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  • PublicaciónAcceso abierto
    Teoría de perturbaciones en teorías de gravedad modificada
    (Universidad de los Andes, 2022-08-03) Molano Moreno, Daniel Alejandro
    Argumentos teóricos y observacionales sugieren que la Relatividad General (RG) quizás no es el ultimo modelo gravitacional. Por ende, han surgido una serie de extensiones al modelo de RG en donde encontrar soluciones exactas a las ecuaciones propuestas es una tarea de alta complejidad. Las técnicas perturbativas desarrolladas en RG son importantes para poder encontrar descripciones físicas y matemáticas de desviaciones a soluciones exactas conocidas, también para poder comparar los modelos gravitacionales modificados con RG y poder discriminar entre estas teorías extendidas. En este trabajo, usando una base matemática formal bien definida de la teoría de perturbaciones, se demuestra que las familias de soluciones en algunos modelos de teorías de gravedad modificada f(R), están desconectadas de las soluciones en RG en el vacío [1] y se muestra como puede ampliarse este resultado a cualquier modelo de f(R) en tanto f sea analítica. Adicionalmente, se muestra como tendríamos resultados similares en otras teorías extendidas, suponiendo algunas conjeturas las cuales son discutidas. Por otro lado, desde el formalismo de teoría de perturbaciones invariante gauge de Nakamura se encuentran las ecuaciones perturbadas a primer y segundo orden y se usan en cosmología a primer orden en f(R). Posteriormente, se encuentran cantidades invariantes gauge y se comparan con resultados particulares encontrados en la literatura. Finalmente, en el este marco de las perturbaciones cosmológicas invariantes gauge, se discute como ampliar estos resultados a segundo orden y en teorías escalar tensor.
  • PublicaciónAcceso abierto
    Fabricación y caracterización de nanodiscos y nanopelículas de FeOx depositados por e-beam para aplicaciones médicas y basadas en Exchange Bias
    (Universidad de los Andes, 2023-12-13) Hernández Gómez, Daniel Fabián
    Los compuestos de hierro nanoestructurados ofrecen alta magnetización y biocompatibilidad, facilitando avances en medicina. Aunque su funcionalización otorga propiedades superparamagnéticas cruciales, la dificultad para lograrlas limita su uso en favor de opciones menos biocompatibles. Además, las interacciones magnéticas en las fases de óxidos de hierro aún requieren investigación, ya que su manipulación podría simplificar el control magnético y facilitar la obtención de propiedades superparamagnéticas. En este sentido, en este trabajo se fabrican nanopelículas por e-beam y nanodiscos mediante litografía de interferencia con e-beam de óxidos de hierro (FeOx) que presentan un inesperado comportamiento superparamagnetico a temperatura ambiente, o cuanto menos tipo-superparamagnético (superparamagnetic-like). Además, encontramos fenómenos de Exchange Bias a más bajas temperaturas, lo que da luz sobre los mecanismos a lugar que dan origen al comportamiento tipo-superparamagnetico encontrado. Las películas fabricadas fueron caracterizadas mediante magnetometría SQUID en función de la temperatura y campo magnético, espectroscopia Raman, XPS, reflectometría XRR. Las curvas de histéresis en función de la Temperatura (u(H) Vs T) medidas por SQUID ilustran un Exchange Bias de hasta 200 Oe por debajo de 175K para campos H_FC de 7, 0 y -7 T en películas de FeOx. Los nanodiscos de FeOx (11.1nm) de 2um de diámetro fueron fabricados por litografía de interferencia óptica. Ellos fueron observados por AFM para determinar la forma y topografía. Los nanodiscos exhiben una respuesta tipo-superparamagnética evidenciada por medidas de MOKE longitudinal. Entonces, preservan el comportamiento observado en forma de película. Adicionalmente, se realizo una prueba de concepto en donde los nanodiscos se dispersan en una solución, y responden magnéticamente a pequeños campos magnéticos del orden de unos pocos cientos de oersted. Estos resultados nos permiten mostrar su potencial uso en aplicaciones médicas, como en el desarrollo de tratamientos selectivos para enfermedades como el cáncer.
