October 16, 2008, N. Lütkenhaus. IQC, Waterloo.

"Testing Optical Quantum Devices via Entanglement Verification" 

11:30 in IFAE seminar room

The verification of entanglement is a useful tool to make sure that quantum devices such as quantum memories, quantum channels, teleportation devices etc indeed operate in the quantum domain. We are interested in devices that operate on infinite-dimensional Hilbert spaces, typically represented by optical modes. In these situations, a complete process tomography is out of question. Instead, we construct procedures based on only two non-orthogonal test states and a limited set of measurements, such as first and second moments of quadrature operators. The corresponding information can be entered in a so-called expectation value matrix, which is now low-dimensional. The question of operation in the quantum domain of our devices (memories, channels) can now be based on discussions of this expectation value matrix. In this talk I outline the expectation value matrix formalism and give applications to quantum key distribution with continuous variables and to tests of quantum memories. 

September 26, 2008, Augusto Roncaglia. FCEyN, Buenos Aires.

"Dynamics of the entanglement between two oscillators in the same environment"

14:30 in IFAE seminar room

We provide a complete characterization of the evolution of the entanglement between two oscillators coupled to a common environment. For resonant oscillators in initial Gaussian states there are three phases with different qualitative long time behavior: Entanglement may undergo a sudden death, it may experience an infinite sequence of events of sudden death and revival or it may persist for long times. The phase diagram is described and analytic expressions for the boundaries between phases are obtained. We provide a simple physical interpretation to understand the origin of entanglement. Our results are exact and applicable to a large variety of non--Markovian environments. The case of non--resonant oscillators is analyzed and the existence of eternal entanglement is proved in such case. To conclude, we discuss the implications of our results on the nature of the correlations between the system and the environment during the process of decoherence.    

September 23, 2008, Thomas Seligman. CICC, Cuernavaca.

"Random Matrix Theory of decoherence and fidelity decay."

14:00 in IFAE seminar room

Random matrix theory seems a natural way to describe uncontrolled perturbations of a quantum system, either internal to describe fidelity decay or external to describe decoherence. We present the corresponding theory, and show, that fidelity decay, where experiments are available, is well described.  In teh case of decohernece, we predict for small near environments deviations from the usual expoenential decay.    

June 12, 2008, Julia Stasinska. GIQ

"Entropic-like inequalities from positive maps"

15:00 in IFAE seminar room

The recently established relation between standard entropic inequalities and the reduction map leads to a natural question: Is it possible to derive inequalities of similar structure from other positive maps?. We give a positive answer showing that the Breuer-Hall map can lead to entropic-like criteria detecting both distillable and bound PPT entanglement. We also give a general scheme for construction of such scalar separability tests from other positive maps and test their efficiency on few classes of states. Moreover, the possible experimental realization with many-copy entanglement witnesses is outlined.   

June 12, 2008, Ognyan Oreshkov. GIQ

"Fault-tolerant holonomic quantum computation"

14:30 in IFAE seminar room

Holonomic quantum computation (HQC) is an adiabatic, all-geometric method of computation that uses non-abelian generalizations of the Berry phase. Due to its geometric nature, this method is known to provide a degree of built-in 

robustness against various types of errors in the control parameters driving the evolution. In this study, we propose a scheme for fault-tolerant HQC on stabilizer codes, which combines the virtues of quantum error correction with those of the geometric approach. The scheme establishes the in-principle scalability of HQC, putting it on equal footing with other models of computation.  

May 15, 2008, Pietro Massignan. Utrecht Institute for Theoretical Physics

"Strong interactions and pairing in imbalanced Fermi gases: what can we learn from RF spectroscopy?"

