arXiv subjects

Paul-Antoine Hervieux

Publications and source records attributed to Paul-Antoine Hervieux.

At least 37 records · Page 2Linked to original sources

arXiv2023

Relaxation to quantum equilibrium and the Born rule in Nelson's stochastic dynamics

Nelson's stochastic quantum mechanics provides an ideal arena to test how the Born rule is established from an initial probability distribution that is not identical to the square modulus of the wave function. Here, we investigate numerically this problem for three relevant cases: a double-slit interference setup, a harmonic oscillator, and a quantum particle in a uniform gravitational field. For all cases, Nelson's stochastic trajectories are initially localized at a definite position, thereby violating the Born rule. For the double slit and harmonic oscillator, typical quantum phenomena, such as interferences, always occur well after the establishment of the Born rule. In contrast, for the case of quantum particles free-falling in the gravity field of the Earth, an interference pattern is observed \emph{before} the completion of the quantum relaxation. This finding may pave the way to experiments able to discriminate standard quantum mechanics, where the Born rule is always satisfied, from Nelson's theory, for which an early subquantum dynamics may be present before full quantum relaxation has occurred. Although the mechanism through which a quantum particle might violate the Born rule remains unknown to date, we speculate that this may occur during fundamental processes, such as beta decay or particle-antiparticle pair production.

quant-ph↗

arXiv2023

Optimal time-entropy bounds and speed limits for Brownian thermal shortcuts

By controlling in real-time the variance of the radiation pressure exerted on an optically trapped microsphere, we engineer temperature protocols that shortcut thermal relaxation when transferring the microsphere from one thermal equilibrium state to an other. We identify the entropic footprint of such accelerated transfers and derive optimal temperature protocols that either minimize the production of entropy for a given transfer duration or accelerate as much as possible the transfer for a given entropic cost. Optimizing the trade-off yields time-entropy bounds that put speed limits on thermalization schemes. We further show how optimization expands the possibilities for accelerating Brownian thermalization down to its fundamental limits. Our approach paves the way for the design of optimized, finite-time thermodynamic cycles at the mesoscale. It also offers a platform for investigating fundamental connections between information geometry and finite-time processes.

cond-mat.stat-mech↗

arXiv2022

Harvesting information to control non-equilibrium states of active matter

We propose to use a correlated noise bath to drive an optically trapped Brownian particle that mimics active biological matter. Thanks to the flexibility and precision of our setup, we are able to control the different parameters that drive the stochastic motion of the particle with unprecedented accuracy, thus reaching strongly correlated regimes that are not easily accessible with real active matter. In particular, by using the correlation time (i.e., the "color") of the noise as a control parameter, we can trigger transitions between two non-equilibrium steady states with no expended work, but only a calorific cost. Remarkably, the measured heat production is directly proportional to the spectral entropy of the correlated noise, in a fashion that is reminiscent of Landauer's principle. Our procedure can be viewed as a method for harvesting information from the active fluctuations.

cond-mat.soft↗

arXiv2021

Fluid descriptions of quantum plasmas

Quantum fluid (or hydrodynamic) models provide an attractive alternative for the modeling and simulation of the electron dynamics in nano-scale objects. Compared to more standard approaches, such as density functional theory or phase-space methods based on Wigner functions, fluid models require the solution of a small number of equations in ordinary space, implying a lesser computational cost. They are therefore well suited to study systems composed of a very large number of particles, such as large metallic nano-objects. They can be generalized to include the spin degrees of freedom, as well as semirelativistic effects such as the spin-orbit coupling. Here, we review the basic properties, advantages and limitations of quantum fluid models, and provide some examples of their applications.

quant-ph↗

arXiv2021

Probing quantum effects with classical stochastic analogs

We propose a method to construct a classical analog of an open quantum system, namely a single quantum particle confined in a potential well and immersed in a thermal bath. The classical analog is made out of a collection of identical wells where classical particles of mass $m$ are trapped. The distribution $n(x,t)$ of the classical positions is used to reconstruct the quantum Bohm potential $V_{\rm Bohm} = -\frac{\hbar^2}{2 m} \frac{Δ\sqrt{n}}{\sqrt{n}}$, which in turn acts on the shape of the potential wells. As a result, the classical particles experience an effective "quantum" force. This protocol is tested with numerical simulations using single- and double-well potentials, evidencing typical quantum effects such as long-lasting correlations and quantum tunneling. For harmonic confinement, the analogy is implemented experimentally using micron-sized dielectric beads optically trapped by a laser beam.

quant-ph↗

arXiv2020

Noise and ergodic properties of Brownian motion in an optical tweezer: looking at the crossover between Wiener and Ornstein-Uhlenbeck processes

We characterize throughout the spectral range of an optical trap the nature of the noise at play and the ergodic properties of the corresponding Brownian motion of an overdamped trapped single microsphere, comparing experimental, analytical and simulated data. We carefully analyze noise and ergodic properties $(i)$ using the Allan variance for characterizing the noise and $(ii)$ exploiting a test of ergodicity tailored for experiments done over finite times. We derive these two observables in the low-frequency Ornstein-Uhlenbeck trapped-diffusion regime and study analytically their evolution towards the high-frequency Wiener free-diffusion regime, in a very good agreement with simulated and experimental results. This leads to reveal noise and ergodic spectral signatures associated with the distinctive features of both regimes.