  • PublicaciónRestringido
    Multimodal X-ray edge illumination imaging for medical applications with polychromatic sources
    (Universidad de los Andes, 2024-01-30) Jurado Romero, Gibran David
    Conventional X-ray imaging relies on the absorption of X-rays by different tissues, which can result in low contrast images. A solution to this issue is the detection of the phase signal due to interactions of photons with matter, which is highly sensitive for low-attenuating tissues and samples. The consistent improvement of the quality of optical elements, the incorporation of precision instruments and the optimization of imaging techniques are responsible for the significant advancement of Phase Contrast Imaging (PCI) methods. Due to limitations in technology and imaging techniques, phase contrast was restricted to the use of synchrotron X-ray radiation sources. However, in the last two decades new solutions to the problem of spatial and temporal resolution have allowed the implementation of phase contrast techniques with conventional polychromatic X-ray sources. Among the different PCI methods are the In-line propagation, speckle-based and edge illumination phase contrast X-ray imaging technique. The latter is of great interest since edge illumination is particularly adapted to visualize details that present low photon absorption as well as being able to extend spatial resolution by exploiting the dithering process, which is crucial in application areas such as biology and medicine. Edge illumination PCI consists of illuminating the sample using the collimation of the photon flux through a mask, the beamlets then are deflected by the sample via photon diffraction, this effect makes beam signals to have positive or negative fringes on detection. The detector lecture of each fringe may be retrieved by exploiting the pixel pitch and mask spatial periodicity, which can lead to the retrieval of absorption, differential phase and dark field signals that bring a complementary description of the observed sample. Recent works make use of two masks to achieve the edge lighting configuration, one of these collimates the photon flux and the other mask limits the photon counting area in the detector pixels. However, the second mask can be avoided if the pixel size is sufficiently small to gather the diffraction pattern. This subpixel effect is achieved by slightly de-aligning the detector from the mask, which produces a positive and negative fringe on the signal (50-50) or a fully fringed signal (100-0). In this work, a readout system for multi-modal X-ray imaging is characterized to study the feasibility a of non-invasive visualization method of biological specimens for medical applications, which is developed by using the Edge-Illumination technique. The absorbtion, phase contrast and dark field signals are effectively recovered using the proposed imaging system for a \textit{Poecilia reticulata} ex-vivo specimen, with a significant CNR increase by a factor of about 7 as compared to conventional X-ray low-dose radiography signal, while also reducing the absorbed dose per mass unit. Furthermore, a comparative analysis with Speckle-Based X-ray imaging is performed on this specimen, successfully recovering the absorption, phase, and dark field signals.
  • PublicaciónAcceso abierto
    Multiphoton interference of distinguishable photons based on inner mode structure
    (Universidad de los Andes, 2023-06-27) Alarcón Carvajal, Jeimmy Alejandra
    Quantum interference is a fundamental phenomenon required by quantum technologies such as quantum computing and quantum communication. Quantum interference using linear optics has been a candidate to simulate a quantum computer and to demonstrate advantages over a classical computer. The interference phenomena can be studied using the inner mode structure of light; for example, in time, polarization, and spatial variables such as orbital angular momentum and transverse momentum. The quality of interference depends on the indistinguishability of the photons that are interfering. This point is detrimental for implementation of applications due to the fact that photonic sources do not produce completely indistinguishable photons. One way to avoid this strong condition of indistinguishability when dealing with interference is to consider that interference can be observed even for distinguishable photons if resolved measurements are done in a particularly chosen variable. Specifically, if one has photons that are distinguishable in time, interference can still be observed in the frequency domain; analogously, for photons that are distinguishable in position, it is possible to observe interference in the transverse momentum variable. This phenomenon has been experimentally observed in the case of a classical light source in the temporal and the spatial domains. Experiments to measure such effect in the quantum regime have been performed using both time-resolved measurements, in the case of two and three photons, and frequency-resolved measurements