing an n-party Hardy paradox. With the help of the Majorana representation, suitable measurement settings can be chosen for these symmetric states which satisfy the paradox. An extended CH inequality can be derived from the probabilistic conditions of the paradox. This inequality is shown to be violated by all symmetric states. The nonlocality properties and entanglement properties of symmetric states are also discusses and compared, notably with respect to persistency and monogamy. It is shown that the degeneracy of some symmetric states is linked to the persistency, which provides a way to use device independent tests to separate nonlocality classes. It is also shown that the inequalities used to show the nonlocality of all symmetric states are not strictly monogamous. A new inequality for Dicke states is shown to be monogamous when the number of parties goes to infinity. But all these inequalities can not detect genuine nonlocality. Applications of nonlocality to communication complexity and Bayesian game theory are also discussed.
Quantum query complexity: Adversaries, polynomials and direct product theorems
Speaker: Jérémie Roland, QuIC, ULB
Date: Friday, February 2, 2013, 11:00.
Abstract: Quantum query complexity has recently seen a lot of progress. One of the most prominent results is certainly that the generalized adversary bound characterizes the bounded-error quantum query complexity for any function. This fundamental result however does not answer all questions about quantum query complexity as it suffers from two limitations. First, the result is non constructive, and there still exist functions for which we can prove a tight lower bound using the polynomial method, the other main lower bound technique, but for which the optimal adversary matrix is unknown. Secondly, it is sometimes necessary to prove lower bounds for exponentially low success probabilities, a regime where the generalized adversary method might not be tight. A typical example is proving a direct product theorem, which essentially states that for solving k instances of a problem in parallel, one cannot do much better than solving each instance independently.
The goal of this talk will be to provide new techniques to tackle those limitations. More precisely, we will show that the multiplicative adversary method, a variation of the original adversary method, generalizes not only the generalized adversary method, but also the polynomial method, so that it essentially encompasses all known lower bound methods. Therefore, this provides a constructive approach to cast polynomial lower bounds into the adversary method framework. Moreover, since the multiplicative adversary method satisfies a strong direct product theorem and is at least as strong as the generalized adversary bound which is tight, this implies that the quantum query complexity of any function satisfies a direct product theorem.
Based on joint work with Andris Ambainis, Loïck Magnin, Martin Rötteler and Troy Lee
Measurement-Device-Independent Entanglement Witnesses
Speaker: Cyril Branciard, University of Queensland, Australia
Date: Friday, January 25, 2013, 11:00.
Abstract: In a recent paper, F. Buscemi claimed that “All entangled quantum states are nonlocal” [1]. I will start by clarifying what he meant by that, namely that any entangled state can bring some advantage over separable states in the context of “semi-quantum nonlocal games”, which use quantum states as inputs. While Buscemi noticed that this result could be used to certify entanglement, his proof does not provide any explicit recipe for that.
Inspired by his work, I will precisely show how, starting from any standard entanglement witness, one can construct an explicit criterion to witness entanglement in Buscemi's scenario with quantum inputs. Such criterions are independent of the measurements apparatuses used in the experiment, and therefore provide “Measurement-Device-Independent Entanglement Witnesses” (MDI-EWs) [2]. Examples will be given to illustrate our construction. Time permitting, I will extend Buscemi's results and our construction to a situation where classical communication is allowed between separated parties, and show that correlations from entangled states cannot be simulated classically [3].
[1] F. Buscemi, PRL 108, 200401 (2012).
[2] C. Branciard, D. Rosset, Y.-C. Liang, N. Gisin, arXiv:1210.8037.
[3] D. Rosset, C. Branciard, N. Gisin, Y.-C. Liang, arXiv:1211.3560.
Heisenberg-limited metrology without entanglement
Speaker: Daniel Braun, Laboratoire de Physique Théorique, Université Paul Sabatier, Toulouse, France
Date: Thursday, March 1st, 2012, 11:00.
Abstract: One of the most promising spin-offs of quantum information theory for science is the idea of using quantum information processing in order to increase the sensitivity of precision measurements. The major goal of these “Quantum Enhanced Measurements” (QEM) is to achieve the Heisenberg limit (HL) - a scaling of the sensitivity as 1/N with the number N of quantum resources. This would represent a major improvement over the standard quantum limit (SQL) in which the sensitivity scales as the inverse square root of N, with a corresponding huge interest for technological applications. However, despite about 30 years of efforts, the SQL has been surpassed only by very few experiments so far, and only for small values of N. Indeed, the standard protocols of QEM require highly entangled states that are typically very prone to decoherence, and are therefore unlikely to scale up to the large numbers of N required before QEM can compete with classical precision measurements.
In this talk, I show that by having the N quantum systems interact with a common “quantum-bus”, one can achieve Heisenberg limited sensitivity without using or ever creating entanglement. The quantum bus can be as simple as a single qubit, or as complex as an environment with many degrees of freedom over which we have only partial control and which creates therefore decoherence. The scaling with N is robust under local decoherence, and decoherence itself can be used as a signal that allows one to achieve the HL. I will present a general theory of this new kind of measurements, based on the fundamental Quantum Cramer-Rao bound, and illustrate the effect in some detail for the example of the measurement of the length of a cavity using superradiance.
Analytical estimate of realistic implementations of a QKD protocol
Speaker: Christina Giarmatzi, ENS Cachan, France
Date: Friday, July 8, 2011, 14:00.
Abstract: We studied realistic implementations of the BB84 protocol with imperfect single-photon source and detectors. After taking into account photon-number splitting attacks and the optimization of the single-photon source, we give a simple analytic formula giving the range of the protocol as a function of the single-photon source anticorrelation parameter and the dark-count probability.
Quantum Bit-Commitment and Coin Flipping in a Device-Independent Setting
Speaker: Jonathan Silman, LIQ, ULB
Date: Friday, May 13, 2011, 11:15.
Abstract: In the distrustful quantum cryptography model the parties have conflicting interests and do not trust one another. Nevertheless, they trust the quantum devices in their labs. The aim of the device-independent approach to cryptography is to do away with the latter assumption, and, consequently, significantly increase security. It is an open question whether the scope of this approach also extends to protocols in the distrustful cryptography model. In this talk, we show that for bit-commitment - one of the most basic primitives within the model - the answer is positive. We present a device-independent (imperfect) bit-commitment protocol, where Alice's and Bob's cheating probabilities are 0.854 and 0.75, which we then use to construct a device-independent coin with cheating probability no greater than 0.836.
Symmetry-assisted adversaries for quantum state generation
Speaker: Loïck Magnin, QuIC, ULB and LRI, Paris
Date: Friday, May 13, 2011, 14:15.
