Person: Rivas Vargas, Ángel
Universidad Complutense de Madrid
Faculty / Institute
Now showing 1 - 10 of 21
PublicationDensity-matrix Chern insulators: finite-temperature generalization of topological insulators(American Physical Society, 2013-10-31) Rivas Vargas, Ángel; Viyuela García, Óscar; Martín-Delgado Alcántara, Miguel ÁngelThermal noise can destroy topological insulators (TI). However, we demonstrate how TIs can be made stable in dissipative systems. To that aim, we introduce the notion of band Liouvillian as the dissipative counterpart of band Hamiltonian, and show a method to evaluate the topological order of its steady state. This is based on a generalization of the Chern number valid for general mixed states (referred to as density-matrix Chern value), which witnesses topological order in a system coupled to external noise. Additionally, we study its relation with the electrical conductivity at finite temperature, which is not a topological property. Nonetheless, the density-matrix Chern value represents the part of the conductivity which is topological due to the presence of quantum mixed edge states at finite temperature. To make our formalism concrete, we apply these concepts to the two-dimensional Haldane model in the presence of thermal dissipation, but our results hold for arbitrary dimensions and density matrices. PublicationLa física y los medios audiovisuales(2022-09-12) Ruiz Cembranos, José Alberto; Álvarez Luna, Clara; Ángel Ortega, Sergio; Fernández Sanz, David; López Maroto, Antonio; Pantaleoni González, Michelangelo; Rivas Vargas, Ángel; Rodrigo Somolinos, Pablo FernandoEn este documento se resumen los principales resultados del proyecto de innovación docente titulado “La física y los medios audiovisuales”. Dicho proyecto fue llevado a cabo en la Facultad de Ciencias Físicas de la Universidad Complutense entre los años 2021 y 2022. PublicationAngular-momentum nonclassicality by breaking classical bounds on statistics(American Physical Society, 2011-10-12) Luis Aina, Alfredo; Rivas Vargas, ÁngelWe derive simple practical procedures revealing the quantum behavior of angular momentum variables by the violation of classical upper bounds on the statistics. Data analysis is minimum and definite conclusions are obtained without evaluation of moments, or any other more sophisticated procedures. These nonclassical tests are very general and independent of other typical quantum signatures of nonclassical behavior such as sub-Poissonian statistics, squeezing, or oscillatory statistics, being insensitive to the nonclassical behavior displayed by other variables. PublicationGeneralized toric codes coupled to thermal baths(IOP Publishing, 2012-03-30) Viyuela García, Óscar; Rivas Vargas, Ángel; Martín-Delgado Alcántara, Miguel ÁngelWe have studied the dynamics of a generalized toric code based on qudits at finite temperature by finding the master equation coupling the code’s degrees of freedom to a thermal bath. We find that in the case of qutrits, new types of anyons and thermal processes appear that are forbidden for qubits. These include creation, annihilation and diffusion throughout the system code. It is possible to solve the master equation in a short-time regime and find expressions for the decay rates as a function of the dimension d of the qudits. While we provide an explicit proof that the system relaxes to the Gibbs state for arbitrary qudits, we also prove that above a certain crossover temperature the qutrits’ initial decay rate is smaller than the original case for qubits. Surprisingly, this behavior only happens for qutrits and not for other qudits with d > 3. PublicationDynamics of thermal effects in the spin-wave theory of quantum antiferromagnets(Elsevier Masson, 2013-04) Rivas Vargas, Ángel; Martín-Delgado Alcántara, Miguel ÁngelWe derive a master equation that allows us to study non-equilibrium dynamics of a quantum antiferromagnet. By resorting to spin-wave theory, we obtain a closed analytic form for the magnon decay rates. These turn out to be closely related to form factors, which are experimentally accessible by means of neutron and Raman scattering. Furthermore, we compute the time evolution of the staggered magnetization showing that, for moderate temperatures, the magnetic order is not spoiled even if the coupling is fully isotropic. PublicationQuantum Thermodynamics in the Refined Weak Coupling Limit(MDPI, 2019-07-25) Rivas Vargas, ÁngelWe present a thermodynamic framework for the refined weak coupling limit. In this limit, the interaction between system and environment is weak, but not negligible. As a result, the system dynamics becomes non-Markovian breaking divisibility conditions. Nevertheless, we propose a derivation of the first and second law just in terms of the reduced system dynamics. To this end, we extend the refined weak coupling limit for allowing slowly-varying external drivings and reconsider the definition of internal energy due to the non-negligible interaction. PublicationSU(2)-invariant depolarization of quantum states of light(American Physical Society, 2013-11-18) Rivas Vargas, Ángel; Luis Aina, AlfredoWe develop an SU(2)-invariant approach to the depolarization of quantum systems as the effect of random unitary SU(2) transformations. From it we derive an SU(2)-invariant Markovian master equation. This is applied to several quantum states examining whether nonclassical states are more sensible to depolarization than the classical ones. Furthermore, we show that this depolarization model provides a nontrivial generalization of depolarization channels to states of arbitrary dimension. PublicationRelaxation times do not capture logical qubit dynamics(Verein Forderung Open Access Publizierens Quantenwissenschaf, 2022-01-24) Rivas Vargas, Ángel; Martín Delgado, Miguel ÁngelQuantum error correction procedures have the potential to enable faithful operation of large-scale quantum computers. They protect information from environmental decoherence by storing it in logical qubits, built from ensembles of entangled physical qubits according to suitably tailored quantum error correcting encodings. To date, no generally accepted framework to characterise the behaviour of logical qubits as quantum memories has been developed. In this work, we show that generalisations of well-established figures of merit of physical qubits, such as relaxation times, to logical qubits fail and do not capture dynamics of logical (plias. We experimentally illustrate that, in particular, spatial noise correlations can give rise to rich and counter-intuitive dynamical behavior of logical qubits. We show that a suitable set of observables, formed by code space population and logical operators within the code space, allows one to track and characterize the dynamical behaviour of logical qubits. Awareness of these effects and the efficient characterisation tools used in this work will help to guide and benchmark experimental implementations of logical qubits. PublicationSub-Heisenberg estimation of non-random phase shifts(IOP Publishing, 2012) Rivas Vargas, Ángel; Luis Aina, AlfredoWe provide evidence that the uncertainty in detection of small and deterministic phase-shift deviations from a working point can be lower than the Heisenberg bound, for a fixed finite mean number of photons. We achieve that by exploiting nonlinearity of estimators and coherence with the vacuum. PublicationRobust nonequilibrium surface currents in the three-dimensional Hofstadter model(American Physical Society, 2022-10-27) Mitchison, Mark T.; Rivas Vargas, Ángel; Martín Delgado, Miguel ÁngelGenuinely two-dimensional robust crosscurrents-which flow against the natural direction of heat flux- have been missing since the discovery of their one-dimensional counterpart. We provide a setup to realize them on a cubic three-dimensional (3D) lattice hosting a Hofstadter model coupled to two heat baths with different temperatures. We show that these currents exhibit dissipative robustness; they are stable against the presence of impurities and tilting of the gauge field in certain nonequilibrium configurations. Moreover, we find protected boundary currents with genuinely 3D robustness, i.e., they are only stable if tunneling can occur in all three spatial directions. The model also presents generic surface currents, which are robust for both bosonic and fermionic systems. We identify the underlying qualitative mechanism responsible for the robustness of the surface currents and the crucial role played by certain discrete symmetries.