cond-mat.stat-mech↗

arXiv2019

Molecular size effects on diffraction resonances in positronium formation from fullerenes

We previously predicted [P.A. Hervieux et al., Phys. Rev. A \textbf{95}, 020701 (2017)] that owing to predominant electron capture by incoming positrons from the molecular shell, C$_{60}$ acts like a spherical diffractor inducing resonances in the positronium (Ps) formation as a function of the positron impact energy. By extending the study for a larger C$_{240}$ fullerene target, we now demonstrate that the diffraction resonances compactify in energy in analogy with the shrinking fringe separation for larger slit size in classical single-slit experiment. The result brings further impetus for conducting Ps spectroscopic experiments with fullerene targets, including target- and/or captured-level differential measurements. The ground states of the fullerenes are modeled in a spherical jellium frame of the local density approximation (LDA) method with the exchange-correlation functional based on the van Leeuween and Baerends (LB94) model potential, while the positron impact and Ps formation are treated in the continuum distorted-wave final state (CDW-FS) approximation.

physics.atm-clus↗

arXiv2019

Phase-space modelling of solid-state plasmas

Conduction electrons in metallic nano-objects ($\rm 1\,nm = 10^{-9}\, m$) behave as mobile negative charges confined by a fixed positively-charged background, the atomic ions. In many respects, this electron gas displays typical plasma properties such as screening and Langmuir waves, with more or less pronounced quantum features depending on the size of the object. To study these dynamical effects, the mathematical artillery of condensed-matter theorists mainly relies on wave function $ψ(r,t)$ based methods, such as the celebrated Hartree-Fock equations. The theoretical plasma physicist, in contrast, lives and breaths in the six-dimensional phase space, where the electron gas is fully described by a probability distribution function $f(r,p,t)$ that evolves according to an appropriate kinetic equation. Here, we illustrate the power and flexibility of the phase-space approach to describe the electron dynamics in small nano-objects. Starting from classical and semiclassical scenarios, we progressively add further features that are relevant to solid-state plasmas: quantum, spin, and relativistic effects, as well as collisions and dissipation. As examples of applications, we study the spin-induced modifications to the linear response of a homogeneous electron gas and the nonlinear dynamics of the electrons confined in a thin metal films of nanometric dimensions.

cond-mat.mes-hall↗

arXiv2019

Optimal protocols and universal time-energy bound in Brownian thermodynamics

We propose an optimization strategy to control the dynamics of a stochastic system transferred from one thermal equilibrium to another and apply it experimentally to a Brownian particle in an optical trap under compression. Based on a variational principle that treats the transfer duration and the expended work on an equal footing, our strategy leads to a family of protocols that are either optimally cheap for a given duration or optimally fast for a given energetic cost. This approach unveils a universal relation $Δt\,ΔW \ge (Δt\,ΔW)_{\rm opt}$ between the transfer duration and the expended work. We verify experimentally that the lower bound is reached only with the optimized protocols.

cond-mat.stat-mech↗

arXiv2018

Plasmonic breathing modes in $\rm C_{60}$ molecules -- A quantum hydrodynamic approach

We propose and illustrate a quantum hydrodynamic (QHD) model for the description of plasmonic oscillations in the $\rm C_{60}$ molecule. Although simpler than competing approaches such as time-dependent density functional theory (TDDFT), the model contains the key ingredients to characterize plasmonic modes, namely the Hartree, exchange and correlation potentials, as well as nonlocal, nonlinear and quantum effects to the lowest order. A variational technique is used to solve analytically the QHD model for the case of breathing (monopolar) plasmonic oscillations, revealing a bulk mode near the plasmon frequency. Numerical simulations of both the QHD equations and a TDDFT model confirm the existence of this mode and highlight a second collective mode at lower energy. Such monopolar modes may be measured experimentally using electron energy loss spectroscopy.

cond-mat.mes-hall↗

arXiv2018

Magnetic moment generation in small gold nanoparticles via the plasmonic inverse Faraday effect

We theoretically investigate the creation of a magnetic moment in gold nanoparticles by circularly polarized laser light. To this end, we describe the collective electron dynamics in gold nanoparticles using a semiclassical approach based on a quantum hydrodynamic model that incorporates the prin- cipal quantum many-body and nonlocal effects, such as the electron spill-out, the Hartree potential, and the exchange and correlation effects. We use a variational approach to investigate the breathing and the dipole dynamics induced by an external electric field. We show that gold nanoparticles can build up a static magnetic moment through the interaction with a circularly polarized laser field. We analyze that the responsible physical mechanism is a plasmonic, orbital inverse Faraday effect, which can be understood from the time-averaged electron current that contains currents rotating on the nanoparticles surface. The computed laser-induced magnetic moments are sizeable, of about 0.35 muB/atom for a laser intensity of 450 GW/cm2 at plasmon resonance.