Abstract: We introduce a new quantum adversary method to prove lower bounds on the query complexity of the quantum state generation problem. This problem encompasses both, the computation of partial or total functions and the preparation of target quantum states. There has been hope for quite some time that quantum state generation might be a route to tackle the Graph Isomorphism problem. We show that for the related problem of Index Erasure our method leads to a lower bound of Omega(sqrt(N)) which matches an upper bound obtained via reduction to quantum search on N elements. This closes an open problem first raised by Shi [FOCS'02]. Our approach is based on two ideas: (i) on the one hand we generalize the known additive and multiplicative adversary methods to the case of quantum state generation, (ii) on the other hand we show how the symmetries of the underlying problem can be leveraged for the design of optimal adversary matrices and dramatically simplify the computation of adversary bounds. Taken together, these two ideas give the new result for Index Erasure by using the representation theory of the symmetric group. Also, the method can lead to lower bounds even for small success probability, contrary to the standard adversary method. Furthermore, we answer an open question due to Spalek [CCC'08] by showing that the multiplicative version of the adversary method is stronger than the additive one for any problem. Finally, we prove that the multiplicative bound satisfies a strong direct product theorem, extending a result by Spalek to quantum state generation problems.
Entanglement enhancement by photon addition and subtraction: a step towards solving the minimum entropy conjecture
Speaker: Carlos Navarrete-Benlloch, University of Valencia, Spain.
Date: Thursday, February 17, 2011, 16:00.
Abstract: In view of the recent progress towards on-demand photon addition and subtraction (see [1]), we have systematically studied how these operations, despite being local, can enhance the entanglement present in a two-mode vacuum squeezed state, which a the state customarily used in quantum protocols involving light. I will also explain our efforts towards a proof of the so-called “minimum entropy conjecture”, which states that the capacity of Gaussian single-mode communication channels such as lossy fibers or amplifiers is attained by using input coherent states [2]. I will show how this latter problem is related to the former one, and can be analytically addressed by exploiting the concept of majorization.
[1] A. Zavatta, V. Parigi, M. S. Kim, H. Jeong, and M. Bellini; Phys. Rev. Lett. 103, 140406 (2009).
[2] V. Giovannetti, S. Guha, S. Lloyd, L. Maccone, and J. H. Shapiro; Phys. Rev. A 70, 032315 (2004).
Quantum correlations, computation and (quantum) phase transitions
Speaker: Janet Anders, University College London, UK.
Date: Wednesday, April 28, 2010, 12:00.
Abstract: Quantum particles have the ability to be very closely correlated, a phenomenon called entanglement. I will first explain how its presence can assist in performing computational tasks. For instance, a calculator with limited logical operations is boosted beyond its own capabilities to a full power computer when given access to quantum correlated particles. Moreover, with the help of quantum correlations a single quantum particle can be used as an external control to evolve and measure any quantum system remotely.
While these are man made, externally controlled applications of quantum correlations one can wonder if existing natural processes exploit their power. Indeed quantum fields and other many-body models support entanglement at a macroscopic scale below a specific transition temperature. The role of the entanglement here is to help “squeeze” the energy below the classical level, by forcing collective behaviour. This links the presence of entanglement in natural systems to the occurrence of Bose-Einstein condensation and quantum phase transitions.
Quantum computing with collective ensembles of multi-level systems
Speaker: Etienne Brion, Laboratoire Aimé Cotton, Orsay.
Date: Friday, April 16, 2010, 11:00.
Abstract: We propose a new physical approach for encoding and processing of quantum information in ensembles of multilevel quantum systems, where the different bits are not carried by individual particles but associated with the collective population of different internal levels. One- and two-bit gates are implemented by collective internal state transitions taking place in the presence of an excitation blockade mechanism, which restricts the population of each internal state to the values zero and unity. Quantum computers with 10-20 bits can be built via this scheme in single trapped clouds of ground state atoms subject to the Rydberg excitation blockade mechanism, and the linear dependence between register size and the number of internal quantum states in atoms offers realistic means to reach larger registers. (Coauthors: L.H. Pedersen, K. Mølmer & M. Saffman.)
Fiber-Optic Quantum Communications and Information Processing
Speaker: Prem Kumar, Northwestern University, USA.
Date: Friday, March 26, 2010, 14:30.
Location: Solbosch, building C, 3rd floor, OPERA-Photonics
Abstract: Recognizing the ubiquitous standard optical fiber for long-distance transmission and the widespread availability of efficient active and passive fiber devices, there are significant efforts underway to develop practical resources for quantum communications and information processing in optical fiber networks. Entanglement, which refers to the nonclassical dependency of physically separated quantum systems, is one such resource that is essential for implementing many of the novel functions of quantum information processing. Therefore, the efficient generation and distribution of quantum entanglement in fiber optical systems is of prime importance. Entanglement has been historically produced by use of the spontaneous parametric down-conversion process in second-order nonlinear crystals, wherein one higher-frequency pump photon splits into two lowerfrequency daughter photons which can be entangled. Coupling such down-converted photons into optical fibers without degrading entanglement, however, has remained a challenging task. Fortunately, the prospects for ready availability of entanglement in the telecom band have dramatically improved in the last few years by the emergence of a new technique for generating entanglement directly in the fiber itself. This technique utilizes the Kerr nonlinearity of standard optical fiber to produce quantum correlated photons through the spontaneous four-wave mixing process. The correlated photons can be entangled in various ways by incorporating indistinguishable pathways in the fourwave mixing amplitude. In this lecture, I will review the status of this field by describing recent experiments that demonstrate the generation and distribution of quantum entanglement is wave-division multiplexed optical fiber systems. I will also present some recent results on utilizing such entanglement for quantum communications and information processing tasks.
Block entropy analysis by lumping: Measuring the uncomputable
Speaker: Kostas Karamanos, University of Athens, Greece.
Date: Monday, January 25, 2010, 12:00.
Abstract: A given finite sequence of letters over a finite alphabet can always be algorithmically generated, in particular by a Turing machine. This fact is at the heart of complexity theory in the sense of Kolmogorov and Chaitin. A relevant question in this context is whether, given a statistically sufficiently long sequence, there exists a deterministic finite automaton that generates it. We will present a simple criterion, based on measuring block entropies by lumping, which is satisfied by all automatic sequences. On the basis of this, one can determine that a given sequence is not automatic and obtain interesting information when the sequence is automatic. Following previous work on the Feigenbaum sequence, we give a necessary entropy-based condition valid for all automatic sequences read by lumping. Applications of these ideas to representative examples are discussed. In particular, we establish new entropic decimation schemes for the Thue-Morse, the Rudin-Shapiro and the paperfolding sequences read by lumping.
