arXiv subjects

Martin E. Garcia

Publications and source records attributed to Martin E. Garcia.

At least 19 recordsLinked to original sources

arXiv2026

Practical quantum tokens: challenges and perspectives

The concept of quantum tokens dates back alongside quantum cryptography to Stephen Wiesner's seminal work in 1983[1]. Already this initial work proposes society-relevant applications such as secure quantum banknotes, which can be exchanged between a bank and a customer. This quantum currency is based on various physical states that can be easily verified but is protected from being copied by the fundamental quantum laws. Four decades later, these ideas have flourished in the field of quantum information, and the concept of quantum banknotes has not only adopted many varying names, such as quantum money, quantum coins, quantum-digital payments, and quantum tokens, but also reached its first experimental demonstrations. In this perspective article, we discuss the current state-of-the-art of quantum tokens in the field of quantum information, as well as their future perspectives. We present a number of physical realizations of quantum tokens with integrated quantum memories and their applicability scenarios in detail. Finally, we discuss how quantum tokens fit into the information security ecosystem and consider their relationship to post-quantum cryptography.

quant-ph↗

arXiv2026

Multiscale Numerical Modelling of Ultrafast Laser-Matter Interactions: Maxwell Two Temperature Model Molecular Dynamics (M-TTM-MD)

In this work, we present a comprehensive numerical framework that couples numerical solutions of Maxwell's equations using the Finite-Difference Time-Domain (FDTD) approach, Molecular Dynamics (MD), and the Two-Temperature Model (TTM) to describe ultrafast laser-matter interactions in metallic systems at the atomic scale. The proposed Maxwell-Two-Temperature Model-Molecular Dynamics (M-TTM-MD) bridges the gap between electromagnetic field propagation, electron-phonon energy exchange, and atomic motion, allowing for a self-consistent treatment of energy absorption, transport, and structural response within a unified simulation environment. The calculated electromagnetic fields incorporate dispersive dielectric properties derived using the Auxiliary Differential Equation (ADE) technique, while the electronic and lattice subsystems are dynamically coupled through spatially and temporally resolved energy exchange terms. The changes in the material topography are then reflected in the updated grid for the FDTD scheme. The developed M-TTM-MD model provides a self-consistent numerical framework that offers insights into laser-induced phenomena in metals, including energy transport and surface dynamics under extreme nonequilibrium conditions.

cond-mat.mtrl-sci↗

arXiv2025

Ensemble-Based Quantum Token Protocol Benchmarked on IBM Quantum Processors

Quantum tokens envision to store unclonable quantum states in a physical device, with the goal of being used for personal authentication protocols, as required by banks. Still, the experimental realization of such devices faces many technical challenges, which can be partially mitigated using ensembles instead of single qubits. In this work, we thus propose an ensemble-based quantum token protocol, describing it through a simple yet general model based on a quantum mechanical observable. The protocol is benchmarked on five IBM quantum processors and a general hacker attack scenario is analyzed, in which the attacker attempts to read the bank token and forge a fake one, based on the information gained from this measurement. We experimentally demonstrate that the probability that the bank erroneously accepts a forged coin composed of multiple tokens can reach values below $10^{-22}$, while the probability that the bank accepts its own coin is above 0.999. The overall security of the protocol is therefore demonstrated within a hardware-agnostic framework, confirming the practical viability of the protocol in arbitrary quantum systems and thus paving the way for future applications with different ensembles of qubits, such as color center defects in solids.

quant-ph↗

arXiv2025

Pausing ultrafast melting by timed multiple femtosecond-laser pulses

An intense femtosecond-laser excitation of a solid induces highly nonthermal conditions. In materials like silicon, laser-induced bond-softening leads to a highly incoherent ionic motion and eventually nonthermal melting. But is this outcome an inevitable consequence, or can it be controlled? Here, we performed ab initio molecular dynamics simulations of crystalline silicon after timed multiple femtosecond-laser pulse excitations with fluence above the nonthermal melting threshold. Our results demonstrate an excitation mechanism that pauses nonthermal melting and creates a metastable state instead, with an electronic structure similar to the ground state. This mechanism can be generalized to other materials, potentially enabling structural and/or electronic transitions to metastable phases in the high-excitation regime. In addition, our approach could be used to switch off nonthermal contributions in experiments, allowing reliable electron-phonon coupling constants to be obtained more easily.

