Modelling Disorder: the Cases of Wetting and DNA Denaturation

We study the effect of the composition of the genetic sequence on the melting temperature of double stranded DNA, using some simple analytically solvable models proposed in the framework of the wetting problem. We review previous work on disordered versions of these models and solve them when there were not preexistent solutions. We check the solutions with Monte Carlo simulations and transfer matrix numerical calculations. We present numerical evidence that suggests that the logarithmic corrections to the critical temperature due to disorder, previously found in RSOS models, apply more generally to ASOS and continuous models. The agreement between the theoretical models and experimental data shows that, in this context, disorder should be the crucial ingredient of any model while other aspects may be kept very simple, an approach that can be useful for a wider class of problems. Our work has also implications for the existence of correlations in DNA sequences.Comment: Final published version. Title and discussion modified. 6 pages, 3 figure

Loschmidt Echo and the Local Density of States

Loschmidt echo (LE) is a measure of reversibility and sensitivity to perturbations of quantum evolutions. For weak perturbations its decay rate is given by the width of the local density of states (LDOS). When the perturbation is strong enough, it has been shown in chaotic systems that its decay is dictated by the classical Lyapunov exponent. However, several recent studies have shown an unexpected non-uniform decay rate as a function of the perturbation strength instead of that Lyapunov decay. Here we study the systematic behavior of this regime in perturbed cat maps. We show that some perturbations produce coherent oscillations in the width of LDOS that imprint clear signals of the perturbation in LE decay. We also show that if the perturbation acts in a small region of phase space (local perturbation) the effect is magnified and the decay is given by the width of the LDOS.Comment: 8 pages, 8 figure

Observed quantum dynamics: classical dynamics and lack of Zeno effect

We examine a case study where classical evolution emerges when observing a quantum evolution. By using a single-mode quantum Kerr evolution interrupted by measurement of the double-homodyne kind (projecting the evolved field state into classical-like coherent states or quantum squeezed states), we show that irrespective of whether the measurement is classical or quantum there is no quantum Zeno effect and the evolution turns out to be classical.Comment: 7 pages, 1 figur

The evolution of P2P networks for file exchange: the interaction between social controversy and technical change

Since the irruption of Napster in 1999, Peer-to-Peer computer networks for file exchange have been at the heart of a heated debate that has eventually evolved into a wide social controversy across the world, involving legal, economical, and even political issues. This essay analyzes the effects of this controversy on the technical innovations that have shaped the evolution of those systems. It argues that the usual image of a single two-sided conflict does not account for most of the technical changes involved. P2P entrepreneurs and creators show a wide range of motivations and business strategies -if any- and users are not a monolithic group with a common set of goals and values. As a result, the actual historical evolution of those networks does not follow a simple linear path but a more complex and multidirectional development

Universal Response of Quantum Systems with Chaotic Dynamics

The prediction of the response of a closed system to external perturbations is one of the central problems in quantum mechanics, and in this respect, the local density of states (LDOS) provides an in- depth description of such a response. The LDOS is the distribution of the overlaps squared connecting the set of eigenfunctions with the perturbed one. Here, we show that in the case of closed systems with classically chaotic dynamics, the LDOS is a Breit-Wigner distribution under very general perturbations of arbitrary high intensity. Consequently, we derive a semiclassical expression for the width of the LDOS which is shown to be very accurate for paradigmatic systems of quantum chaos. This Letter demonstrates the universal response of quantum systems with classically chaotic dynamics.Comment: 4 pages, 3 figure