Publications (51)151.58 Total impact

Article: Quantumness of spin1 states
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ABSTRACT: We investigate quantumness of spin1 states, defined as the HilbertSchmidt distance to the convex hull of spin coherent states. We derive its analytic expression in the case of pure states as a function of the smallest eigenvalue of the Bloch matrix and give explicitly the closest classical state for an arbitrary pure state. Numerical evidence is provided that the exact formula for pure states provides an upper bound on the quantumness of mixed states. Due to the connection between quantumness and entanglement we obtain new insights into the geometry of symmetric entangled states. 
Dataset: 1509.08300v2
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ABSTRACT: We investigate multiqubit permutationsymmetric states with maximal entropy of entanglement. Such states can be viewed as particular spin states, namely anticoherent spin states. Using the Majorana representation of spin states in terms of points on the unit sphere, we analyze the consequences of a pointgroup symmetry in their arrangement on the quantum properties of the corresponding state. We focus on the identification of anticoherent states (which are states with maximally mixed reduced density matrices in the symmetric subspace) associated with pointgroup symmetric sets of points. We provide three different characterizations of anticoherence, and establish a link between symmetries, anticoherence and classes of states equivalent through stochastic local operations with classical communication (SLOCC). We then investigate in detail the case of few numbers of qubits, and construct infinite families of anticoherent states with pointgroup symmetry of their Majorana points, showing that anticoherent states do exist to arbitrary order.  [Show abstract] [Hide abstract]
ABSTRACT: We present a comprehensive study of the destruction of quantum multifractality in the presence of perturbations. We study diverse representative models displaying multifractality, including a pseudointegrable system, the Anderson model and a random matrix model. We apply several types of natural perturbations which can be relevant for experimental implementations. We construct an analytical theory for certain cases, and perform extensive largescale numerical simulations in other cases. The data are analyzed through refined methods including double scaling analysis. Our results confirm the recent conjecture that multifractality breaks down following two scenarios. In the first one, multifractality is preserved unchanged below a certain characteristic length which decreases with perturbation strength. In the second one, multifractality is affected at all scales and disappears uniformly for a strong enough perturbation. Our refined analysis shows that subtle variants of these scenarios can be present in certain cases. This study could guide experimental implementations in order to observe quantum multifractality in real systems.  [Show description] [Hide description]
DESCRIPTION: arXiv:1506.05720, 20 pages, 27 figures. 
Article: Tensor Representation of Spin States
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ABSTRACT: We propose a generalisation of the Bloch sphere representation for mixed spin states based on covariant matrices introduced by Weinberg in 1964 in the context of quantum field theory. We show that these matrices form a tight frame, enabling a compact and transparent representation of density matrices of arbitrary spin in terms of tensors that share the most important properties of Bloch vectors. We investigate the properties of this representation, and give various applications, notably a novel characterization of anticoherent spin states.  [Show abstract] [Hide abstract]
ABSTRACT: In the introductory section of the article we give a brief account of recent insights into statistics of high and extreme values of disordergenerated multifractals following a recent work by the first author with P. Le Doussal and A. Rosso (FLR) employing a close relation between multifractality and logarithmically correlated random fields. We then substantiate some aspects of the FLR approach analytically for multifractal eigenvectors in the RuijsenaarsSchneider ensemble (RSE) of random matrices introduced by E. Bogomolny and the second author by providing an ab initio calculation that reveals hidden logarithmic correlations at the background of the disordergenerated multifractality. In the rest we investigate numerically a few representative models of that class, including the study of the highest component of multifractal eigenvectors in the RuijsenaarsSchneider ensemble.  [Show abstract] [Hide abstract]
ABSTRACT: We expose two scenarios for the breakdown of quantum multifractality under the effect of perturbations. In the first scenario, multifractality survives below a certain scale of the quantum fluctuations. In the other one, the fluctuations of the wave functions are changed at every scale and each multifractal dimension smoothly goes to the ergodic value. We use as generic examples a onedimensional dynamical system and the threedimensional Anderson model at the metalinsulator transition. Our results suggest that quantum multifractality breakdown is universal and follows one of these two scenarios depending on the perturbation. We also discuss the experimental implications.  [Show abstract] [Hide abstract]
ABSTRACT: We analyze the game of go from the point of view of complex networks. We construct three different directed networks of increasing complexity, defining nodes as local patterns on plaquettes of increasing sizes, and links as actual successions of these patterns in databases of real games. We discuss the peculiarities of these networks compared to other types of networks. We explore the ranking vectors and community structure of the networks and show that this approach enables to extract groups of moves with common strategic properties. We also investigate different networks built from games with players of different levels or from different phases of the game. We discuss how the study of the community structure of these networks may help to improve the computer simulations of the game. More generally, we believe such studies may help to improve the understanding of human decision process.  [Show abstract] [Hide abstract]
ABSTRACT: For random matrices with treelike structure there exists a recursive relation for the local Green functions whose solution permits to find directly many important quantities in the limit of infinite matrix dimensions. The purpose of this note is to investigate and compare expressions for the spectral density of random regular graphs, based on easy approximations for real solutions of the recursive relation valid for trees with large coordination number. The obtained formulas are in a good agreement with the results of numerical calculations even for small coordination number.  [Show abstract] [Hide abstract]
ABSTRACT: We investigate the set of quantum channels acting on a single qubit. We provide a compact generalization of the FujiwaraAlgoet conditions for complete positivity to nonunital qubit channels, which we then use to characterize the possible geometric forms of the pure output of the channel. We provide universal sets of quantum channels for all unital qubit channels as well as for all extremal (not necessarily unital) qubit channels, in the sense that all qubit channels in these sets can be obtained by concatenation of channels in the corresponding universal set. We also show that our universal sets are essentially minimal.  [Show abstract] [Hide abstract]
