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Search Results: 1 - 10 of 224158 matches for " R. Portugal "
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Group-theoretic Approach for Symbolic Tensor Manipulation: I. Free Indices
R. Portugal,B. F. Svaiter
Physics , 2001,
Abstract: We describe how Computational Group Theory provides tools for manipulating tensors in explicit index notation. In special, we present an algorithm that puts tensors with free indices obeying permutation symmetries into the canonical form. The method is based on algorithms for determining the canonical coset representative of a subgroup of the symmetric group. The complexity of our algorithm is polynomial on the number of indices and is useful for implementating general purpose tensor packages on the computer algebra systems.
Solution to the Hidden Subgroup Problem for a Class of Noncommutative Groups
D. N. Goncalves,R. Portugal
Mathematics , 2011,
Abstract: The hidden subgroup problem (HSP) plays an important role in quantum computation, because many quantum algorithms that are exponentially faster than classical algorithms can be casted in the HSP structure. In this paper, we present a new polynomial-time quantum algorithm that solves the HSP over the group $\Z_{p^r} \rtimes \Z_{q^s}$, when $p^r/q= \up{poly}(\log p^r)$, where $p$, $q$ are any odd prime numbers and $r, s$ are any positive integers. To find the hidden subgroup, our algorithm uses the abelian quantum Fourier transform and a reduction procedure that simplifies the problem to find cyclic subgroups.
The QWalk Simulator of Quantum Walks
F. L. Marquezino,R. Portugal
Computer Science , 2008, DOI: 10.1016/j.cpc.2008.02.019
Abstract: Several research groups are giving special attention to quantum walks recently, because this research area have been used with success in the development of new efficient quantum algorithms. A general simulator of quantum walks is very important for the development of this area, since it allows the researchers to focus on the mathematical and physical aspects of the research instead of deviating the efforts to the implementation of specific numerical simulations. In this paper we present QWalk, a quantum walk simulator for one- and two-dimensional lattices. Finite two-dimensional lattices with generic topologies can be used. Decoherence can be simulated by performing measurements or by breaking links of the lattice. We use examples to explain the usage of the software and to show some recent results of the literature that are easily reproduced by the simulator.
Quantum Hitting Time on the Complete Graph
R. A. M. Santos,R. Portugal
Physics , 2009,
Abstract: Quantum walks play an important role in the area of quantum algorithms. Many interesting problems can be reduced to searching marked states in a quantum Markov chain. In this context, the notion of quantum hitting time is very important, because it quantifies the running time of the algorithms. Markov chain-based algorithms are probabilistic, therefore the calculation of the success probability is also required in the analysis of the computational complexity. Using Szegedy's definition of quantum hitting time, which is a natural extension of the definition of the classical hitting time, we present analytical expressions for the hitting time and success probability of the quantum walk on the complete graph.
Stochastic Simulation of Grover's Algorithm
L. R. U. Manssur,R. Portugal
Physics , 2003,
Abstract: We simulate Grover's algorithm in a classical computer by means of a stochastic method using the Hubbard-Stratonovich decomposition of n-qubit gates into one-qubit gates integrated over auxiliary fields. The problem reduces to finding the fixed points of the associated system of Langevin differential equations. The equations are obtained automatically for any number of qubits by employing a computer algebra program. We present the numerical results of the simulation for a small search space.
Group-theoretic Approach for Symbolic Tensor Manipulation: II. Dummy Indices
L. R. U. Manssur,R. Portugal
Physics , 2001, DOI: 10.1142/S0129183102004571
Abstract: Computational Group Theory is applied to indexed objects (tensors, spinors, and so on) with dummy indices. There are two groups to consider: one describes the intrinsic symmetries of the object and the other describes the interchange of names of dummy indices. The problem of finding canonical forms for indexed objects with dummy indices reduces to finding double coset canonical representatives. Well known computational group algorithms are applied to index manipulation, which allow to address the simplification of expressions with hundreds of indices going further to what is needed in practical applications.
Testing nonlinearities between Brazilian exchange rate and inflation volatilities
Albuquerque, Christiane R.;Portugal, Marcelo S.;
Revista Brasileira de Economia , 2006, DOI: 10.1590/S0034-71402006000400001
Abstract: there are few studies, directly addressing exchange rate and inflation volatilities, and lack of consensus among them. however, this kind of study is necessary, especially under an inflation-targeting system where the monetary authority must know well price behavior. this article analyses the relation between exchange rate and inflation volatilities using a bivariate garch model, and therefore modeling conditional volatilities, fact largely unexplored by the literature. we find a semi-concave relation between those series, and this nonlinearity may explain their apparently disconnection under a floating exchange rate system. the article also shows that traditional tests, with non-conditional volatilities, are not robust.
Quantum Search Algorithms on Hierarchical Networks
F. L. Marquezino,R. Portugal,S. Boettcher
Physics , 2012, DOI: 10.1109/ITW.2011.6089429
Abstract: The "abstract search algorithm" is a well known quantum method to find a marked vertex in a graph. It has been applied with success to searching algorithms for the hypercube and the two-dimensional grid. In this work we provide an example for which that method fails to provide the best algorithm in terms of time complexity. We analyze search algorithms in degree-3 hierarchical networks using quantum walks driven by non-groverian coins. Our conclusions are based on numerical simulations, but the hierarchical structures of the graphs seems to allow analytical results.
Renormalization and Scaling in Quantum Walks
S. Boettcher,S. Falkner,R. Portugal
Physics , 2013, DOI: 10.1103/PhysRevA.90.032324
Abstract: We show how to extract the scaling behavior of quantum walks using the renormalization group (RG). We introduce the method by efficiently reproducing well-known results on the one-dimensional lattice. As a nontrivial model, we apply this method to the dual Sierpinski gasket and obtain its exact, closed system of RG-recursions. Numerical iteration suggests that under rescaling the system length, $L^{\prime}=2L$, characteristic times rescale as $t^{\prime}=2^{d_{w}}t$ with the exact walk exponent $d_{w}=\log_{2}\sqrt{5}=1.1609\ldots$. Despite the lack of translational invariance, this is very close to the ballistic spreading, $d_{w}=1$, found for regular lattices. However, we argue that an extended interpretation of the traditional RG formalism will be needed to obtain scaling exponents analytically. Direct simulations confirm our RG-prediction for $d_w$ and furthermore reveal an immensely rich phenomenology for the spreading of the quantum walk on the gasket. Invariably, quantum interference localizes the walk completely with a site-access probability that declines with a powerlaw from the initial site, in contrast with a classical random walk, which would pass all sites with certainty.
Mixing Times in Quantum Walks on Two-Dimensional Grids
F. L. Marquezino,R. Portugal,G. Abal
Computer Science , 2010, DOI: 10.1103/PhysRevA.82.042341
Abstract: Mixing properties of discrete-time quantum walks on two-dimensional grids with torus-like boundary conditions are analyzed, focusing on their connection to the complexity of the corresponding abstract search algorithm. In particular, an exact expression for the stationary distribution of the coherent walk over odd-sided lattices is obtained after solving the eigenproblem for the evolution operator for this particular graph. The limiting distribution and mixing time of a quantum walk with a coin operator modified as in the abstract search algorithm are obtained numerically. On the basis of these results, the relation between the mixing time of the modified walk and the running time of the corresponding abstract search algorithm is discussed.
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