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Quantum tokens envision to store unclonable authentication keys in quantum states that are issued by a bank for example. In contrast to quantum communication, the information is not transmitted, but rather used for personal authentication in a physical device. Still, its experimental realization faces many technical challenges. In this work, we propose an ensemble-based quantum-token protocol, making these applications technologically less-demanding. A simple and minimal model is developed to describe the quantum token hardware, while the protocol is fully benchmarked and compared on five different IBM quantum processors. First, the uncertainties of the hardware are characterized, from which the main quality parameters that describe the token can be extracted. Following that, the fraction of qubits which the bank prepares and measures successfully is benchmarked. These fractions are then compared with the values obtained from an attacker who attempts to read the bank token and prepare a forged key. From which we experimentally demonstrate an acceptance probability of 0.057 for a forged token, in contrast to 0.999 for the bank's own tokens. These values can be further optimized by increasing the number of tokens in the device. Finally, we show that minor improvements in the hardware quality lead to significant increases in the protocol security, denoting a great potential of the protocol to scale with the ongoing quantum hardware evolution. We provide an open source tool with graphical user interface to benchmark the protocol with custom ensemble based qubits. This work demonstrates the overall security of the protocol within a hardware-agnostic framework, further confirming the interoperability of the protocol in arbitrary quantum systems and thus paving the way for future applications with different qubits., Comment: 14 pages, 10 figures, associated Github repository https://github.com/lucas-tsunaki/quantum-token

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., Comment: 12 pages, 9 figures

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., Comment: 12 pages, 2 figures, 20 equations

Owing to their multiple applications, lithium ferrites are relevant materials for several emerging technologies. For instance, LiFeO2 has been spotted as an alternative cathode material in Li-ion batteries, while LiFe5O8 is the lowest damping ferrite, holding promise in the field of spintronics. The Li-content in lithium ferrites has been shown to greatly affect the physical properties, and in turn, the performance of functional devices based on these materials. Despite this, lithium content is rarely accurately quantified, as a result of the low number of electrons in Li hindering its identification by means of routine materials characterization methods. In the present work, magnetic lithium ferrite powders with Li:Fe ratios of 1:1, 1:3 and 1:5 have been synthesized, successfully obtaining phase-pure materials (LiFeO2 and LiFe5O8), as well as a controlled mixture of both phases. The powders have been compacted and subsequently sintered by thermal treatment (Tmax = 1100 {\deg}C) to fabricate dense pellets which preserve the original Li:Fe ratios. Li-content on both powders and pellets has been determined by two independent methods: (i) Rutherford backscattering spectroscopy combined with nuclear reaction analysis and (ii) Rietveld analysis of powder X-ray diffraction data. With good agreement between both techniques, it has been confirmed that the Li:Fe ratios employed in the synthesis are maintained in the sintered ceramics. The same conclusion is drawn from spatially-resolved confocal Raman microscopy experiments on regions of a few microns. Field emission scanning electron microscopy has evidenced the substantial grain growth taking place during the sintering process - mean particle sizes rise from about 600 nm in the powders up to 3.8(6) um for dense LiFeO2 and 10(2) um for LiFe5O8 ceramics.

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., Comment: Letter plus Suppl. Mat

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., Comment: 13 pages, 4 figures (main), 16 pages, 4 figures (supplemental)

Li metal has been deposited on the surface of a Ru(0001) single crystal containing patches of monolayer-thick epitaxial graphene islands. The use of low-energy electron microscopy and diffraction allowed us to {\em in situ} monitor the process by measuring the local work function as well as to study the system in real and reciprocal space, comparing the changes taking place on the graphene with those on the bare Ru(0001) surface. It is found that Li deposition decreases the work function of the graphene islands but to a much smaller degree than of the Ru(0001) surface, as corresponds to its intercalation below the graphene overlayer. Finally, the diffusion process of Li out of the graphene islands has been monitored by photoelectron microscopy using a visible-light laser., Comment: 6 pages, 5 figures. Submitted to Applied Surface Science