11:30 in IFAE seminar room

We calculate the radio-frequency (RF) spectrum of balanced and imbalanced ultracold Fermi gases in the normal phase across the BEC-BCS crossover. In the BEC regime, we find that the spectrum separates in two components, which correspond to bound pairs and thermally excited unbound atoms, respectively. At low temperatures, Fermi-blocking effects strongly modify the shape of the molecular component. We evaluate the number of pairs in the gas as a function of imbalance, temperature and interaction strength. For the balanced case at unitarity, our calculation favourably compares with the data recently published by the MIT group.

April 21-25, 2008, Prof. Janos Bergou, Hunter College, CUNY

Specail Lecture Series  on "Quantum Measurements and Selected Applications"

10:00-12:00 in IFAE seminar room

April 15, 2008, Geza Toth. ICFO

"Optimal spin squeezing inequalities detect bound entanglement in spin models"

14:30 in IFAE seminar room

We determine the complete set of generalized spin squeezing inequalities. These are entanglement criteria that can be used for the experimental detection of entanglement in a system of spin-1/2 particles in which the spins cannot be individually addressed. They can also be used to show the presence of bound entanglement in the thermal states of several spin models. Ref:  Phys. Rev. Lett. 99, 250405 (2007)

February 5, 2008,  Barabara Kraus. University of Innsbruck

"Quantum Kolomgorov Complexity"

14:30 in IFAE seminar room

Kolmogorov complexity is a measure of the information contained in a binary string. We investigate here the notion of quantum Kolmogorov complexity, a measure of the information required to describe a quantum state. We show that for any definition of quantum Kolmogorov complexity measuring the number of classical bits required to describe a pure quantum state, there exists a pure n-qubit state which requires exponentially many bits of description. This is shown by relating the classical communication complexity to the quantum Kolmogorov complexity. Furthermore we give some examples of how quantum Kolmogorov complexity can be applied to

prove results in different fields, such as quantum computation and thermodynamics, and we generalize it to the case of mixed quantum states.

December 20, 2007, Sergio Boixo. University of New Mexico

"Quantum Simulated Annealing"

14:30 in IFAE seminar room

We develop a quantum algorithm to solve combinatorial optimization problems through quantum simulation of a classical annealing process. Our algorithm combines techniques from quantum walks, quantum phase estimation, and quantum Zeno effect. It can be viewed as a quantum analogue of the discrete-time Markov chain Monte Carlo implementation of classical simulated annealing. Our implementation scales with the inverse of the square root of the minimum spectral gap of the stochastic matrix used in the classical simulation. The quantum algorithm outperforms the classical one, which scales with the inverse of the gap.

December 17-18, Geza Giedke, Max-Planck-Institute für Quantenoptik, Garching (Germany).

Special lectures on "Quantum Information with Continuous Variables"

17:30-19:00 in Sala de Seminaris del Departament de Fisica.

December 18, 2007. Joan Baguda,Universitat Autònoma de Barcelona

"Applications of the de Broglie-Bohm interpretation to the study of dynamics of quantum systems"

14:30 in IFAE seminar room

The de Broglie-Bohm interpretation of Quantum Mechanics constitutes a causal, hidden variable theory completely equivalent at a prediction level to the usual Copenhagen interpretation. Well beyond its ontologic implications, some recent developments indicate that its formalism could provide a source of novel algorithms for the study of mesoscopic quantum systems.

November 13, 2007. M. Hayashi. Japan Science and Technology Agency 

"State discrimination of Entangled States by Local Operations"

14:30 in IFAE seminar room

We consider state dicrimination among several entangled states by using local operation, one-way LOCC, two-way LOCC, and separable operations. The following are our settings:

1) Pure entangled state v.s. the maximally entangled states

2) Maximally entangled state v.s. other states

3) Discriminating among pulural entangled states perfectly

We also treat the possibility of cloning the state bilocal operation, and that of channel.