cond-mat.mtrl-sci↗

arXiv2024

Tuned ionic mobility by Ultrafast-laser pulses in Black Silicon

Highly non-equilibrium conditions in femtosecond-laser excited solids cause a variety of ultrafast phenomena that are not accessible by thermal conditions, like sub-picosecond solid-to-liquid or solid-to-solid phase transitions. In recent years the microscopic pathways of various laser-induced crystal rearrangements could be identified and led to novel applications and/or improvements in optoelectronics, photonics, and nanotechnology. However, it remains unclear what effect a femtosecond-laser excitation has on ionic impurities within an altered crystal environment, in particular on the atomic mobility. Here, we performed ab-initio molecular dynamics (AIMD) simulations on laser-excited black silicon, a promising material for high-efficient solar cells, using the Code for Highly excIted Valence Electron Systems (CHIVES). By computing time-dependent Bragg peak intensities for doping densities of 0.16% and 2.31% we could identify the overall weakening of the crystal environment with increasing impurity density. The analysis of Si-S bond angles and lengths after different excitation densities, as well as computing interatomic forces allowed to identify a change in ion mobility with increasing impurity density and excitation strength. Our results indicate the importance of impurity concentrations for ionic mobility in laser-excited black silicon and could give significant insight for semiconductor device optimization and materials science advancement.

cond-mat.mtrl-sci↗

arXiv2024

Unified theoretical description of thermal and nonthermal laser-induced ultrafast structural changes in solids

The ultrafast dynamics of ions in solids following intense femtosecond laser excitation is governed by two fundamentally distinct yet interplaying effects. On one hand, the significant generation of hot electron-hole pairs by the light pulse alters the strength and nature of interatomic bonding, resulting in nonthermal ionic motion. On the other hand, incoherent electron-ion collisions drives an equilibration of the electrons and the ions, which reach a common temperature on a picosecond timescale. This letter introduces, for the first time, a unified theory that comprehensively accounts for both phenomena. Our approach is versatile, applicable to both ab-initio and large-scale molecular dynamics simulations, and leads to a generalization of the two-temperature model-molecular-dynamics equations of motion. To illustrate the effectiveness of our method, we apply it to describe the laser excitation of silicon thin films. Our simulations reproduce the time evolution of the Bragg peaks in excellent agreement with the experiments.

physics.comp-ph↗

arXiv2023

Biological rhythms generated by a single activator-repressor loop with heterogeneity and diffusion

Common models of circadian rhythms are constructed as compartmental reactions of well mixed biochemicals involving a negative-feedback loop containing several intermediate reaction steps in order to enable oscillations. Spatial transport of reactants is mimicked as an extra compartmental reaction step. In this letter, we show that a single activation-repression biochemical reaction pair is enough to produce sustained oscillations, if the sites of both reactions are spatially separated and molecular transport is mediated by diffusion. Our proposed scenario is the simplest possible one in terms of the participating chemical reactions and provides a conceptual basis for understanding biological oscillations and triggering in-vitro assays aimed at constructing minimal clocks.

physics.bio-ph↗

arXiv2021

Correction of Density-Functional-Theory based polynomial interatomic potentials to reproduce experimental melting properties

Recently, we developed a method to construct polynomial interatomic potentials from ab-initio calculations in order to accurately describe laser excited solids [PRL 124, 085501 (2020)]. However, ab-initio methods, and therefore analytical potentials derived from them, commonly do not provide an accurate prediction of the melting temperature. In order to reproduce the experimental melting properties, but keeping the accuracy in the laser excited case, we present here an approach to modify few key coefficients of polynomial interatomic potentials constructed from ab-initio data. We show that, with the help of such corrections, the electronic-temperature dependent interatomic potential for silicon can, at the same time, describe nonthermal laser induced effects with ab-initio accuracy and also provide the correct experimental melting temperature and slope $dT/dp$.

