Farias, M. Belén and Groenendijk, Solofo and Schmidt, Thomas L., Generalized Chern Numbers Based on Open System Green's Functions, arXiv:2102.08283 [cond-mat.mes-hall] (2021)
We present an alternative approach to studying topology in open quantum systems, relying directly on Green's functions and avoiding the need to construct an effective non-Hermitian Hamiltonian. We define an energy-dependent Chern number based on the eigenstates of the inverse Green's function matrix of the system which contains, within the self-energy, all the information about the influence of the environment, interactions, gain or losses. We explicitly calculate this topological invariant for a system consisting of a single 2D Dirac cone and find that it is half-integer quantized when certain assumptions over the damping are made. Away from these conditions, which cannot or are not usually considered within the formalism of non-Hermitian Hamiltonians, we find that such a quantization is usually lost and the Chern number vanishes, and that in special cases, it can change to integer quantization.
Wu, Kunmin and Schmidt, Thomas L. and Farias, M. Belén, Quantum Friction in the Hydrodynamic Model, arXiv:2012.10204 [quant-ph] (2020)
We study the phenomenon of quantum friction in a system consisting of a polarizable atom moving at a constant speed parallel to a metallic plate. The metal is described using a charged hydrodynamic model for the electrons. This model featuring long-range interactions is appropriate for a clean metal in a temperature range where scattering due to Coulomb interactions dominates over the scattering of electron by impurities. We find that a quantum friction force between the atom and the metal surface exists even in the absence of intrinsic damping in the metal, but that it only starts once the velocity of the atom exceeds the effective speed of sound in the metal. We argue that this condition can be fulfilled most easily in metals with nearly empty or nearly filled bands. We make quantitative predictions for the friction force to the second and fourth order in the atomic polarizability, and show that the threshold behavior persists to all orders of the perturbation theory.
Farias, M. Belén and Zyuzin, Alexander A. and Schmidt, Thomas L., Casimir force between Weyl semimetals in a chiral medium, Phys. Rev. B 101, 235446 (2020)
Farı́as, M Belén and Lombardo, Fernando C and Soba, Alejandro and Villar, Paula I and Decca, Ricardo S, Towards detecting traces of non-contact quantum friction in the corrections of the accumulated geometric phase, npj Quantum Information 6, 1--7 (2020)
Drechsler, Martín and Farías, M. Belén and Freitas, Nahuel and Schmiegelow, Christian Tomás and Paz, Juan Pablo, State dependent motional squeezing of a trapped ion: new method and applications, arXiv:1911.05810 (2019)
We show that the motion of a cold trapped ion can be squeezed by modulating the intensity of a phase-stable optical lattice placed inside the trap. As this method is reversible and state selective it effectively implements a controlled-squeeze gate. We show how to use this resource, that can be useful for quantum information processing with continuous variables, in order to prepare coherent superpositions of states which are squeezed along complementary quadratures. We show that these states, which we denote as "$Ξ$-states", exhibit high sensitivity to small displacements along two complementary quadratures which make them useful for quantum metrology.
Farías, M. Belén and Fosco, C. D. and Lombardo, Fernando C. and Mazzitelli, Francisco D., Motion induced radiation and quantum friction for a moving atom, Phys. Rev. D 100, 036013 (2019)
We study quantum dissipative effects that result from the nonrelativistic motion of an atom, coupled to a quantum real scalar field, in the presence of a static imperfect mirror. Our study consists of two parts: in the first, we consider accelerated motion in free space, namely, switching off the coupling to the mirror. This results in motion induced radiation, which we quantify via the vacuum persistence amplitude. In the model we use, the atom is described by a quantum harmonic oscillator (QHO). We show that its natural frequency poses a threshold which separates different regimes, involving or not the internal excitation of the oscillator, with the ulterior emission of a photon. At higher orders in the coupling to the field, pairs of photons may be created by virtue of the dynamical Casimir effect (DCE). In the second part, we switch on the coupling to the mirror, which we describe by localized microscopic degrees of freedom. We show that this leads to the existence of quantum contactless friction as well as to corrections to the free space emission considered in the first part. The latter are similar to the effect of a dielectric on the spontaneous emission of an excited atom. We have found that, when the atom is accelerated and close to the plate, it is crucial to take into account the losses in the dielectric in order to obtain finite results for the vacuum persistence amplitude.
Viotti, Ludmila and Farías, M. Belén and Villar, Paula I. and Lombardo, Fernando C., Thermal corrections to quantum friction and decoherence: A closed-time-path approach to atom-surface interaction, Phys. Rev. D 99, 105005 (2019)
In this paper we study the dissipative effects and decoherence induced on a particle moving at constant speed in front of a dielectric plate in quantum vacuum, developing a closed-time-path (CTP) integral formulation in order to account for the corrections to these phenomena generated by finite temperatures. We compute the frictional force of the moving particle and find that it contains two different contributions: a pure quantum term due to quantum fluctuations (even present at vanishing temperatures) and a temperature-dependent component generated by thermal fluctuations (the bigger the contribution, the higher the temperature). We further estimate the decoherence timescale for the internal degree of freedom of the quantum particle. As expected, decoherence time is reduced by temperature; however, this feature is stronger for large velocities and for resonant situations. When the particle approaches relativistic speed, decoherence time becomes independent of temperature. The finite temperature corrections to the force or even in the decoherence timescale could be used to track traces of quantum friction through the study of the velocity dependence since the sole evidence of this dependence provides an indirect testimony of the existence of a quantum frictional force.