September 27, 2007. Martí Cuquet, Universitat Autònoma de Barcelona

"Quantumness of correlations and local broadcasting"

14:30 in IFAE seminar room

Broadcasting is distributing the information encoded in M inputs equally prepared over M > N users. When the broadcast states are pure, ideal broadcasting coincides with quantum cloning, where the output is the tensor product of identical states. In the case of input mixed states, only the local state of each final user has to be equal to the input state. This is impossible for 1->2 broadcasting, but can be done if there are enough input copies. Broadcasting of mixed states can be implemented by means of spin networks. We have studied broadcasting of phase covariant qubits using a spin star network with couplings described by the XY Model with an external magnetic field. In this framework we achieved fidelities up to about 0.9, depending on the parameters of the network and the length of the input Bloch vector. Other intial states and models for the network could be checked so that a fidelity F=1 may be obtained.

July 18, 2007. Matteo Paris. Università di Milano

"Quantifying the non-Gaussian character of a quantum state"

12:00 in IFAE seminar room

We address the issue of quantifying the non-Gaussian character of a bosonic quantum state and introduce a non-Gaussianity measure based on the Hilbert-Schmidt distance between the state under examination and a reference Gaussian state. We analyze in details the properties of the proposed measure and exploit it to evaluate the non-Gaussianity of some relevant single- and multi-mode quantum states. The evolution of non-Gaussianity is also analyzed for quantum states undergoing the processes of Gaussification by loss and de-Gaussification by photon-subtraction. The suggested measure is easily computable for any state of a bosonic system and allows to define a corresponding measure for the non-Gaussian character of a quantum operation.

July 6, 2007. Alex Monras. UAB

PhD Thesis defence: "Optimal estimation of quantum states and operations"

12:00 in IFAE seminar room

The author will discuss the methods in quantum statistics used  in the estimation of quantum states and operations. Special emphasis will be made on the analogies between the quantum formalism  and the (classical) measure theory. The published works of the author on this subject will be presented.

July 5, 2007. Janos Bergou. Hunter College. City University of New York

"Complementarity and entanglement in multipartite systems"

14:30 in IFAE seminar room

Complementarity constitutes one of the basic concepts of quantum mechanics.  It was originally formulated for single quantum systems which may possess properties that are equally real but mutually exclusive.  For most of us the best known manifestation is wave-particle duality, the complementary aspect between propagation and detection.  Recently, significant progress has been made towards a quantitative formulation of duality.  As a byproduct, complementarity can now be viewed as a concep tin quantum information theory since complementarity relations provide us with a quantification of the resources for quantum information.  In particular, we show that complementarity defines quantitative entanglement measures as mutually exclusive properties in a tripartite qubit system. This allows us to identify different types of entanglement.  For example, genuine bi- and tri-partite entanglement measures form complementary quantities.  Taken together, they are mutually exclusive to single-partite realities.  Further, we demonstrate that the total entanglement of a quantum system, composed of an arbitrary number of qubits, generates a mutually exclusive property to the single-partite realities of the system. This enables us to propose a strategy to define genuine n-partite entanglement measures.

June 6, 2007. Lluis Masanes. Cambridge University

"Correlations in ground states and energy gap"

14:30 in IFAE seminar room

We consider translationally invariant pure states in one-dimensional lattices whose distant subsystems are uncorrelated, i.e. they split. We find a relation between the overlap of two given close states and their correlation length. This can be used for obtaining better bounds in the Clustering Theorem, which relates the energy gap G and the correlation length L as G<1/L. For instance, we find that when the exponential decay of correlations has a power-law correction of order Z, a tighter bound holds G<1/L^(Z+1).