physics.comp-ph↗

arXiv2018

Ultrafast spin current generation in ferromagnetic thin films

Spin currents have been shown to play a key role in the ultrafast laser-driven demagnetization process in ferromagnetic thin films. Here, we show that an oscillating spin current can be generated in the film via the application of a femtosecond laser pulse in the visible range. In particular, we prove that the spin current is due to ballistic electrons traveling back and forth in the film. The dependence of the current intensity on the driving electric field is also investigated.

cond-mat.str-el↗

arXiv2018

Quantum hydrodynamics for nanoplasmonics

Quantum effects play a significant role in nanometric plasmonic devices, such as small metal clusters and metallic nanoshells. For structures containing a large number of electrons, ab-initio methods such as the time-dependent density functional theory (TD-DFT) are often impractical because of severe computational constraints. Quantum hydrodynamics (QHD) offers a valuable alternative by representing the electron population as a continuous fluid medium evolving under the action of the self-consistent and external fields. Although relatively simple, QHD can incorporate quantum and nonlinear effects, nonlocal effects such as the electron spillout, as well as exchange and correlations. Here, we show an application of the QHD methods to the plasmonic breathing oscillations in metallic nanoshells. We illustrate the main advantages of this approach by comparing systematically the QHD results with those obtained with a TD-DFT code.

cond-mat.mes-hall↗

arXiv2017

Spin-torque switching and control using chirped oscillating currents

We propose to use oscillating spin currents with slowly varying frequency (chirp) to manipulate and control the magnetization dynamics in a nanomagnet. By recasting the Landau-Lifshitz-Slonczewski equation in a quantum-like two-level formalism, we show that a chirped spin current polarized in the direction normal to the anisotropy axis can induce a stable precession of the magnetic moment at any angle (up to $90^\circ$) with respect to the anisotropy axis. The drive current can be modest ($10^6\,\rm A/cm^2$ or lower) provided the chirp rate is sufficiently slow. The induced precession is stable against thermal noise, even for small nano-objects at room temperature. Complete reversal of the magnetization can be achieved by adding a small external magnetic field antiparallel to the easy axis. Alternatively, a combination of chirped ac and dc currents with different polarization directions can also be used to trigger the reversal.

cond-mat.mes-hall↗

arXiv2016

Ubiquitous diffraction resonances in positronium formation from fullerenes

Due to the dominant electron capture by positrons from the molecular wall and the spatial dephasing across the wall-width, a powerful diffraction effect universally underlies the positronium (Ps) formation from fullerenes. This results into trains of resonances in the Ps formation cross section as a function of the positron beam energy, producing uniform structures in recoil momenta in analogy with classical single-slit diffraction fringes in the configuration space. The prediction opens a hitherto unknown avenue of Ps spectroscopy with nanomaterials.

physics.atm-clus↗

arXiv2016

Phase space methods for the spin dynamics in condensed matter systems

Using the phase-space formulation of quantum mechanics, we derive a four-component Wigner equation for a system composed of spin-1/2 fermions (typically, electrons) including the Zeeman effect and the spin-orbit coupling. This Wigner equation is coupled to the appropriate Maxwell equations to form a self-consistent mean-field model. A set of semiclassical Vlasov equations with spin effects is obtained by expanding the full quantum model to first order in the Planck constant. The corresponding hydrodynamic equations are derived by taking velocity moments of the phase-space distribution function. A simple closure relation is proposed to obtain a closed set of hydrodynamic equations.

cond-mat.mes-hall↗

arXiv2016

High-harmonic generation in a quantum electron gas trapped in a nonparabolic and anisotropic well

An effective self-consistent model is derived and used to study the dynamics of an electron gas confined in a nonparabolic and anisotropic quantum well. This approach is based on the equations of quantum hydrodynamics, which incorporate quantum and nonlinear effects in an approximate fashion. The effective model consists of a set of six coupled differential equations (dynamical system) for the electric dipole and the size of the electron gas. Using this model we show that: (i) High harmonic generation is related to the appearance of chaos in the phase space, as attested by related Poincaré sections; (ii) Higher order harmonics can be excited efficiently and with relatively weak driving fields by making use of chirped electromagnetic waves.

cond-mat.mes-hall↗

arXiv2014

Autoresonant control of the magnetization switching in single-domain nanoparticles

The ability to control the magnetization switching in nanoscale devices is a crucial step for the development of fast and reliable techniques to store and process information. Here we show that the switching dynamics can be controlled efficiently using a microwave field with slowly varying frequency (autoresonance). This technique allowed us to reduce the applied field by more than $30%$ compared to competing approaches, with no need to fine-tune the field parameters. For a linear chain of nanoparticles the effect is even more dramatic, as the dipolar interactions tend to cancel out the effect of the temperature. Simultaneous switching of all the magnetic moments can thus be efficiently triggered on a nanosecond timescale.

cond-mat.mes-hall↗