Farias, M. Belén and Kort-Kamp, Wilton J. M. and Dalvit, Diego A. R., Quantum friction in two-dimensional topological materials, Phys. Rev. B 97, 161407 (2018)
We develop the theory of quantum friction in two-dimensional topological materials. The quantum drag force on a metallic nanoparticle moving above such systems is sensitive to the nontrivial topology of their electronic phases, shows a novel distance scaling law, and can be manipulated through doping or via the application of external fields. We use the developed framework to investigate quantum friction due to the quantum Hall effect in magnetic field biased graphene, and to topological phase transitions in the graphene family materials. It is shown that topologically nontrivial states in two-dimensional materials enable an increase of two orders of magnitude in the quantum drag force with respect to conventional neutral graphene systems.
Farias, M. Belén and Fosco, César D. and Lombardo, Fernando C. and Mazzitelli, Francisco D., Quantum friction between graphene sheets, Phys. Rev. D 95, 065012 (2017)
We study the Casimir friction phenomenon in a system consisting of two flat, infinite, and parallel graphene sheets, which are coupled to the vacuum electromagnetic (EM) field. Those couplings are implemented, in the description we use, by means of specific terms in the effective action for the EM field. They incorporate the distinctive properties of graphene, as well as the relative sliding motion of the sheets. Based on this description, we evaluate two observables due to the same physical effect: the probability of vacuum decay and the frictional force. The system exhibits a threshold for frictional effects; namely, they only exist if the speed of the sliding motion is larger than the Fermi velocity of the charge carriers in graphene.
Klatt, J. and Farías, M. Belén and Dalvit, D. A. R. and Buhmann, S. Y., Quantum friction in arbitrarily directed motion, Phys. Rev. A 95, 052510 (2017)
Quantum friction, the electromagnetic fluctuation-induced frictional force decelerating an atom which moves past a macroscopic dielectric body, has so far eluded experimental evidence despite more than three decades of theoretical studies. Inspired by the recent finding that dynamical corrections to such an atom's internal dynamics are enhanced by one order of magnitude for vertical motion—compared with the paradigmatic setup of parallel motion—we generalize quantum friction calculations to arbitrary angles between the atom's direction of motion and the surface in front of which it moves. Motivated by the disagreement between quantum friction calculations based on Markovian quantum master equations and time-dependent perturbation theory, we carry out our derivations of the quantum frictional force for arbitrary angles by employing both methods and compare them.
Farías, M. Belén and Lombardo, Fernando C., Dissipation and decoherence effects on a moving particle in front of a dielectric plate, Phys. Rev. D 93, 065035 (2016)
In this work, we consider a particle moving in front of a dielectric plate and study two of the most relevant effects of the vacuum field fluctuations: the dissipation and the decoherence of the particle’s internal degrees of freedom. We consider the particle to follow a classical, macroscopically fixed trajectory. To study the dissipative effects, we calculate the in-out effective action by functionally integrating over the vacuum field and the microscopic degrees of freedom of both the plate and the particle. This in-out effective action develops an imaginary part and, hence, a nonvanishing probability for the decay (because of friction) of the initial vacuum state. We analyze how the dissipation is affected by the relative velocity between the particle and the plate and the properties of the microscopic degrees of freedom. In order to study the effects of decoherence over the internal degrees of freedom of the particle, we calculate the closed time path or Schwinger-Keldysh influence action, by functionally integrating over the vacuum field and the microscopic degrees of freedom of the plate. We estimate the decoherence time as the time needed by two different quantum configurations (of the internal degree of freedom of the particle) to be possible to differentiate from one another. We analyze the way in which the presence of the mirror affects the decoherence and the possible ways to maximize or reduce its effects.
Farías, M. Belén and Fosco, César D. and Lombardo, Fernando C. and Mazzitelli, Francisco D. and Rubio López, Adrián E., Functional approach to quantum friction: Effective action and dissipative force, Phys. Rev. D 91, 105020 (2015)
We study the Casimir friction due to the relative, uniform, lateral motion of two parallel semitransparent mirrors coupled to a vacuum real scalar field ϕ. We follow a functional approach, whereby nonlocal terms in the action for $ψ$, concentrated on the mirrors’ loci, appear after functional integration of the microscopic degrees of freedom. This action for $ψ$, which incorporates the relevant properties of the mirrors, is then used as the starting point for two complementary evaluations: Firstly, we calculate the in-out effective action for the system, which develops an imaginary part, hence a nonvanishing probability for the decay (because of friction) of the initial vacuum state. Secondly, we evaluate another observable: the vacuum expectation value of the frictional force, using the in-in or closed time path formalism. Explicit results are presented for zero-width mirrors and half-spaces, in a model where the microscopic degrees of freedom at the mirrors are a set of identical quantum harmonic oscillators, linearly coupled to $ψ$.
Farias, María Belén and Quinteiro, Guillermo Federico and Tamborenea, Pablo Ignacio, Photoexcitation of graphene with twisted light, The European Physical Journal B 86, 432 (2013)
We study theoretically the interaction of twisted light with graphene. The light-matter interaction matrix elements between the tight-binding states of electrons in graphene are determined near the Dirac points. We examine the dynamics of the photoexcitation process by posing the equations of motion of the density matrix and working up to second order in the field. The time evolution of the angular momentum of the photoexcited electrons and their associated photocurrents are examined in order to elucidate the mechanisms of angular momentum transfer. We find that the transfer of spin and orbital angular momentum from light to the electrons is more akin here to the case of intraband than of interband transitions in semiconductors, due to the fact that the two relevant energy bands of graphene originate from the same atomic orbitals.