June 1, 2007. Tommaso Roscilde. Max-Planck Institute for Quantum Optics

"Localization of ultracold bosons in optical lattices"

16:00 in IFAE seminar room

The study of correlated ultracold atoms in a disordered environment represents one of the most recent frontiers in the growing interface between atomic/molecular physics and condensed matter physics. The possibility of introducing disorder in a controlled way, namely tuning its statistics, its strength vs. the strength of the inter-particle interactions etc., opens the route for the direct observation of long-sought phenomena in quantum systems with randomness: localization of coherent matter waves, localization-delocalization transitions, novel insulating quantum phases in strongly correlated quantum systems with randomness.  I will present recent theoretical and numerical work investigating realistic setups to observe equilibrium and non-equilibrium disordered phases of ultra-cold bosons in one-dimensional optical lattices. In particular, I will discuss the realization of randomized metastable states in binary mixtures of bosons with strongly unequal effective masses, and the rich phase diagram of ultracold bosons in  a pseudo-random potential realized by an incommensurate superlattice.

June 1, 2007. Marco Piani. University of Innsbruck

"Quantumness of correlations and local broadcasting"

14:30 in IFAE seminar room

In this work we study the dynamics of cold atoms in an optical lattice when crossing the insulator to superfluid phase transition. Compared to previous approaches, this work focuses on an initial state which contains entangled atom pairs and tries to get information about the final state.

May 17, 2007. Gerardo Adesso. University of Salerno

"The social life of modes"

14:30 in IFAE seminar room

April 16-20 and May 2-4 , 2007. Prof. M. A. Martin-Delgado

Universidad Complutense de Madrid

"Lecture Series on Topological Effects in Quantum Information and Condensed Matter"

14:30-17:00  in IFAE seminar room

April 27, 2007. Mini-workshop "Local vs Collective access to quantum information"

R. Gill (Leiden University)

S. Virmani (Imperial College)

V. Buzek (Slovak Academy of Sciences Bratislava)

15:00-19:00 in IFAE seminar room

March 30, 2007. Antonio Picon. UAB

Tesina defense: "Classical and Quantum Aspects of Light with Orbital Angular Momentum"

16:00 in IFAE seminar room

 Light carries and transfers energy as well as linear and angular momentum. The angular momentum contains a spin contribution, associated with polarization, and an orbital component, linked with the spatial profile and phase distribution of light. Entangled photons prepared in a superposition of states bearing a well-defined orbital angular momentum provide access to multidimensional encoding. This feature has attracted considerable interest in quantum information and particularly for certain quantum communication protocols because with these states more information can be stored and there is less sensitivity to decoherence. In this talk, I will present my recent work on the quantization and phase space characteristics of photon states having orbital angular momentum, and elucidate nonclassical aspects such as entanglement and how it can be measured.

March 1, 2007. Juan Jose Garcia-Ripoll. Universidad Complutense de Madrid

"Melting of entangled pairs in an optical lattice"

14:30 in IFAE seminar room

In this work we study the dynamics of cold atoms in an optical lattice when crossing the insulator to superfluid phase transition. Compared to previous approaches, this work focuses on an initial state which contains entangled atom pairs and tries to get information about the final state.

December 15, 2006. Mini-Workshop:  Pawel Horodecki (U. Gdansk), Gerhard Birkl (TU Darmstadt), and Belén Paredes (Uni. Mainz)
15:00-19:00 in IFAE seminar room

December 1, 2006. David Porras. Max-Planck-Institut für Quantenoptik
"Studying spin chain models with matrix product states and DMGR"
15:00 in IFAE seminar room
In this talk I will present a numerical algorithm inspired by the Density Matrix Renormalization Group method and the Matrix Product States. It allows us to calculate the spectrum of excitations with a definite linear momentum in a quantum chain in a controlled way, as well as energy gaps and correlation functions [Phys. Rev. B 73, 014410 (2006)].  I will also discuss the application of the method to the study of the quantum phases of spin chains with S=1.

October 25, 2006. Jake Taylor. MIT
"Quantum control of coupled spins in a mesoscopic environment"
14:30 in IFAE seminar room
Isolated spins in solid state devices are promising systems for implementing ideas from quantum information science.  However, any solid state system will be coupled to nearby degrees of freedom, such as the lattice nuclear spins of the host material.  In this talk we explore approaches for robust control of localized spin systems in such a mesoscopic environment, with a focus on electron spins in quantum dots and color centers.  We develop methods for mitigating decoherence, improving quantum operations, using the nuclear spin environment as a resource, and engineering long-range interactions. Finally, implications for the realization of fault-tolerant quantum information processing will be discussed.