cond-mat.mtrl-sci↗

arXiv2021

The SARS-CoV-2 Spike Protein is vulnerable to moderate electric fields

Most of the ongoing projects aimed at the development of specific therapies and vaccines against COVID-19 use the SARS-CoV-2 spike (S) protein as the main target [1-3]. The binding of the spike protein with the ACE2 receptor (ACE2) of the host cell constitutes the first and key step for virus entry. During this process, the receptor binding domain (RBD) of the S protein plays an essential role, since it contains the receptor binding motif (RBM), responsible for the docking to the receptor. So far, mostly biochemical methods are being tested in order to prevent binding of the virus to ACE2 [4]. Here we show, with the help of atomistic simulations, that external electric fields of easily achievable and moderate strengths can dramatically destabilise the S protein, inducing long-lasting structural damage. One striking field-induced conformational change occurs at the level of the recognition loop L3 of the RBD where two parallel beta sheets, believed to be responsible for a high affinity to ACE2 [5], undergo a change into an unstructured coil, which exhibits almost no binding possibilities to the ACE2 receptor (Figure 1a). Remarkably, while the structural flexibility of S allows the virus to improve its probability of entering the cell, it is also the origin of the surprising vulnerability of S upon application of electric fields of strengths at least two orders of magnitude smaller than those required for damaging most proteins. Our findings suggest the existence of a clean physical method to weaken the SARS-CoV-2 virus without further biochemical processing. Moreover, the effect could be used for infection prevention purposes and also to develop technologies for in-vitro structural manipulation of S. Since the method is largely unspecific, it can be suitable for application to mutations in S, to other proteins of SARS-CoV-2 and in general to membrane proteins of other virus types.

physics.bio-ph↗

arXiv2018

Self-learning analytical interatomic potential describing laser-excited silicon

We develop an electronic-temperature dependent interatomic potential $Φ(T_\text{e})$ for unexcited and laser-excited silicon. The potential is designed to reproduce ab initio molecular dynamics simulations by requiring force- and energy matching for each time step. $Φ(T_\text{e})$ has a simple and flexible analytical form, can describe all relevant interactions and is applicable for any kind of boundary conditions (bulk, thin films, clusters). Its overall shape is automatically adjusted by a self-learning procedure, which finally finds the global minimum in the parameter space. We show that $Φ(T_\text{e})$ can reproduce all thermal and nonthermal features provided by ab initio simulations. We apply the potential to simulate laser-excited Si nanoparticles and find critical damping of their breathing modes due to nonthermal melting.

cond-mat.mtrl-sci↗

arXiv2017

Coherent and Incoherent Structural Dynamics in Laser-Excited Antimony

We investigate the excitation of phonons in photoexcited antimony and demonstrate that the entire electron-lattice interactions, in particular coherent and incoherent electron-phonon coupling, can be probed simultaneously. Using femtosecond electron diffraction (FED) with high temporal resolution, we observe the coherent excitation of the fully symmetric \Ag\ optical phonon mode via the shift of the minimum of the atomic potential energy surface. Ab initio molecular dynamics simulations on laser excited potential energy surfaces are performed to quantify the change in lattice potential and the associated real-space amplitude of the coherent atomic oscillations. Good agreement is obtained between the parameter-free calculations and the experiment. In addition, our experimental configuration allows observing the energy transfer from electrons to phonons via incoherent electron-lattice scattering events. The electron-phonon coupling is determined as a function of electronic temperature from our DFT calculations and the data by applying different models for the energy-transfer.

cond-mat.mtrl-sci↗

arXiv2013

Selecting fast folding proteins by their rate of convergence

We propose a general method for predicting potentially good folders from a given number of amino acid sequences. Our approach is based on the calculation of the rate of convergence of each amino acid chain towards the native structure using only the very initial parts of the dynamical trajectories. It does not require any preliminary knowledge of the native state and can be applied to different kinds of models, including atomistic descriptions. We tested the method within both the lattice and off-lattice model frameworks and obtained several so far unknown good folders. The unbiased algorithm also allows to determine the optimal folding temperature and takes at least 3--4 orders of magnitude less time steps than those needed to compute folding times.

physics.bio-ph↗

arXiv2010

Exact and approximate symmetries for light propagation equations with higher order nonlinearity