with two photons. In this context, this thesis presents a study in the framework of the role of distinguishability in multiphoton interference. In the first chapter, an introduction is presented giving a general context. The second chapter made a review about the role of distinguishability in interference from a classical description. In the third chapter, the researches in the landscape of the interference of two distinguishable photons using temporal variables are studied. In the fourth chapter, the interference of two distinguishable photons from the spatial variables is proposed. In fifth chapter, the experimental design of an interferometer to measure the spatial interference of two distinguishable photons is exposed and partially implemented. Therefore, the contribution of this thesis is to explore multi-photon interference for two photons in the spatial inner mode structure. The interference is calculated theoretically for two distinguishable photons in transverse momentum, using the second-order correlation function. In regards to the experimental development of multi-photon interference based on spatial inner mode structure, in this thesis, the design of an interferometric system is presented, and measurements of the generation of uncorrelated photon pairs are reported. The design of the experiment is divided into four stages: First, the source consisting of pairs of non-spatially correlated photons. Second, an optical arrangement to control the spatial variables of the photons. Third, the two photon interferometer, and finally the detection scheme that performs the spatially resolved correlation measurements. Specifically, the experimental contributions made in the development of this thesis are to propose the experimental setup, and to accomplish the complete implementation of the first stage or the interferometer. The photon source is based on Type II collinear Spontaneous Parametric Down Conversion (SPDC), designed to produce spatially uncorrelated photon. Therefore, a complete theoretical description is reported regarding the spatial correlations presented in the photon pair generation process using collinear Type II SPDC. The experimental spatial characterization consists on measuring the second order correlations in the transverse momentum variable. This is done by means of a rasterization method. The experimental results are compared with the theoretical model.
  • PublicaciónAcceso abierto
    Non - Invasive visualization of plant structures by x-ray phase contrast imaging
    (Universidad de los Andes, 2024-01-31) Rueda Pérez, Camilo Andrés
    Medicine, biology and the industrial area have found in X-rays a great tool for creating high-quality images, hence, new methods involving X-ray imaging have emerged intending to improve the current techniques and reduce the invasiveness of the performed processes. An example of a new branch of methods that seeks these objectives is X-ray phase contrast imaging (XPCi). Even though its implementation has not transcended to the medical field and its use is constrained to experimental investigations due to the cost of optical instrumentation and complexity in its setups, recently published studies suggest that applying XPCi-based techniques brings an improvement in the image quality even with a lower radiation dose compared to classical attenuation-based techniques and without implementing external contrast agents with high atomic numbers. Other fields like biology and botany have also started to explore the use of XPCi, in which diverse investigations have shown that internal vegetal structures in the plant's stem, as well as the veins, seeds, midribs, or reproductive system, can be clearly depicted employing these alternative methods. This represents significant progress in this area because traditional methods are highly invasive and lead to the full destruction of the analyzed sample because of the implementation of acids or contrast agents which are harmful to this type of tissue. The present work implements two XPCi methods (propagation-based PCi and Edge Illumination PCi) at the High-Energy Laboratory at Universidad de los Andes to visualize major and minor veins in the leaves of three different plant species commonly known in the Colombian environments and with different types of leaf venations, which are: coffee, cannabis, and eucalyptus. Besides, both methods' impact on the image quality is described for three different preparations that each kind of leaf underwent given the attenuation and phase contrast effects on the X-rays. On the other hand, phase retrieval algorithms were applied to recover the phase information that the samples printed on the X-rays. This method increases the image quality as well as the contras-to-noise ratio value for all the visualized major and minor veins. Finally, simulations carried out by the Monte-Carlo software of McXtrace permitted to emulate the laboratory conditions and simulate the attenuation and phase contrast effects that the leaf’s veins induced to the incident X-rays.