October 20, 2006. Gabriel Molina Terriza . ICFO
"Exploring high dimensional spaces (with the transversal shape of photons)"
14:30 in IFAE seminar room
Quantum mechanics shows that particles and systems which lie in a Hilbert space of many dimensions have properties which cannot be found in usual spin-1/2 systems. Some of these properties have found applications in the field of quantum information. In this talk I will show a system which easily allows to implement controllable operations in high-dimensional spaces: the transversal shape of the photon. In particular I will talk about the Orbital Angular Momentum of Photons, its applications to quantum communications and the future possibilities of such a system.

July 4, 2006.  Montserrat Casas. Universitat de les Illes Balears
"Maximum entangled mixed states and the speed of quantum evolution"
14:30 in IFAE seminar room
By performing a systematic numerical survey of the space of pure and mixed states of bipartite systems of any dimension, we investigate the relationship between quantum separability and the violation of the classical q-entropic inequalities. In two-qubit systems we focused our attention on the properties that link a particular class of states, the so called Maximally Entangled Mixed States (MEMS) [1,2], with the violation of the entropic and Bell inequalities. The correlation between the concurrence and the speed of quantum evolution determined by the action of a local Hamitonian has been systematically studied [3].  [1] J. Munro et al., Phys. Rev A 64, 030302 (2001); [2] J. Batle et al., Phys. Rev. A 71, 024301 (2005); [3] J. Batle et al.,  Phys. Rev A 72, 032337 (2005), Phys. Rev A 73, 049904 (2006)

June 29, 2006. M. A. Martin-Delgado. Universidad Complutense de Madrid
"Topological Quantum Computation and Condensed Matter"
14:30 in IFAE seminar room
An introduction to topological quantum computation as a promising instance of fault-tolerant quantum computation and its relation to condensed matter models. Summary: I. Quantum Communication Briefing II. Quantum Computation Briefing III. Quantum Error Correction and Topology IV. Topological Orders in Condensed Matter V. Topological Quantum Computation and Condensed Matter Systems..

June 13, 2006. Sandu Popescu. University of Bristol
"Entanglement and the foundations of statistical mechanics"
14:30 in IFAE seminar room
Statistical mechanics is one of the most successful areas of physics. Yet, almost 150 years since its inception, its foundations and basic postulates are still the subject of debate. Here we suggest that the main postulate of statistical mechanics, the equal a priori probability postulate, should be abandoned - it is misleading and unnecessary. We argue that it should be replaced by a general canonical principle, whose physical content is fundamentally different from the postulate it replaces: It refers to individual states, rather than to ensemble or time averages. Furthermore, while the original postulate is an unprovable assumption, the principle we propose is mathematically proven. The key element of our principle is the quantum entanglement between the system and its environment.

May 29, 2006. Enrique Rico Ortega. University of Innsbruck
"Scale perturbation in valence bond ground states"
14:30 in IFAE seminar room
A simple and efficient method for calculating the ground state for a class of anti- ferromagnet systems is presented. It combines the valence bond structure of the ground state for this class of systems and real space renormalization group. As an example, the entire Haldane phase for a spin-1 chain is described, form the AKLT model through the Heisenberg model, ending at the critical WZNW SU_2(2) model. The picture that emerges from this new method leads us to consider the relationship between the hidden topological order that characterized the Haldane phase and potential applications in quantum communication technology.