A pictorial approach to measurement-based quantum computing
Speaker: Ross Duncan, University of Oxford, UK.
Date: Thursday, November 19, 2009, 16:00.
Abstract: Recent work in the foundations of quantum mechanics carried out in Oxford has emphasised the role that algebraic structures - hidden in Hilbert space - play in various stereotypically quantum behaviours. I will demonstrate the role that these structures can play in elucidating the most “intrinsically quantum” computing model, that of measurement based quantum computation via a series of diagrammatic proofs demonstrating: the equivalence of various entangled quantum states, and correctness of various measurement-based quantum computations.
Methods for estimating transmission characteristics of Gaussian quantum channels with memory
Speaker: Oleg Pilyavets, P. N. Lebedev Physical Institute, Moscow, Russia.
Date: Friday, November 13, 2009, 11:00.
Abstract: Methods for analytical and numerical estimation of classical capacity for lossy bosonic Gaussian channel with memory are proposed. The methods above are also demonstrated to be useful for calculating of achievable rates. New phenomena namely, violations of quadrature and mode symmetries for classical capacity are explained. The role of channel memory is analyzed. Relation between quantum channels problems and quantum tomography (probability representation) is discussed for continuous variable and spin systems.
Proof of principle of a new algorithm for factoring arbitrary numbers with a single recorded interferogram
Speaker: Vincenzo Tamma, University of Maryland, Baltimore, USA.
Date: Tuesday, October 20, 2009, 12:00.
Abstract: We factorize, in a single experiment, up to a eight-digits number, using a new factorization algorithm, based on the polychromatic optical interference in a continuous truncated exponential sum (CTES) interferometer. We demonstrate that this algorithm allows, in principle, to factorize arbitrary numbers exploiting the periodicity of a single recorded CTES interferogram.
Dissipation in Laplacian fields across irregular boundaries
Speaker: Kostas Karamanos, University of Athens, Greece.
Date: Wednesday, August 12, 2009, 12:00.
Abstract: The entropy production associated to a Laplacian field distributed across irregular boundaries is studied. In the context of the active zone approximation an explicit expression is given for the entropy production in terms of geometry, whose relation to the variational formulation is discussed. It is shown that the entropy production diminishes for successive prefractal generations of the same fractal generator, so that the final fractal object is expected to dissipate less than all previous ones. The relevance of this result in the abundance of fractal surfaces or interfaces observed in nature is discussed.
Positive Wigner functions and uncertainty relation in the space of mixed states
Speaker: Aikaterini Mandilara, QuIC.
Date: Friday, June 19, 2009, 12:00.
Abstract: According to Hudson's theorem, any pure quantum state with a positive Wigner function is necessarily a Gaussian state. In addition, it is well known that Gaussian states are the only pure states which satisfy the Heisenberg uncertainty relation. In this talk, recent results will be presented on the extension of Hudson's theorem to mixed one-dimensional quantum states with a positive Wigner function. These results permit us to reproduce and extend the “Purity-bounded uncertainty relationship” for mixed quantum states and shed light on the connection of these two fundamental problems. Possible applications to the problem of generating non-Gaussian states with non-positive Wigner functions in continuous-variable quantum information will be also presented.
Repeatable quantum memory channels
Speaker: Tomas Rybar, Research Center for Quantum Information, Slovak Academy of Sciences, Slovakia.
Date: Friday, June 12, 2009, 10:00.
Abstract: Within the framework of quantum memory channels, we introduce the notion of repeatability of quantum channels. In particular, a quantum channel is called repeatable if there exists a memory device implementing the same channel on each individual input. We show that random unitary channels can be implemented in a repeatable fashion, whereas the nonunital channels cannot.
On continuous variable quantum algorithms for oracle identification problems
Speaker: Barry Sanders, iCORE Chair of Quantum Information Science, University of Calgary, Canada.
Date: Tuesday, June 9, 2009, 15:00.
Abstract: We establish a framework for oracle identification problems in the continuous variable setting, where the stated problem necessarily is the same as in the discrete variable case. Continuous variables are manifested through a continuous representation in an infinite-dimensional Hilbert space. We apply this formalism to the Deutsch-Jozsa problem and show that, due to an uncertainty relation between the continuous representation and its Fourier-transform dual representation, the corresponding Deutsch-Jozsa algorithm is probabilistic hence forbids an exponential speed-up.
Capacity of a Bosonic channel with Gauss-Markov memory
Speaker: Joachim Schäfer, QuIC.
Date: Thursday, April 30, 2009, 14:30.
Abstract: We present the model of a Gauss-Markov memory and derive the capacity of a classical and a quantum channel with such a memory: the Gaussian additive-noise channel and a bosonic Gaussian noise channel. For both cases, we show that the capacity is found by a so-called “water filling” solution, and present explicit results in certain input power regions. Finally, we show that the results for the quantum channel translate in the classical limit to the results for its classical counterpart.
Coherent-state quantum key distribution with multiletter alphabets
Speaker: Denis Sych, Max Planck Institute for the Science of Light, Erlangen, Germany.
Date: Friday, April 17, 2009, 12:00.
On Symmetric Informationally-Complete Positive-Operator Valued Measures (SIC POVMs)
Speaker: Thomas Durt, TONA, VUB.
Date: Friday, June 27, 2008, 12:00.
Abstract: SIC POVMs present interesting applications in Quantum Tomography. Covariant SIC POVMs under the Heisenberg-Weyl group are of particular interest and have been the object of intensive research. They seemingly exist for all finite dimensions, although most often their existence was merely established by numerical methods. Moreover they satisfy a property that was conjectured by Zauner, the so-called Zauner's conjecture. As has been shown by Appleby and Fuchs, in prime dimensions p, they minimize the quadratic Renyi entropy averaged over a maximal set of p+1 Mutually Unbiased Bases.
Thick Quantum Hologram
Speaker: Denis Vasilyev, V. A. Fock Physics Institute, St.-Petersburg, Russia.
Date: Thursday, April 24, 2008, 15:00.
Quantum compatible information as an effective tool for physical problems
Speaker: Denis Sych, Max-Planck Research Group, Institute of Optics, Information and Photonics, University of Erlangen-Nuremberg, Germany.
Date: Tuesday, February 19, 2008, 10:00.
Abstract: We present a special measure of quantum information - quantum compatible information. It is shown that it can be effectively used in analysis of various physical problems. We discuss the basic ideas and properties of quantum compatible information, along with several examples of its application, including analysis of structure and properties of bipartite quantum states, calculation of quantum information exchange between dipole-dipole interacting atoms or molecules, improving of quantum key distribution protocols, etc.