  • PublicaciónAcceso abierto
    On entanglement entropy in quantum field theory
    (Universidad de los Andes, 2023-12-14) Roa Rodríguez, Diego Alberto
    Entanglement entropy in quantum field theory has become a topic of increasing interest in several fields of physics, such as condensed matter, conformal field theory and statistical physics, high-energy and gravitational physics and also mathematical physics. Sorkin proposed a novel method for the computation of entanglement entropy of a scalar field in globally hyperbolic spacetimes, in a covariant way. The entropy formula proposed by Sorkin requires the solution of a spectral problem involving both the Wightmann two-point function as well as the Pauli-Jordan function, each one regarded as the kernel of an integral operator.
  • PublicaciónEmbargo
    Electronic and topological properties of TaTe4 and Cu doped TaTe4
    (Universidad de los Andes, 2023-12-13) Rojas Castillo, Julián David
    This work explores the quest for topological phases in novel materials. Such phases, including topological insulators, topological superconductors, and Weyl semimetals, hold great promise for various applications. Among the potential hosts for these states, this study focuses on transition metal chalcogenides (TMCs), with a specific emphasis on TaTe4 since it is a Weyl semimetal candidate. Additionally, TaTe4 exhibits charge density wave (CDW) behavior below room temperature. This study investigates the coexistence of topological phases with CDW in TaTe4, particularly focusing on the intriguing axionic states, which are predicted to arise in Weyl semimetals with CDW. To unravel these complex phenomena, we synthesized single crystals of TaTe4 and Cu-doped TaTe4.Our research followed two main pathways: first, characterizing the Fermi surface of TaTe4 through the analysis of Shubnikov de Haas (SdH) oscillations, providing novel insights into the topological features of TaTe4; second, conducting current-voltage (IV) curve measurements on Cu-doped TaTe4 to investigate CDW sliding, especially under the influence of high magnetic fields with varying orientations. Our findings reveal non-linear behavior, hinting at the presence of CDW sliding and its potential connections to topological states.
  • PublicaciónAcceso abierto
    Controlled fusion process for charged particles in a dyson gas with log-coulomb interaction
    (Universidad de los Andes, 2022-12-09) Mateus Rubio, John Fredy
    The behaviour out of equilibrium for controlled fusion processes in a N charged particle system, based on the Brownian motion in a Dyson Gas, is currently under analysis. The system is modeled in a reservoir of temperature T, where the charges q are randomly placed on a unitary circle interacting via their time-independent 2D-Coulomb log-potential. In addition, the drag of the system with its surroundings, which fixes the rate of diffusion, is considered. The dynamics of the k-th particle is then governed by a Langevin equation, which allows to perform the computational simulation of the Brownian motion of the charges. Additionally, a critical fusion angle and probability fusion parameter (random fusion control) have been considered to make a two-particle fusion when they are separated by an angle less or equal to this critical angle. When a fusion process occurs, the total number of particles in the system is reduced by one, and the charge of the merged particles is added while the total charge of the system is invariant. It then becomes interesting to study how the density n decays, as well as how the charge-clusters ck (particles with charge q=k) are created over time.
  • PublicaciónAcceso abierto
    Control protocols for a system of N Brownian mutual interacting particles
    (Universidad de los Andes, 2022-12-01) Rengifo Munoz, Diego Fernando
    The concept of thermodynamic transformation is central to classical thermodynamics. Equilibrium states are usually connected by quasi-static thermodynamic transformations, which take a long (virtually infinite) time to be completed because the system is always near the equilibrium. This idealization can be removed by considering transformations that change an externally controlled parameter in a finite time, and the system is left in a non-equilibrium state; therefore, the system needs a finite time to reach the final equilibrium state under the new external conditions. For a sudden (step process) change in the external parameter, the time is called \tau_{relax}. A relevant optimization question in control theory is if this final state can be reached in a shorter time than \tau_{relax} with a suitable control on the external parameter. This project aims to achieve this goal for a system of N-interacting Brownian particles subject to a noisy environment at inverse temperature \beta. The evolution of the system is governed by the Langevin equation and its corresponding Fokker-Planck equation. This system is mesoscopic; consequently, the laws of macroscopic thermodynamics do not apply. On the other hand, the system is sufficiently complex to use a detailed description given by non-equilibrium statistical mechanics. For this reason, the work, and heat become stochastic variables, and we are interested in computing their probability distribution during the process \cite{seifer}. This will be done using a path integral formulation and Fokker-Planck-type partial differential equations. The techniques used will be analytical and computational. Once we have the probability distributions, we can extract useful physical information contained in the probability distribution moments.
