Optical implementability of the two-dimensional Quantum Walk

We propose an optical cavity implementation of the two-dimensional coined quantum walk on the line. The implementation makes use of only classical resources, and is tunable in the sense that a large number of different unitary transformations can be implemented by tuning some parameters of the device.Comment: 9 pages, 3 figure

Arcsine Laws in Stochastic Thermodynamics

We show that the fraction of time a thermodynamic current spends above its average value follows the arcsine law, a prominent result obtained by L\'evy for Brownian motion. Stochastic currents with long streaks above or below their average are much more likely than those that spend similar fractions of time above and below their average. Our result is confirmed with experimental data from a Brownian Carnot engine. We also conjecture that two other random times associated with currents obey the arcsine law: the time a current reaches its maximum value and the last time a current crosses its average value. These results apply to, inter alia, molecular motors, quantum dots and colloidal systems.Comment: 11 pages, 11 figure

Spin-density-wave instability in graphene doped near the van Hove singularity

We study the instability of the metallic state towards the formation of a new ground state in graphene doped near the van Hove singularity. The system is described by the Hubbard model and a field theoretical approach is used to calculate the charge and spin susceptibility. We find that for repulsive interactions, within the random phase approximation, there is a competition between ferromagnetism and spin-density wave (SDW). It turns out that a SDW with a triangular geometry is more favorable when the Hubbard parameter is above the critical value U_c(T), which depends on the temperature T, even if there are small variations in the doping. Our results can be verified by ARPES or neutron scattering experiments in highly doped graphene.Comment: 5 pages, 5 figures, 1 tabl

Entropy production and Kullback-Leibler divergence between stationary trajectories of discrete systems

The irreversibility of a stationary time series can be quantified using the Kullback-Leibler divergence (KLD) between the probability to observe the series and the probability to observe the time-reversed series. Moreover, this KLD is a tool to estimate entropy production from stationary trajectories since it gives a lower bound to the entropy production of the physical process generating the series. In this paper we introduce analytical and numerical techniques to estimate the KLD between time series generated by several stochastic dynamics with a finite number of states. We examine the accuracy of our estimators for a specific example, a discrete flashing ratchet, and investigate how close is the KLD to the entropy production depending on the number of degrees of freedom of the system that are sampled in the trajectories.Comment: 14 pages, 7 figure

Spontaneous symmetry breaking as a resource for noncritically squeezed light

In the last years we have proposed the use of the mechanism of spontaneous symmetry breaking with the purpose of generating perfect quadrature squeezing. Here we review previous work dealing with spatial (translational and rotational) symmetries, both on optical parametric oscillators and four-wave mixing cavities, as well as present new results. We then extend the phenomenon to the polarization state of the signal field, hence introducing spontaneous polarization symmetry breaking. Finally we propose a Jaynes-Cummings model in which the phenomenon can be investigated at the single-photon-pair level in a non-dissipative case, with the purpose of understanding it from a most fundamental point of view.Comment: Review for the proceedings of SPIE Photonics Europe. 11 pages, 5 figures

Antecedents and consequences of knowledge management performance: the role of IT infrastructure

Purpose: In this paper, we assess the role of knowledge management (KM) practices as a key antecedent of KM performance. Also, we examine how Information technology (IT) infrastructure is used as a driver of KM performance, organizational performance and innovation. In addition, the effects of IT infrastructure can be indirect. Specifically, we show that KM performance is a mediator between organizational performance and innovation. Design/methodology/approach: Applying a variance-based structural equation modelling (PLS), we have carried out a study among a sample of 82 Andalusian technology-intensive innovative companies. Findings: First, KM practices and IT infrastructure are significant antecedents of KM performance. Second, KM performance has a direct influence on business performance and innovation outcomes. Third, IT infrastructure does not have a direct influence on business performance and innovation outcomes, but does have a significant indirect effect on them via KM performance. Practical implications: This research provides insights for why some firms may not be realizing benefits from investing in IT infrastructure. KM performance is strongly needed for the successful implementation of IT infrastructure in the organizations. Originality/value: The findings are important for practitioners and researchers because this study makes a contribution to the literature in KM by supporting the perspective that the business and organizational performance are function of the KM performance, a complementary resource through the value of IT infrastructure is enhanced.Peer Reviewe

Síntesis y caracterización del nitruro ternario de titanio y vanadio (TixV1-xN)

En el presente trabajo se aborda la síntesis del nitruro ternario de titanio y vanadio por reducción carbotérmica de una mezcla de óxidos o bien por nitruración directa de la mezcla de metales, en todo el rango de composiciones empleando el método de análisis térmico a velocidad controlada (ATVC). La caracterización de los productos obtenidos se ha llevado a cabo mediante difracción de rayos-X y microscopia electrónica. El empleo de la técnica EELS ha mostrado su capacidad para la cuantificación de este tipo de materiales, así como la utilización de la técnica espectroscópica XAS nos ha servido para corroborar los datos obtenidos a partir de los diagramas de difracción de rayos-X sobre distancias de enlace, número de coordinación y grado de desorden, esto último no detectado por las otras técnicas usadas en la caracterización de los productos. Los valores de microdureza registrados para las muestras sintetizadas se encuentran dentro del rango de valores reportados para este tipo de materiales.Peer reviewe