May 13, 2006. Stephen Barnett. University of Strathclyde
"Optimal quantum measurements: from minimum error to maximum confidence"
14:30 in IFAE seminar room
The superposition principle carries with it the existence of quantum states that are similar in the sense that no measurement strategy can distinguish between them with certainty. Under these circumstances it is both interesting and of practical importance to ask what is the best that can be done. In my talk I will concentrate on quantum communications problems in which prior information about the set of possible states and their associated probabilities is known.  Within this I will discuss state detection with minimum error and unambiguous, or error-free state discrimination, which works by including the possibility of an inconclusive result.  I shall also describe a new strategy, which is optimised so that we can be as confident as possible of our state identification within the rules of quantum theory.  I will show how such measurements can be performed by reference to experiments using optical polarisation.

April 6, 2006. William Wootters. Williams College
"Picturing Qubits in Phase Space"
12:00 in  in IFAE seminar room

April 4, 2006. William Wootters. Williams College
"Quantum Entanglement as a Resource for Communication"
15:00 in Sala de Graus, Faculty of Sciences, UAB

April 3, 2006. William Wootters. Williams College
"Optimal Discrimination among Product States"
12:00 in  in IFAE seminar room

April 3, 2006. Adan Cabello. Universidad de Sevilla
"Bell inequalities based on equalities"
15:00 in IFAE seminar room
We show that bipartite Bell inequalities based on the Einstein-Podolsky-Rosen criterion for elements of reality allow, when applied to some specific hyper-entangled states, feasible experimental verifications that quantum nonlocality grows exponentially with the size of the subsystems, and Bell loophole-free tests with current photo-detection efficiencies.

March 9, 2006. Susana Huelga. University of Hertfordshire
"Quantum dynamics and Noise-Assisted processes in chains of solid-state qubits"
14:30 in IFAE seminar room
The operation of solid state electrical circuits in the quantum regime has experienced a significant experimental progress in recent years. I will review the main ideas behind the design of superconducting (Josephson) qubits and discuss the quantum dynamics of arrays of coupled Josephson qubits. Different decoherence sources and the suitable theoretical framework to describe them will be analyzed in some detail. The second part of the talk explores noise-assisted processes. I will show that driven, coupled spin systems exhibit stochastic resonance according to both dynamical and information-theoretic figures of merit. The talk is planned to be reasonably self-contained.

March 9, 2006. Martin Plenio. Institute for Mathematical Sciences &  Quantum Optics and Laser Science Group Imperial College London.
"Entanglement in Many Body Systems."
14:30 in IFAE seminar room
One focus of attention in quantum information science was and still is the study of the structure entanglement between two parties. A reasonably detailed understanding has been reached. The same cannot be said however about the detailed study of three or more party entanglement and progress appears to have slowed considerably. In recent years emphasis has therefore shifted towards the investigation a special class of states, namely those occurring in quantum many body systems, eg ground and thermal states. In this talk I will present some of the results that we have obtained in this direction, including scaling laws and a novel approach for the approximation of ground states of quantum many body systems in arbitrary spatial dimensions.

Jun. 28, 2005. Mark Raizen. University of Texas.
"Experiments with a Particle in a Box: Bose Einstein condensates, and Maxwell's Demon."
16:00 Room C3/016, Faculty of Sciences, UAB.
The "particle in a box" is at the heart of quantum mechanics and is a paradigm for many problems in physics. In this talk, I will describe recent experiments conducted by my group which confine a Bose-Einstein condensate to a one-dimensional optical box. These conditions should enable the experimental realization of a "quantum tweezer" for atoms and allow preparation of atomic number states. More generally, we have demonstrated the capability to measure atom statistics by single-atom counting, paving the way for the new field of quantum atom optics.The concept of a "particle in a box" is also widely used in thermodynamics. The historic paradox of Maxwell's demon suggested a way to cause particles to accumulate in one side of the box, in an apparent violation of the Second Law. Motivated by these ideas we have developed a method to form an optical "one-way" barrier for atoms. This device would allow atoms coming from one side of the barrier to pass through, but those coming from the other side to be reflected. I will show how this idea can be used for phase space compression and cooling, as an optical realization of Maxwell's demon.