  • PublicaciónAcceso abierto
    Segment-based feature proposal for the morphological classification of T Tauri star light curves
    (Universidad de los Andes, 2023-07-14) León, Benjamín
    T Tauri stars are young stellar objects that exhibit a wide range of morphological variability in their light curves, product of multiple physical processes. Numerical features can be designed to identify the characteristics of these brightness variations. The identification process is automatized in the literature through the use of machine-learning algorithms. This Master thesis aims to utilize supervised machine-learning algorithms for morphological classification of T Tauri stars with a set of proposed fea- tures based on segmentation strategies. The features are tested on an ex- ternal data set for evaluation and the best parameters for classification are discussed. We evaluate the features through the Welch t-test, the Mann-Whitney U-test and the Levene test for equal variance. After the testing, seven algo- rithms are trained with light curves from the Orion star formation complex obtained from the TESS project. The algorithms are subject to sequential reduction of feature space, hyper parameter grid search and recurrent im- portance calculations to optimize classification results in terms of F1 score and Cohen kappa. The optimized algorithms are then applied to a sample of over 2000 hand-classified confirmed T Tauri stars. In this work, we propose 61 features based upon robust statistics, pseudo- time-series analysis and auto-correlation measurements. These features were utilized in 13 implementations of binary and multi-class classifiers and opti- mized taking advantage of a high-performance cluster. We implement statis- tical tests as feature evaluation and light curve filtering strategies innovative to astronomical feature design. The highest achieving features on the test- ing data set were analyzed individually and were conceptually connected to physical processes, signal crowding and systematic effects. The algorithms obtained F1 scores higher than 0.4 for all classes with maximum feature dimension of 10. This work contributes a new set of useful features that consistently achieve high importance when compared to features used in the litera- ture. Innovative feature design, evaluation stages and algorithm optimiza- tion pipelines were implemented.
  • PublicaciónAcceso abierto
    Spatial variables of light: From controlled decoherence in quantum key distribution to the spatial Franson interferometer
    (Universidad de los Andes, 2023-07-07) Sabogal Pérez, Daniel Ricardo
    In this thesis, the spatial variables of light are used in the practical and fundamental realms. In the practical domain, a theoretical and experimental study of the method that is referred to as the controllable decoherence assisted scheme is presented. The scheme is based on the possibility of introducing decoherence in a controllable way. Theoretically, it is shown that the method allows reducing the amount of information that an eavesdropper can obtain in the BB84 protocol under the entangling probe attack. Experimentally, two proof-of-principle experiments using heralded single photons were performed. One in which the BB84 protocol is implemented without adding decoherence, and another in which the controllable decoherence assisted scheme is used in the BB84 protocol. In the first one, it is found an average value of QBER= 3.9 ± 0.3 % for five keys of ¿ 1000 bits each one. In the second experiment, it is observed that the controllable decoherence introduced in Alice's site is indeed canceled, allowing to recover low values of the QBER. Regarding the study of fundamental concepts by means of light spatial variables, the generation of spatial-bin entanglement is addressed. Specifically, the spatial analog of the Franson interferometer is presented. The Franson interferometer is used to obtain time-bin entanglement. This is achieved by using pairs of temporarily correlated photons and two Mach-Zehnder interferometers to have the option of light traveling by long or short paths that will constitute the basis of time-bin entanglement. In the spatial version proposed, the interferometers are replaced by tunable beam displacers to obtain left or right spatial modes that will constitute the spatial-bin entanglement. Moreover, it is explained how to violate the Bell inequality in the position-transverse momentum domain using the spatial Franson interferometer.