Quantum gates by adiabatic passage
Speaker: Xavier Lacour, Université de Bourgogne, Dijon, France.
Date: Friday, June 29, 2007, 12:00.
Abstract: Adiabatic passage is a powerful technique that allows the manipulation of atomic populations in a robust way, in the sense that it is insensitive to fluctuations of experimental parameters or to partial knowledge of the system. We will present some new strategies to implement universal sets of gates, based on adiabatic processes of STIRAP type. We will also present some designs that allow the implementation of specific more complex operations, in a faster way than by the composition of elementary gates of a universal set.
Phase space methods for discrete quantum systems
Speaker: David Gross, Imperial College, UK.
Date: Wednesday, April 25, 2007, 11:00.
Abstract: In this talk, I will highlight the similarities between various structures for discrete and continuous variable systems. All these objects have connections to (discrete or continuous) phase spaces. E.g. we will see that stabilizer states are a natural analogue of Gaussian pure states in that both a) have state vectors described by quadratic forms, b) are the unique pure states to allow for non-negative Wigner functions c) minimize suitable entropic uncertainty relations d) exhibit non-trivial entanglement structures despite having an efficient classical description. Depending on the interests of the audience, I can address the relation of Gaussian operations with the Clifford group, properties of discrete Wigner functions or potential shortcomings of the analogy for mixed states.
On quantum de Finetti theorems
Speaker: Robert Koenig, CQC, University of Cambridge, UK.
Date: Monday, March 19, 2007, 16:30.
Abstract: In its most basic form, the finite quantum de Finetti theorem states that the reduced k-partite density operator of an n-partite symmetric state can be approximated by a convex combination of k-fold product states. I will discuss various forms of this statement including Renner's “exponential” approximation by “almost-product”-states, the highest-weight theorem which deals with certain triples of representations of the unitary group, and D'Cruz et al.'s result for infinite-dimensional systems. I will show how these theorems follow from a single de Finetti-type statement when applied to the corresponding symmetry group. This gives some insight into the nature of the set of approximating states, and leads to a new “exponential” theorem for coherent states. (Joint work with Graeme Mitchison.)
Limites quantiques de la résolution optique et l'information à priori
Speaker: Mikhail Kolobov, PhLAM, Université de Lille 1, France.
Date: Friday, January 19, 2007, 11:00.
Search via quantum walk
Speaker: Jérémie Roland, QuIC.
Date: Friday, December 8, 2006, 14:00.
Abstract: By suppressing details, any search problem may be cast as the problem of finding a “marked” element from a set X with n elements. Let M^X be the set of marked elements. One approach to finding from M, if it is not empty, is to repeatedly sample from X uniformly until a marked element is picked. A more cost-effective approach re-uses resources expended in generating the first sample (time, random bits, black-box queries, etc.) by simulating the steps of a Markov chain with state space X to generate the next sample. This approach often takes advantage of some structure present in the ground set X and the Markov chain, and leads to a more efficient algorithm. In this talk, we study quantum analogues of this randomized scheme.
In particular, we propose a new method for designing quantum search algorithms for finding a “marked” element in the state space of a classical Markov chain. The algorithm is based on a quantum walk à la Szegedy [FOCS'04, pp 32-41] that is defined in terms of the Markov chain. The main new idea is to apply quantum phase estimation to the quantum walk in order to implement an approximate reflection operator, which is used in an amplitude amplification scheme.
(Joint work with F. Magniez, A. Nayak and M. Santha.) arXiv: quant-ph/0608026
Casimir-Polder intermolecular forces
Speaker: Roberto Passante, University of Palermo, Italy.
Date: Friday, June 16, 2006, 9:00.
Hyper-entangled two-photon states for quantum communication applications
Speaker: Marco Barbieri, Quantum Optics Group, Department of Physics, University of Roma “La Sapienza”, Italy.
Date: Monday, May 8, 2006, 11:00.
Abstract: Two photon states entangled in polarization and momentum, hyper-entangled, have been generated by using linear optics and a single Type I nonlinear crystal. These states have been completely characterized and their nonlocal behaviour has been verified by independent Bell's inequalities tests performed in the two degrees of freedom of entanglement and by an “all versus nothing” test of local realism. The manipulation of these states may represent a useful control in quantum state engineering and Bell state measurements and, more in general, in Quantum Information applications. (Joint work with F. De Martini and P. Mataloni.)
Quantum realizability criteria for joint probabilities
Speaker: Stefano Pironio, ICFO, Barcelona, Spain.
Date: Thursday, April 20, 2006, 12:00.
Abstract: We introduce a family of criteria for a distribution of joint probabilities P(ab|xy) to be interpreted as the probabilities for two separate observers to obtain outcomes a and b when making local measurements x and y on a shared quantum state. Each test in the family is a semidefinite program, a particular convex optimization problem for which powerful computational methods exist. These tests can be ordered in a hierarchical structure, each test being at least as powerful as the preceding one. The hierarchy is complete in the sense that any non-quantum probability distribution necessarily fails a test at some point in the hierarchy. Our approach can be used to obtain upper-bounds on the quantum violation of arbitrary Bell inequalities. It yields, for instance, tight bounds for the violation of the Collins et al inequalities.
Local equivalence of stabilizer states
Speaker: Maarten Van den Nest, ESAT, KULeuven.
Date: Wednesday, December 21, 2005, 11:00.
Abstract: Stabilizer states are multi-party quantum states that are of interest in several important applications in quantum information theory, such as quantum error-correction and measurement-based quantum computation. It is well known that stabilizer states have a high degree of genuine multi-party entanglement, and that this entanglement is one of the key ingredients responsible for the success with which stabilizer states are used in their applications. Therefore, a detailed study of the multipartite entanglement properties of stabilizer states is called for. When considering the entanglement properties of stabilizer states, a natural subject is a study of the action of local operations on stabilizer states and a classification of stabilizer states under local equivalence. In our PhD we have performed a systematic study of this topic and in this talk an overview of our work is given. We provide answers to basic questions in the context of local equivalence of stabilizer states, such as: when are two stabilizer states equal up to a local transformation? Can one efficiently decide whether two stabilizer states are locally equivalent? What are the relevant local invariants? How is the action of a local transformation on a graph state translated into an action on the underlying graph? A central, very recent result is a proof that the notions of SLOCC equivalence, local unitary equivalence and local Clifford equivalence all coincide for stabilizer states, showing that there is in fact only one type of local equivalence of stabilizer states. The proof that local unitary equivalence of stabilizer states coincides with local Clifford equivalence (joint work with David Gross, Imperial College, London) has been obtained very recently and settles a conjecture that has been around for several years.