Thin epitaxial films of metals on insulating substrates are essential for many applications, as conducting layers, in magnetic devices or as templates for further growth. In this work, we report on the growth of epitaxial Ru films on single-crystalline Al$_2$O$_3$(0001) substrates by magnetron sputtering and their subsequent systematic characterization using Rutherford backscattering spectrometry of He ions both in random and in channeling conditions. We include results of a Ru(0001) single crystal for comparison. Analysis of channeling shows that films thicker than 35 nm grow with (0001) orientation, a well-defined epitaxial relation with the substrate and a high degree of crystal quality, comparable to the Ru(0001) single crystal. Thinner films of down to 7 nm in thickness, for which relaxation of epitaxial strain is not complete, produce a similar degree of dechanneling. The surface of the films can be prepared in a clean and ordered state in order to allow further epitaxial growth on top., Comment: 8 pages (two columns), 5 figures, submitted to Applied Surface Science

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 $dT/dp$., Comment: 27 pages, 14 figures, 4 tables

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., Comment: 27 pages (main paper and extended data), 7 figures (3 in main paper and 4 in extended data)

We develop an electronic-temperature dependent interatomic potential $\Phi (T_\text{e})$ 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. $\Phi (T_\text{e})$ 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 $\Phi (T_\text{e})$ 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.

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.

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.

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., Comment: 25 pages, 5 figures

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., Comment: 3 figures

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., Comment: 9 pages, 5 figures Changes to content To be published in New Journal of Physics (accepted for publication)

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., Comment: 5 pages, 2 figures

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., Comment: 20 pages, 13 figures

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., Comment: 5 pages, 3 figures

Using first principles, all-electron calculations and dynamical simulations we study the behavior of the A_1g and E_g coherent phonons induced in Bi by intense laser pulses. We determine the potential landscapes in the laser heated material and show that they exhibit phonon-softening, phonon-phonon coupling, and anharmonicities. As a consequence the E_g mode modulates the A_1g oscillations and higher harmonics of both modes appear, which explains recent isotropic reflectivity measurements. Our results offer a unified description of the different experimental observations performed so far on bismuth., Comment: 3 figures

The pairwise quantum entanglement of sites in disordered electronic one-dimensional systems (rings) is studied. We focus on the effect of diagonal and off diagonal disorder on the concurrence $C_{ij}$ between electrons on neighbor and non neighbor sites $i,j$ as a function of band filling. In the case of diagonal disorder, increasing the degree of disorder leads to a decrease of the concurrence with respect to the ordered case. However, off-diagonal disorder produces a surprisingly strong enhancement of entanglement. This remarkable effect occurs near half filling, where the concurrence becomes up to 15% larger than in the ordered system., Comment: 21 pages, 9 figures

We present a detailed theoretical study of scanning tunneling imaging and spectroscopy of \Csixty on silver and gold surfaces, motivated by the recent experiments and discussion by X. Lu et al. [PRL \textbf{90}, 096802 (2003) and PRB \textbf{70}, 115418 (2004)]. The surface/sample/tip system is described within a self--consistent DFT based tight--binding model. The topographic and conductance images are computed at constant current from a full self--consistent transport theory based on nonequilibrium Green's functions and compared with those simulated from the local density of states. The molecular orbitals of \Csixty are clearly identified in the energy resolved maps, in close correspondence with the experimental results. We show how the tip structure and orientation can affect the images. In particular, we consider the effects of truncated tips on the energy resolved maps., Comment: 9 pages, 8 figures

The optimal control of two-level systems by time-dependent laser fields is studied using a variational theory. We obtain, for the first time, general analytical expressions for the optimal pulse shapes leading to global maximization or minimization of different physical quantities. We present solutions which reproduce and improve previous numerical results., Comment: 12 pages, 2 figures

We present variational theory for optimal control over a finite time interval in quantum systems with relaxation. The corresponding Euler-Lagrange equations determining the optimal control field are derived. In our theory the optimal control field fulfills a high order differential equation, which we solve analytically for some limiting cases. We determine quantitatively how relaxation effects limit the control of the system. The theory is applied to open two level quantum systems. An approximate analytical solution for the level occupations in terms of the applied fields is presented. Different other applications are discussed.