For the first time exact analytical solutions to the eikonal equations in (1+1) dimensions with a refractive index being a saturated function of intensity are constructed. It is demonstrated that the solutions exhibit collapse; an explicit analytical expression for the self-focusing position, where the intensity tends to infinity, is found. Based on an approximated Lie symmetry group, solutions to the eikonal equations with arbitrary nonlinear refractive index are constructed. Comparison between exact and approximate solutions is presented. Approximate solutions to the nonlinear Schrodinger equation in (1+2) dimensions with arbitrary refractive index and initial intensity distribution are obtained. A particular case of refractive index consisting of Kerr refraction and multiphoton ionization is considered. It is demonstrated that the beam collapse can take place not only at the beam axis but also in an off-axis ring region around it. An analytical condition distinguishing these two cases is obtained and explicit formula for the self-focusing position is presented.

math-ph↗

arXiv2008

Laser-induced solid-solid phase transition in As under pressure: A theoretical prediction

In Arsenic a pressure-induced solid-solid phase transition from the A7 into the simple cubic structure has been experimentally demonstrated [Beister et al., Phys. Rev. B 41, 5535 (1990)]. In this paper we present calculations, which predict that this phase transition can also be induced by an ultrashort laser pulse in As under pressure. In addition, calculations for the pressure-induced phase transition are presented. Using density functional theory in the generalized gradient approximation, we found that the pressure-induced phase transition takes place at 26.3 GPa and is accompanied by a volume change "Delta V" = 0.5 bohr^3/atom. The laser-induced phase transition is predicted for an applied pressure of 23.8 GPa and an absorbed laser energy of 2.8 mRy/atom.

cond-mat.other↗

arXiv2008

Exact description of self-focusing in highly nonlinear geometrical optics

We demonstrate that laser beam collapse in highly nonlinear media can be described, for a large number of experimental conditions, by the geometrical optics approximation within high accuracy. Taking into account this fact we succeed in constructing analytical solutions of the eikonal equation, which are exact on the beam axis and provide: i) a first-principles determination of the self-focusing position, thus replacing the widely used empirical Marburger formula, ii) a benchmark solution for numerical simulations, and iii) a tool for the experimental determination of the high-order nonlinear susceptibility. Successful comparison with several experiments is presented.

physics.optics↗

arXiv2007

Optimized Gaussian exponents for Goedecker-Teter-Hutter pseudopotentials

We have optimized the exponents of Gaussian s and p basis functions for the elements H, B-F, and Al-Cl using the pseudopotentials of Goedecker, Teter, and Hutter [Phys. Rev. B 54, 1703 (1996)] by minimizing the total energy of dimers. We found that this procedure causes the Gaussian to be somewhat more localized than the usual procedure, where the exponents are optimized for atoms. We further found that three exponents, equal for s and p orbitals, are sufficient to reasonably describe the electronic structure of all elements that we have studied. For Li and Be results are presented for pseudopotentials of Hartwigsen et al. [Phys. Rev. B 58, 3641 (1998)]. We expect that our exponents will be useful for density functional theory studies where speed is important.

cond-mat.other↗

arXiv2007

Negative differential resistance of Styrene on an ideal Si[111] surface: dependence of the I-V characteristics on geometry, surface doping and shape of the STM-tip

We study the electron transport properties through a supported organic molecule styrene (C8H8) on an ideal silicon surface Si[111] and probed by a STM-tip. The I-V characteristics and the differential conductance of the molecule are calculated using a self consistent approach based on non equilibrium Green's functions. Two different adsorption configurations for the molecule on the surface were considered which corresponds to a global and a local minimum of the total energy. In both cases we find a negative differential resistance (NDR) in a given interval of bias voltages. This effect is controlled by the states available close to the Fermi level of the surface and can be manipulated by properly doping the substrate. We also analyze the influence of the tip-shape on the I-V characteristics.

cond-mat.mtrl-sci↗

arXiv2006

Break-down of the density-of-states description of scanning tunneling spectroscopy in supported metal clusters

Low-temperature scanning tunneling spectroscopy allows to probe the electronic properties of clusters at surfaces with unprecedented accuracy. By means of quantum transport theory, using realistic tunneling tips, we obtain conductance curves which considerably deviate from the cluster's density of states. Our study explains the remarkably small number of peaks in the conductance spectra observed in recent experiments. We demonstrate that the unambiguous characterization of the states on the supported clusters can be achieved with energy-resolved images, obtained from a theoretical analysis which mimics the experimental imaging procedure.

cond-mat.other↗