Adiabatic passage for quantum information
Speaker: Stephane Guerin, Laboratoire de Physique, Université de Bourgogne, Dijon, France.
Date: Friday, December 16, 2005, 15:00.
Abstract: Adiabatic passage techniques for the control of atomic and molecular processes by laser and cavity fields will be presented. I will then discuss applications to quantum information processes such as the generation of photon number states on demand, entangled states and quantum gates in a cavity.
Nonsignalling correlations and Quantum Mechanics
Speaker: Nick Jones, QUIC and University of Bristol, UK
Date: Wednesday, November 23, 2005, 14:30.
Abstract: I will discuss some specific results which suggest that there might be a unit resource of all bipartite nonlocality. I will also try to give the audience a broad feel for nonsignalling theories and why they should be interesting to physicists. I will then mention some areas which I am investigating at the moment. This will involve a discussion of some of the following: the dynamics of nonsignalling distributions, using nonsignalling resources to simulate quantum states, and ideas concerning distillation and causality.
Simuler un POVM à l'aide de boîtes non locales et de communication classique
Speaker: Julien Degorre, Laboratoire de Recherche en Informatique (LRI), Orsay, France
Date: Friday, April 22, 2005, 14:30.
Abstract: Une paire de boîtes non locales est un gadget qui prend deux bits (x et y) en entrée et qui produit deux bits (a et b) en sortie, de telle sorte que la parité de a et b est toujours égale à la conjonction de x et y. La magie, bien sûr, vient du fait qu'une des boîtes produit a à partir de x seulement alors que l'autre produit b à partir de y, et que ces deux boîtes ne communiquent pas! Impossible, direz-vous? Eh bien vous avez raison même si les boîtes partagent une quantité illimitée d'intrication! Une utilisation de cette ressource non locale est strictement plus faible que la communication d'un bit classique. On sait pourtant que toute paire de mesures projectives (von Neumann) sur une paire EPR partagée peut être simulée par une seule utilisation d'une paire de boîtes non locales (Cerf-Gisin-Massar-Popescu-04). Qu'en est-il de la simulation d'une paire de POVM sur un paire EPR partagée? (Simulation qu'on sait déjà; faire par la transmission espérée de 6 bits classique (Methot-03).) Je présenterai un résultat, qui répond à cette question si on permet également un peu de communication classique. Aprés avoir introduit les boîtes non locales, je présenterai un protocole qui permet de simuler les POVM avec, en moyenne, 4 bits de communication et 2 utilisations de boîtes non locales. Le fait d'utiliser les boîtes non locales rend à mon goût le protocole plus joli, plus symétrique, et pourrait donner une nouvelle intuition sur le probléme. Le compromis à faire entre le nombre d'utilisations de boîtes non locales et le nombre de bits de communication sera discuté, ainsi que le rôle de chacune de ces ressources.
Key distillation from Gaussian states by Gaussian operations
Speaker: Miguel Navascués, ICFO, Barcelona, Spain
Date: Friday, January 21, 2005, 10:00.
Macroscopic Phenomena of Quantum Entanglement.
Speaker: Caslav Brukner, University of Vienna, Austria
Date: Tuesday, December 21, 2004, 14:30.
Abstract: It is commonly believed that for the understanding of the behaviour of large, macroscopic, objects there is no need to invoke any genuine quantum entanglement. We show that this belief is fundamentally mistaken and that entanglement is crucial to correctly describe some macroscopic properties of solids. We demonstrate that macroscopic thermodynamical properties - such as internal energy, heat capacity or magnetic susceptibility - can detect quantum entanglement in solids in the thermodynamical limit even at nonzero temperatures. We identify the parameter regions (critical values of magnetic field and temperature) within which entanglement is witnessed by these thermodynamical quantities. Finally, we demonstrate that two different experiments performed in 1963 and in 2000 clearly and conclusively indicate that entanglement exits in macroscopic samples of Cooper Nitrate at temperatures below 5 Kelvin.
Lossless quantum compression.
Speaker: Caroline Rogers, University of Warwick, UK
Date: Tuesday, December 14, 2004, 14:00.
Abstract: Classical lossless compression is an everyday application for compressing files so that they can be stored efficiently on a hard disc or sent efficiently in a compressed form over the internet. Lossless classical compression uses a variable length code to encode commonly occurring strings with short code words, reducing the length of the encoding on average to the Shannon entropy of the source of the strings. If two quantum strings in superposition are encoded with a variable length code, the encoded string may be in a superposition of two different lengths. Thus, when mixtures of non-orthogonal quantum strings are encoded with variable length codes, the encoded states may have indeterminate lengths. For lossless coding, the entire superposition must be left intact and the longest part of the superposition is used to measure the length of the encoded string. The minimal average length of an encoding for a mixture is called the lossless entropy of the mixture. The lossless entropy of a mixture can be different for two mixtures with the same density operator and decreases towards the von Neumann entropy when the mixtures represent good purifications. Lossless entropy does not represent any obseravable in a quantum system, however many open questions such as entanglement catalysis try to transform one state into another losslessly subject to some conditions. Perhaps lossless entropy might be useful in tackling some of these open questions.
Extremal covariant maps and optimal cloning of a couple of mutually unbiased bases
Speaker: Giulio Chiribella, University of Pavia, Italy
Date: Monday, November 22, 2004, 14:00.
A general solution to the Mean King's problem in prime power dimensions, Bell states and mutually unbiased bases
Speaker: Thomas Durt, TONA, VUB
Date: Wednesday, November 17, 2004, 14:00.
Abstract: When the dimension N of a finite dimensional Hilbert space is a prime power p^m (with p a prime number, and m a positive integer) it is possible to obtain a set of N+1 mutually unbiased bases. They generalize the X, Y, and Z qubit bases, and are associated to a discrete group of unitary transformations that generalizes the sigma operators of Pauli. This group, which is a discrete version of the Heisenberg-Weyl group (sometimes called Pauli group), contains N2</sup> elements which are in one to one correspondence with (so-called) Bell states; it is generated by discrete realisations of spatial translations and Galilean boosts. When the dimension N is a prime power pm, there exists a finite field of N elements. We derive a new way to express the mutually unbiased bases in terms of the (operations of the) associated finite field of N elements. We also derive the transformation law for the Bell states when a basis transforms into a mutually unbiased basis. In prime dimensions, this reduces to a well-known property of the Jacobi or Clifford group. All this leads to a new solution to the Mean King's problem in arbitrary prime power dimensions, that is, it provides a way to ascertain the values of N+1 observables that are maximally complementary in the sense that they are diagonal in mutually unbiased bases.