ABSTRACT: We calculate analytically, for finitesize matrices, joint probability densities of ratios of level spacings in ensembles of random matrices characterized by their associated confining potential. We focus on the ratios of two spacings between three consecutive real eigenvalues, as well as certain generalizations such as the overlapping ratios. The resulting formulas are further analyzed in detail in two specific cases: the betaHermite and the betaLaguerre cases, for which we offer explicit calculations for small N. The analytical results are in excellent agreement with numerical simulations of usual random matrix ensembles, and with the level statistics of a quantum manybody lattice model and zeros of the Riemann zeta function.  [Show abstract] [Hide abstract]
ABSTRACT: We derive expressions for the probability distribution of the ratio of two consecutive level spacings for the classical ensembles of random matrices. This ratio distribution was recently introduced to study spectral properties of manybody problems, as, contrary to the standard level spacing distributions, it does not depend on the local density of states. Our Wignerlike surmises are shown to be very accurate when compared to numerics and exact calculations in the large matrix size limit. Quantitative improvements are found through a polynomial expansion. Examples from a quantum manybody lattice model and from zeros of the Riemann zeta function are presented.  [Show abstract] [Hide abstract]
ABSTRACT: We study a version of the mathematical RuijsenaarsSchneider model and reinterpret it physically in order to describe the spreading with time of quantum wave packets in a system where multifractality can be tuned by varying a parameter. We compare different methods to measure the multifractality of wave packets and identify the best one. We find the multifractality to decrease with time until it reaches an asymptotic limit, which is different from the multifractality of eigenvectors but related to it, as is the rate of the decrease. Our results could guide the study of experimental situations where multifractality is present in quantum systems.  [Show abstract] [Hide abstract]
ABSTRACT: We construct perturbation series for the qth moment of eigenfunctions of various critical randommatrix ensembles in the strong multifractality regime close to localization. Contrary to previous investigations, our results are valid in the region q<1/2. Our findings allow one to verify, at first leading orders in the strong multifractality limit, the symmetry relation for anomalous fractal dimensions Δ(q)=Δ(1q), recently conjectured for critical models where an analog of the metalinsulator transition takes place. It is known that this relation is verified at leading order in the weak multifractality regime. Our results thus indicate that this symmetry holds in both limits of small and large coupling constant. For general values of the coupling constant we present careful numerical verifications of this symmetry relation for different critical randommatrix ensembles. We also present an example of a system closely related to one of these critical ensembles, but where the symmetry relation, at least numerically, is not fulfilled.  [Show abstract] [Hide abstract]
ABSTRACT: We give an explicit parametrization of the set of mixed quantum states and of the set of mixed classical states for a spin1. Classical states are defined as states with a positive GlauberSudarshan Pfunction. They are at the same time the separable symmetric states of two qubits. We explore the geometry of this set, and show that its boundary consists of a twoparameter family of ellipsoids. The boundary does not contain any facets, but includes straightlines corresponding to mixtures of pure classical states. 
Article: Perturbation approach to multifractal dimensions for certain critical randommatrix ensembles
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ABSTRACT: Fractal dimensions of eigenfunctions for various critical random matrix ensembles are investigated in perturbation series in the regimes of strong and weak multifractality. In both regimes, we obtain expressions similar to those of the critical banded random matrix ensemble extensively discussed in the literature. For certain ensembles, the leadingorder term for weak multifractality can be calculated within standard perturbation theory. For other models, such a direct approach requires modifications, which are briefly discussed. Our analytical formulas are in good agreement with numerical calculations. 
Article: The game of go as a complex network
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ABSTRACT: We study the game of go from a complex network perspective. We construct a directed network using a suitable definition of tactical moves including local patterns, and study this network for different datasets of professional tournaments and amateur games. The move distribution follows Zipf's law and the network is scale free, with statistical peculiarities different from other real directed networks, such as e. g. the World Wide Web. These specificities reflect in the outcome of ranking algorithms applied to it. The fine study of the eigenvalues and eigenvectors of matrices used by the ranking algorithms singles out certain strategic situations. Our results should pave the way to a better modelization of board games and other types of human strategic scheming. 
Article: Integrable random matrix ensembles
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ABSTRACT: We propose new classes of random matrix ensembles whose statistical properties are intermediate between statistics of WignerDyson random matrices and Poisson statistics. The construction is based on integrable Nbody classical systems with a random distribution of momenta and coordinates of the particles. The Lax matrices of these systems yield random matrix ensembles whose joint distribution of eigenvalues can be calculated analytically thanks to integrability of the underlying system. Formulas for spacing distributions and level compressibility are obtained for various instances of such ensembles.  [Show abstract] [Hide abstract]
ABSTRACT: Based on numerical and perturbation series arguments we conjecture that for certain critical random matrix models the information dimension of eigenfunctions D(1) and the spectral compressibility χ are related by the simple equation χ+D(1)/d=1, where d is system dimensionality.
Publication Stats
505  Citations  
151.58  Total Impact Points  
Top Journals
Institutions

2015

Université ParisSaclay
Lutetia Parisorum, ÎledeFrance, France


20112015

Université ParisSud 11
Orsay, ÎledeFrance, France


20072011

French National Centre for Scientific Research
Lutetia Parisorum, ÎledeFrance, France


20052010

University of Toulouse
Tolosa de Llenguadoc, MidiPyrénées, France


20052006

Paul Sabatier University  Toulouse III
 Laboratoire de Physique Théorique  UMR 5152  LPT
Tolosa de Llenguadoc, MidiPyrénées, France


2004

University of Bristol
 School of Mathematics
Bristol, England, United Kingdom