Jun. 1, 2005. Jan Wehr. Dept. of Mathematics, Tucson.
"Probability theory from gambling to disorder systems."
14:30 in IFAE seminar room
I will review two cornerstones of classical probability theory---the law of large numbers and the central limit theorem---starting from the birth of the former during Pascal`s seventeenth century gambling escapades. The same ideas are used today in theory of disordered systems to study their fundamental properties: self-averaging and sample-to-sample fluctuations. This will be illustrated by new results on conductivity of random media, including anomalous critical conductance fluctuations, deviating from the classical behavior. The talk will be aimed at a general audience; no expertise in probability theory or disordered systems will be assumed.

May 30, 2005. Ennio Arimondo. University of Pisa.
"Nonlinear dynamics of a Bose-Einstein condensate within an optical lattice."
12:00 in "Sala d'actes", Faculty of Sciences, UAB.
The transport properties of Bose-Einstein condensates of rubidium atoms loaded within a one-dimensional optical lattice are investigated experimentally. The role played by the nonlinear term describing the interaction between the condensate atoms is investigated. Deformations of the Bloch oscillation contrast, asymmetries in the Landau-Zener probability, and insurgence of instabilities will be presented. A comparison with theoretical models will be performed.

May 13, 2005. J. Eschner. ICFO.
"A single trapped ion in front of a mirror -from cavity QED effects to quantum feedback."
14:30 in IFAE seminar room.

Apr. 8, 2005. H. Jeff Kimble. California Institute of Technology (CALTECH).
"Cavity QED with Single Atoms and Photons."
12:00 in "Sala d'actes", Faculty of Sciences, UAB.

Dec. 21, 2004. Eugenio Roldán. Universitat de Valencia.
"Quantum walks: classical implementations."
13:00 in IFAE seminar room

Dec. 15, 2004. Aditi Sen. University of Hannover.
"Locally accessible information: How much can the parties gain by cooperating?"
16:00 in IFAE seminar room
The problem of local distinguishability of quantum states has direct implications on the use of entanglement as a resource in quantum information theory. It is therefore important to quantify the process of local distinguishability. We find a universal Holevo-like upper bound on the locally accessible information for arbitrary two party ensembles. We analyze our bound and exhibit a class of states which saturate it. Interestingly, this bound leads to a bound on the capacity of distributed dense coding for arbitrary multiparty shared states in several scenarios. This bound also gives a bound on the yield of singlets in some distillation processes.

Jun. 11, 2004. Gabriel F. Calvo. ICFO.
"Orbital Angular Momentum of Photons"
16:00 in IFAE seminar room
It is at the very basis of classical and quantum electrodynamics that particles and fields posses energy, linear and angular momentum. For matter particles it is well known that the angular momentum can be separated into orbital and spin contributions. However, is it possible for optical electromagnetic fields to have such separation? If so, what are the observable effects? In this tutorial-oriented seminar we will try to answer these and other related questions and present some of the current applications of the angular momentum of light in different areas ranging from optical tweezers to quantum information.

May 21, 2004. Richard Gill. Ultrecht U.
"A statistical approach to quantum tomography"
14:30 in IFAE seminar room
I will explain a statistical approach to quantum tomography (quantum state reconstruction; also reconstruction of quantum operations) using the tools of Fisher information and maximum likelihood estimation. Applications will be sketched to the issue of adaptive versus non-adaptive, joint versus separate measurement schemes, and (in state tomography) use of entangled versus non-entangled probe states. I will argue that already in the most simple quantum tomography problem (separate measurements on many copies of a two-level system) there is so much rich structure, that a single number figure of merit (such as average fidelity with respect to a "noninformative" prior) does not reflect the complexity of the problem. The analysis will throw new light on the strange convergence rates recently discovered by Bagan, Baig, Munoz-Tapia and Rodriguez (2004; PRA 69, 010304).