The Second Law and the Holevo Bound
Speaker: Koji Maruyama, QUIC
Date: Monday, November 8, 2004, 14:00.
Abstract: Thermodynamics is a macroscopic physical theory whose two very general laws are independent of any underlying dynamical laws and structures. Nevertheless, its generality enables us to understand a broad spectrum of phenomena in physics, information science, and biology. Recently, it has been realised that information storage and processing based on quantum mechanics can be much more efficient than their classical counterpart. What general bound on storage of quantum information does thermodynamics imply? We show that thermodynamics implies a weaker bound than the quantum mechanical one (the Holevo bound). In other words, if any post-quantum physics should allow more information storage, it could still be under the umbrella of thermodynamics.
Entanglement, Critical Phenomena, and RG Flows
Speaker: Enrique Rico Ortega, University of Barcelona
Date: Tuesday, October 26, 2004, 14:00.
Abstract: A microscopic calculation of ground state entanglement for several spin models shows the emergence of universal scaling behavior at quantum phase transitions. Entanglement is thus controlled by conformal symmetry. Away from the critical point, entanglement gets saturated by a mass scale. We also explore entanglement loss along renormalization group trajectories which is seen to follow from a rigid reordering of the eigenvalues of the reduced density matrix for the spin block.
Zeno effect and non-decaying subspaces in unstable quantum systems
Speaker: Evgueni Karpov, Instituts Internationaux de Physique et de Chimie Solvay
Date: Friday, October 15, 2004, 14:00.
Abstract: Non-exponential short-time behaviour of the survival probability of quantum unstable states is the origin of the Zeno effect (slowing down the decay by measurement). It is one of potential means for struggling decoherence in quantum computations. We have studied this effect in the frame of the Friedrichs-Fano model for different form factors. We have shown that this probability is not necessarily analytic at the time origin. The time when the quantum-Zeno effect could be observed is found to be much smaller than usually estimated. We examine and compare different definitions of the Zeno time. We have also have estimated the duration of the anti-Zeno period. In the framework of the N-level Friedrichs model, we demonstrate that the Zeno era can be considerably enlarged by proper choice of the system parameters. We show that in degenerated and nearly degenerated systems the decay can be considerably slowed down or even stopped by appropriate choice of initial conditions.
Variational perturbation theory
Speaker: Axel Pelster, University of Essen, Germany
Date: Thursday, September 30, 2004, 14:00.
Abstract: The talk provides an introduction to variational perturbation theory, which converts divergent weak-coupling series into convergent strong-coupling series. We elaborate this theory by approximating the ground-state energy of the anharmonic oscillator for all coupling strengths. Then, we determine analytically the proportionality constant in the pressure law of a membrane which thermally fluctuates between two walls. Furthermore, we determine the quantum statistical effective classical potential of the hydrogen atom in a homogeneous magnetic field for all temperatures and field strengths. Finally, we locate the quantum phase transition for a homogeneous Bose gas in the plane of s-wave scattering length a_s and temperature T.
Experimental quantum key distribution over highly noisy channels
Speaker: Louis-Philippe Lamoureux, QUIC
Date: Thursday, September 16, 2004, 12:30.
Abstract: We present an experiment which enables two parties to perform secure quantum key distribution over a noisy communication channel. The security of the key distribution is achieved by using a method called “error filtration”, which makes it possible to reduce the quantum bit error rate from a value where the key distribution is insecure to a value where it is provably secure. The idea behind error filtration is multiplexing, i.e. using more transmission channels than necessary to send a quantum state. We introduce the idea by giving an intuitive approach, then move on to present an optical implementation of error filtration, and conclude with some general remarks.
On the multiplicativity of the maximal output purities of Gaussian channels
Speaker: Alessio Serafini, University of Salerno, Italy
Date: Tuesday, May 11, 2004, 12:30.
Abstract: We address the question of the multiplicativity of the maximal output purity (evaluated by p-norms) of bosonic Gaussian channels under Gaussian inputs. Such a question amounts to asking whether, given a tensor product of “parallel” channels, input entanglement can help to better preserve the coherence of the global output state. We consider products of single-mode channels resulting from symplectic dilations (i.e. from quadratic hamiltonians acting on larger Hilbert spaces). We show that the maximal output purity under Gaussian inputs is multiplicative for any p>1 on a large class of product channels and for p=2 on all channels consisting of products of single-mode channels. Our results strongly suggest the general multiplicativity of the Gaussian maximal output purity for any p and tensor products of single mode channels.
Quantum correlations: knowledge excesses and Bell inequalities
Speaker: Radim Filip, Palacky University, Olomouc, Czech Republic
Date: Wednesday, May 5, 2004, 12:30.
Abstract: A constraint restricting two complementary knowledge excesses measured on a single copy of a mixed state of two qubits (signal S and meter M) is derived. The complementary knowledge excesses quantify the enhancement of ability to predict a result of a pair of complementary projective measurements on the qubit S utilizing data from the projective measurements performed on the qubit M. For any mixed state and for arbitrary measurements, the knowledge excesses are bounded by maximum of the violation of Bell inequalities under single-copy local operations. This result is experimentally demonstrated on two-photon Werner states prepared by means of spontaneous parametric down-conversion.
Programmable quantum multimeters and their optical implementations
Speaker: Miloslav Dusek, Palacky University, Olomouc, Czech Republic
Date: Monday, April 26, 2004, 11:00.
Abstract: Programmable quantum multimeters are devices that can realize any generalized quantum measurement from a chosen set (either exactly or approximately). Their main feature is that the desired positive operator valued measure is selected by the quantum state of a program register. In particular, programmable quantum devices that can accomplish any projective measurement on a single qubit are described. There are two limit cases: Deterministic devices giving an erroneous result from time to time and probabilistic devices that operate error-free but sometimes lead to an inconclusive result. Another example of a quantum multimeter is a programmable device for unambiguous discrimination of pairs of non-orthogonal states. Simple optical implementations of some of these devices are proposed and experimental results are presented.
Thermodynamical Detection of Entanglement by Maxwell's Demons (and some demonic implications)
Speaker: Koji Maruyama, Imperial College, UK
Date: Monday, April 19, 2004, 12:30.
Abstract: Entangled states are said to be more correlated than classically correlated states. Besides tasks in information processing, can we make use of this excess correlation in entanglement to do something physically useful? Generalising classical Szilard's engine, we show we can devise a thermodynamical process that reveals the usefulness of entanglement. The idea leads to the “thermodynamical separability criterion”, which can be seen as a kind of entanglement witness with locally observable quantities. We will also give an analysis on whether we can derive results in quantum information theory by taking an axiomatic approach from the Second Law of thermodynamics, combining with the above process.
