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M. Ruiz-Altaba

Publications and source records attributed to M. Ruiz-Altaba.

8 recordsLinked to original sources
arXiv1999

Noise in Grover's Quantum Search Algorithm

Grover's quantum algorithm improves any classical search algorithm. We show how random Gaussian noise at each step of the algorithm can be modelled easily because of the exact recursion formulas available for computing the quantum amplitude in Grover's algorithm. We study the algorithm's intrinsic robustness when no quantum correction codes are used, and evaluate how much noise the algorithm can bear with, in terms of the size of the phone book and a desired probability of finding the correct result. The algorithm loses efficiency when noise is added, but does not slow down. We also study the maximal noise under which the iterated quantum algorithm is just as slow as the classical algorithm. In all cases, the width of the allowed noise scales with the size of the phone book as N^-2/3.

quant-ph↗
arXiv1999

Stretched Horizon for Non-Supersymmetric Black Holes

We review the idea of stretched horizon for extremal black holes in supersymmetric string theories, and we compute it for non-supersymmetric black holes in four dimensions. Only for small masses of the order of the Veneziano wavelength is the stretched horizon bigger than the event horizon.

gr-qc↗
arXiv1999

Pion Scattering Revisited

Chiral Ward identities lead to consistent accounting for the sigma's width in the linear sigma model's Feynman rules. Reanalysis of pion scattering data at threshold imply a mass for the sigma of 600+ 200 - 100 MeV.

hep-ph↗
arXiv1999

The Linear Sigma Model at Work: Succesful Postdictions for Pion Scattering

We enforce chiral Ward identities on the vertices of the linear sigma model to take into account the width of the scalar sigma, considered as a true physical resonance. We consider pion scattering at very low energies and, from a fit to the measured phase shifts in the various angular momentum and isospin channels, conclude a mass for the sigma of 600+200-100 MeV. Its width turns out to be of the same size.

hep-ph↗
arXiv1998

On the Initial Conditions for Pre-Big-Bang Cosmology

The beautiful scenario of pre-big-bang cosmology is appealling not only because it is more or less derived from string theory, but also because it separates clearly the problem of the initial conditions for the universe from that of high curvatures. Recently, the pre-big-bang program was subject to attack from on the grounds that pre-big-bang cosmology does not solve the horizon and flatness problems in a ``natural'' way, as customary exponential ``new'' inflation does. In particular, it appears that an arbitrarily small deviation from perfect flatness in the initial state can not be accommodated. For this analysis, matter in the universe before the big bang was assumed to be radiation. We perform a similar analysis to theirs, but using the equation of state for ``string matter'' $ρ=-3p$ which seems more appropriate to the physical situation and, also, is motivated by the scale factor duality (in the flat case) with respect to our expanding, radiation dominated, universe. For an open universe we find, exactly, the same time-dependence of the scale factor as in the Milne universe, recently found to represent the universal attractor at $t=-\infty $ of all pre big bang cosmologies. We conclude that our radiation dominated universe comes from a flat rather than a curved region.

hep-th↗
arXiv1998

Pre-Big-Bang in String Cosmology

We compute the amount of inflation required to solve the horizon problem of cosmology in the pre-big-bang scenario. First we give a quick overview of string cosmology as developed by Veneziano and collaborators. Then we show that the amount of inflation in this background solves the horizon problem. We discuss fine-tuning.

hep-th↗
arXiv1996

The q-deformed quark model: quantum isospin and q-hadrons

We consider the possibility that the SU(2) isospin symmetry, exact in strong interactions but only approximate in nature, is in fact a quantum group. Using a doublet of q-quarks, we build the wavefuntions of pi-mesons, nucleons and Delta baryons. We redo the usual quark model computation for the magnetic moments and mass relations, and everything fits with experimental data. We find, nevertheless, that it is impossible to parametrize successfully the large mass difference between the charged and neutral pions with the single q of the quantum group U_q(sl(2)).

hep-ph↗