Apr. 22, 2004. Alexandre Guillaume. Jet Propulsion Laboratory, USA
"Coherent oscillations in a Cooper pair box"
14:30 in IFAE seminar room
We report on the control of the temporal evolution of the quantum state in a superconducting qubit. We have fabricated a single Cooper-pair box (SCB) in close proximity to a single electron transistor (SET) operated in the radio-frequency mode (RF-SET) with an inductor and capacitor lithographed directly on chip. The RF-SET was used to measure the charge state of the SCB revealing a 2e periodic charge quantization. We performed spectroscopy measurements to extract the charging energy (EC) and the Josephson coupling energy (EJ). Control of the temporal evolution of the quantum charge state was achieved by applying fast DC pulses to the SCB gate. The dephasing and relaxation times were extracted from these measurements.

Feb. 6, 2004. Albert Bramon. Universitat Autònoma de Barcelona.
"Quantum mechanics with neutral kaons"
12:00 in IFAE seminar room
When discussing the neutral kaon system in his famous Lectures on Physics, Feynman wrote: " If there is any place where we have a chance to test the main principles of quantum mechanics... this is it!" The purpose of the seminar is to review some recent work of our group on this enlighting but subtle subject. Kaons are shown to be relevant to discuss QM issues such as Bell inequalities, Hardy's argument, quantum marking and erasure, complementarity principle, quantum entanglement,...

Jun. 6, 2003. Iñigo Egusquiza. Universitat del Pais Basc.
"Real clocks ans the Zeno effect"
14:30 in IFAE seminar room
Real clocks are not perfect. This must have an effect in our predictions for the behaviour of a quantum system, an effect for which we present a unified description, encompassing several previous proposals. We study the relevance of clock errors in the Zeno effect, and find that generically no Zeno effect can be present (in such a way that there is no contradiction with currently available experimental data). We further observe that, within the class of stochasticities in time addressed here, there is no modification in emission lineshapes.

Jan. 13, 2003. Adán Cabello. Universidad de Sevilla.
"Violación de la desigualdad de Bell más allá del límite de Cirel'son"
16:00 in IFAE seminar room
La desigualdad de Bell en la versión de Clauser, Horne, Shimony y Holt, establece que el valor absoluto de la suma de los resultados de ciertos experimentos separados por intervalos de género espacio sobre un sistema de dos partículas debe estar acotado por 2 en cualquier teoría de variables ocultas locales (en particular, en cualquier teoría con "elementos de realidad" como los definidos por Einstein, Podolsky y Rosen). Es bien sabido que ciertas predicciones de la mecánica cuántica violan esta limitación. Por otro lado, está muy extendida la opinión de que "Quantum theory does not allow any stronger violation of the CHSH inequality than the one already achieved in Aspect's experiment [2 \sqrt{2}]." (A. Peres, Quantum Theory: Concepts and Methods, p. 174.), o "The violation of the Bell inequality found in (...) [2 \sqrt{2}] is the maximum possible in quantum mechanics." (M. A. Nielsen and I. L. Chuang, Quantum Computation and Quantum Information, p. 118). El valor 2 \sqrt{2} se conoce como el límite de Cirel'son. Las predicciones de la mecánica cuántica para dos partículas extraídas de un sistema de tres partículas preparadas en determinados estados cuánticos violan la desigualdad de Bell-CHSH más allá del límite de Cirel'son.

Nov. 26, 2002. Kai Eckert. Theoretische Quantenoptikgruppe. Universität Hannover.
"Quantum random walks and their implementation in optical microtraps"
14:30 in IFAE seminar room
Quantum random walks are the quantum analogues of classical random walks. We will review the striking features that distinguish the quantum version from the classical one and we will describe a scheme to implement them with neutral atoms stored in an array of optical microtraps.