The no-signalling condition and key distribution
Speaker: Jonathan Barrett, QUIC
Date: Wednesday, March 17, 2004, 12:30
Abstract: It is well known that measurements performed on spatially separated, entangled quantum systems can give rise to correlations that are non-local, in the sense of violating a Bell-type inequality. It is also known that these correlations can be a useful resource, e.g., in communication complexity scenarios. In this talk, we describe a new use for non-local correlations. They can be used as the basis for a key distribution protocol that is secure even against an eavesdropper who can break the laws of quantum mechanics, as long as superluminal signalling is impossible. In particular, we show that (i) quantum correlations can give security against individual attacks by such eavesdroppers and (ii) a hypothetical set of correlations, more non-local than quantum mechanics allows, can give security against collective attacks by such eavesdroppers. We leave open the question of whether a quantum protocol with security against collective attacks exists.
Frequency down-conversion through Bose condensation of light
Speaker: Patrick Navez, QUIC
Date: Tuesday, February 10, 2004, 12:00
Abstract: We propose an experimental setup allowing to convert an input light of wavelengths about 1-2 micron into an output light of a lower frequency. The basic principle of operating relies on the nonlinear optical properties exhibited by a microcavity filled with a photonic bandgap material. The light inside this material behaves like an interacting Bose gas susceptible to reach thermal equilibrium and create a Bose-Einstein condensate. Theoretical estimations show that, using a fiber grating, a conversion of a 1-micron into a 1.5-micron wavelength is achieved with an input pulse of about 1 ns and characterized by a peak power of 10^3 W.
Kinetic Theory of a Bose Condensed Gas
Speaker: Patrick Navez, QUIC
Date: Friday, January 23, 2004, 14:30
Abstract: From the microscopic theory, we derive a number conserving quantum kinetic equation, valid for a dilute Bose gas at any temperature, in which the binary collisions between the quasi-particles are mediated by phonon-like excitations (called ``condenson''). This different approach starts from the many-body Hamiltonian of a Boson gas and uses, in an appropriate way, the generalized random phase approximation (resumation of the ring diagramms). As a result, the collision term of the kinetic equation contains higher-order contributions in the expansion in the interaction parameter. This different expansion shows up that a scattering involves the emission and the absorption of a phonon-like excitation. The major interest of this particular mechanism is that, in a regime where the condensate is stable, the collision process between condensed and non condensed particles is totally blocked due to a total annihilation of the mutual interaction potential induced by the condensate itself. As a consequence, the condensate is not constrained to relax and can be superfluid. Furthermore, a Boltzmann-like H-theorem for the entropy exists for this equation and allows to distinguish between dissipative and non dissipative phenomena (like vortices). We also illustrate the analogy between this approach and the kinetic theory for a plasma, in which the excitations correspond precisely to a plasmon. Finally, we show the equivalence of this theory with the non-number conserving Bogoliubov theory at zero temperature.
Two aspects of non-locality: classical production cost and extremal no-signalling correlations.
Speaker: Stefano Pironio, QUIC
Date: Friday, December 19, 2003, 12:00
Abstract: Considering non-locality as a resource, we examine two questions. First, what is the cost of producing it by classical means, i.e., with communication? Second, what are the maximal non-local correlations?
An experimental implementation of quantum coin tossing
Speaker: Louis-Philippe Lamoureux, QUIC
Date: Thursday, December 11, 2003, 12:00
Interaction, entanglement, and the classical limit
Speaker: Thomas Durt, TONA, Vrije Universiteit Brussel
Date: Tuesday, November 25, 2003, 12:00
Abstract: We briefly define quantum entanglement and present a troubling property associated with it: non-locality. We show that two interacting systems that are initially non-entangled remain so during the interaction provided that the Hamiltonian does not couple them to bi-orthogonal states. Then, we prove that non-entangling Hamiltonians (for all non-entangled states) are de facto separable; in other words, there is no interaction without entanglement. Finally, we show that, in simple cases, the non-entangling regime corresponds to our classical conceptions about what physicists call particles. This confirms the predictability sieve criterion of Zurek, defined in the framework of the decoherence approach, according to which classical islands correspond to the regions of the Hilbert space where the (quantum) information of the reduced system is maximal.
Special QUIC seminar
Speaker: Thomas Durt, TONA, Vrije Universiteit Brussel
Date: Wednesday, October 8, 2003, 12:00
Abstract: For this special QUIC seminar, Thomas Durt will comment the video tape of a program on classical (but also quantum!) cryptography that was broadcast by the Vlaamse Television (Canvas, VRT) in the beginning of 2003. This program (called “Over Leven”) stars people from the VUB (including Thomas Durt) and also made a few shots in the ULB optics lab where quantum information experiments are being carried out.
Statistical algorithmic compressibility analysis of DNA sequences. Towards algorithmic laws for Biology?
Speaker: Kostas Karamanos, CENOLI, ULB
Date: Monday, September 22, 2003, 12:00
On the existence of mutually unbiased bases in arbitrary dimensions
Speaker: Claude Archer, Service de Mathématique, Ecole Polytechnique, ULB
Date: Friday, September 12, 2003, 12:00
Entanglement and spin squeezing in a quantum phase transition
Speaker: Guillaume Palacios, Groupe de Physique des Solides, Université Pierre et Marie Curie, Jussieu, France
Date: Wednesday, August 20, 2003, 3 pm
Cloning the Entanglement of Quantum States
Speaker: Louis-Philippe Lamoureux, QUIC
Date: Wednesday, June 18, 2003, 12:00
Non-Gaussian Cloning of Gaussian States
Speaker: Patrick Navez, QUIC
Date: Wednesday, June 11, 2003, 12:00
Quantum coin tossing in the presence of noise
Speaker: Jonathan Barrett, QUIC
Date: Tuesday, April 8, 2003, 12:00
Abstract: Although quantum coin tossing is known to be impossible in the ideal case, quantum mechanics can offer levels of partial security that are impossible classically without invoking extra assumptions. Hence it is useful to investigate the possibility of carrying out quantum coin tossing in realistic situations where noise will be present. The presence of noise, however, turns out to have considerable security implications that require a rethinking of the problem. This leads us to consider the generation of random n-bit strings, as opposed to single-shot coin tosses. We will consider an explicit protocol for the generation of random n-bit strings and show that security, guaranteed by the laws of physics alone, can be obtained even when noise is present. The amount of security is quantitatively related to noise levels.
Construction of a Shared Secret Key Using Continuous Variables
Speaker: Jean Cardinal (Computer Science Departement, ULB) and Gilles Van Assche (QUIC)
Date: Tuesday, March 11, 2003, 12:00
Abstract: Motivated by recent advances in quantum cryptography with continuous variables, we study the problem of extracting a shared digital secret key from two correlated real values. Alice has access to a real value XA, and Bob to another value XB such that I(XA; XB)>0. They wish to convert their values into a shared secret digital information while leaking as little information as possible to Eve. We show how the problem can be decomposed in two subproblems known in other contexts. The first is the design of a quantizer that maximizes a mutual information criterion, the second is known as coding with side information.
An Introduction to Godel's Incompleteness Theorem
Speaker: Bruno Marchal, IRIDIA, ULB
Date: Friday, 14 December, 2001, 2:30 pm
Abstract: We propose a relatively simple proof of Godel's incompleteness theorem. The proof will give some conceptual motivation for the Church Turing Thesis, and provide clues on why universal computer are “really” universal. We will end up with suggestions about a possible application on Universal Quantum Computing.
Quantum game theory
Speaker: Jiangfeng Du, University of Science and Technology of China, Hefei, P.R. China
Date: Wednesday, 5 December, 2001, 2:30 pm
Abstract: Game theory is a theory of decision making, which provides powerful tools for investigating situations in which several parties make desisions according to their personal interest. I will give an account of how the parties would decide in a situation which involves contest, rivalry, or struggle. Such game-theory problems have been found to be relevant to social science, biology, or economics. Game theory was born in 1944 with the publication of the book “Theory of Games and Economic Behavior” by J. Von Neumann and O. Morgenstern. Recently, game theory was extended to the quantum world, and quantum strategies were discussed and shown to be powerful. In this talk, I will focus on the Prisoner's Dillema, a famous example in classical game theory, and I will introduce some recent results on quantum games.
Bell inequalities for arbitrarily high dimensional systems
Speaker: Serge Massar, QUIC
Date: Wednesday, 26 September, 2001, 16:00
Abstract: We develop a novel approach to Bell inequalities based on a constraint that the correlations exhibited by local realistic theories must satisfy. This is used to construct a family of Bell inequalities for bipartite quantum systems of arbitrarily high dimensionality which are strongly resistant to noise. In particular our work gives an analytic description of numerical results of D. Kaszlikowski, P. Gnacinski, M. Zukowski, W. Miklaszewski, A. Zeilinger, Phys. Rev. Lett. 85, 4418 (2000) and T. Durt, D. Kaszlikowski, M. Zukowski, quant-ph/0101084, and generalizes them to arbitrarily high dimensionality.
A “mechanist” approach to the foundations of quantum mechanics
Speaker: Bruno Marchal, IRIDIA, ULB
Date: Tuesday, 22 May, 2001, 14:00
Abstract: The mechanist hypothesis makes possible self-duplication. This entails a form of indeterminacy from the point of view of the person who undertakes that self-duplication experiment. A comparison with some other form of physical indetermininacy will be proposed. In particular, we will show (without going into too much details) how to quantify that indeterminacy and how Hilbert spaces appears in that setting.
Some interesting properties of quTrits in relation with non-locality and quantum cryptography
Speaker: Thomas Durt, TONA, Vrije Universiteit Brussel
Date: Tuesday, 15 May, 2001, 14:00
Abstract: Unbiased 2N-multiport beamsplitters are devices with the following property: if a photon enters any single input port (out of the N), its chances of exit are equally split between all N output ports. Bell multiports are unbiased multiports with the distinguishing trait that the elements of their unitary transition matrix are solely powers of the N-th root of unity. When N =3, these Bell multiports possess very interesting properties in relation with non-locality and quantum cryptography. The aim of the seminar is to present and discuss the advantages brought by these devices for quantum key distribution in comparison with existing quBits and quTrits protocols.
Linear Optics Quantum Computation
Speaker: Sara Schneider, QUIC
Date: Friday, 11 May, 2001, 14:00
Abstract: I will try to explain Linear Optics Quantum Computation (LOQC), a recent proposal by Knill, Laflamme and Milburn to implement quantum computation using only linear optics, measurement and feedback. The paper reference is Nature 409, 46 (2001).
Optical information storage in an atomic medium
Speaker: Sofyan Iblisdir, QUIC
Date: Friday, 20 April, 2001, 16:00
Abstract: We will present some recent results, showing how in Electromagnetically Induced Transparency, one can theoretically define a field which couples atomic and light excitations. In particular, we will see that information carried by a light pulse can be reversibly decelerated and stored into a collective state of atoms. A related recent experiment will be outlined too.
Overview of Security Proofs for Quantum Key Distribution
Speaker: Gilles Van Assche, QUIC
Date: Thursday, 28 March, 2001, 16:00
Abstract: Quantum Key Distribution is a novel way to make coffee in a secure way. Two persons, Alice and Bob, famous for their cafeine addiction, may secretly agree on the ratio of the various components of their favorite drink. In this presentation, we will simply outline the security proofs as found in the quantum information literature. We also hope to be able to precisely stress out on which assumption such proofs are based.
Quantum computation based on the adiabatic evolution of the Hamiltonian
Speaker: Jérémie Roland, QUIC
Date: Wednesday, March 14, 2001, 4 pm
On the reversible extraction of classical information from a quantum source
Speaker: Andreas Winter, Universitaet Bielefeld, Germany
Date: Thursday, November 30, 2000, 11:00
Some new results on quantum entanglement
Speaker: Frank Verstraete and Koenraad Audenaert, K. U. Leuven
Date: Tuesday, October 10, 2000, 2:30 pm
Simulations and measurements in ion trap quantum computers
Speaker: Sara Schneider, QUIC
Date: Tuesday, October 3, 2000, 2:30 pm
Abstract: Quantum computers are, in a way, measurement devices which allow to perform and also simulate experiements. I will first give a short introduction into ion trap quantum computation. Then I will discuss how to use an ion trap quantum computer to simulate a 'trit' with two qubits and, as an application of this, propose a measurement of an SU(3) geometrical phase in ion traps. The last part will be concerned with the simulation of large angular momentum systems in ion traps. The focus will be on the Dicke-model, a model which includes coherent driving and collective decoherence of a set of two-level systems. I will discuss the semiclassical dynamics of the system and show for the case of two qubits how entanglement arises.
Quantum and/or classical secret sharing
Speaker: Gilles Van Assche, QUIC
Date: Wednesday, September 20, 2000, 2:30 pm