  • PublicaciónEmbargo
    Multiferroicity in alloys of 2D transition-metal dichalcogenides
    (Universidad de los Andes, 2023-05-29) Rojas Páez, Harold Alberto
    Multiferroics are materials that simultaneously exhibit more than one type of ordering such as magnetic, electric, or elastic. Multiferroicity enables the development of new devices for information processing and storage due to the possibility of controlling one type of polarization using a field different from its conjugate field (e.g., magnetic polarization through an electric field). The existence of multiferroic properties in two-dimensional (2D) materials promises advantages in the development of multifunctional devices at smaller scales. These materials, which are intrinsically 2D, are formed by stacking atomically thin layers of crystalline structures with Van der Walls interatomic forces between them, which permits nano-structuration to obtain individual layers from the bulk material. One of the most studied 2D materials families is the Transition-metal dichalcogenides (TMD), composites of the type MX2, with M a transition-metal atom (Mo, W, etc) and X a chalcogen atom (S, Se or Te). The recent discovery of room-temperature multiferroicity in alloys of TMDs opens new possibilities in multifuctional devices at the nanoscale. In this thesis work we propose to study the emergence of multiferroic states in alloys of TMDs and to explore the physical and chemical conditions favorable for their appearance. We want to explore the possibilities opened by substitutions of chalcogen atoms, M(X_xY_{1¿x})_2, or transition metal atoms, (M_xN_{1¿x})X_2, into the structure of the original material MX_2. In particular, we want to deepen our understanding of electric and magnetic properties of W(Se_{1¿x}Te_x)_{2¿¿} from bulk to the nanoscale limit and the dependence of these properties with different tuning parameters.
  • PublicaciónAcceso abierto
    Feasibility studies on the production of new particles with preferential couplings to third generation fermions at the LHC
    (Universidad de los Andes, 2022-06-06) Rodríguez Cruz, Cristian Fernando
    The standard model (SM) of particle physics is the most successful theory we have when it comes to describing the functioning of the subatomic world. Day by day, the SM is put to test by different experiments conducted around the world, aimed to test its validity. The project that is, perhaps, the most important in this frontier is the Large Hadron Collider (LHC). One of the goals of the experiments at the LHC is to accurately measure the parameters of the SM, and also search for any deviation from the SM predictions that could indicate new physics. In recent years, the observations reported by LHCb, Babar, and Belle experiments of the apparent anomalies in B-meson decays, together with the possible anomaly on the magnetic angular momentum of muons reported by the Muon g - 2 experiment at Fermilab, indicate that, perhaps, lepton flavour universality is violated in the SM, in turn being a window to search new physics. Of the new models that intent to extend the SM to explain violation of lepton flavour universality, several of them introduce new particles with preferential couplings to third generation fermions. Some of the most popular models include the hypothetical production of heavy mass particles such as Z', W', and Leptoquarks (LQ). In this project, we seek to conduct feasibility studies for the LHC associated with the production of these new hypothetical particles through different production mechanism and with preferential couplings to third generation fermions. These studies will be conducted using different simulation packages to emulate the LHC conditions and the statistical analysis will use Machine Learning (ML) methods.
  • PublicaciónAcceso abierto
    Superconductivity and charge density wave in the extended fermi-hubbard model with disorder
    (Universidad de los Andes, 2023-01-26) Borja Peña, Cristian Mauricio
    The study of strongly correlated systems provides clues for the understanding of macroscopic quantum phenomena. One of the most attractive and active fields in this matter is high-temperature superconductivity (HTSC) and the search for mechanisms that strengthen the superconductivity (SC) in quantum materials. One possible avenue for this enhancement is to weaken competing electronic phases, among which charge density waves (CDW) rise as feasible candidates, as these seem to be ubiquitous in the phase diagrams of, for example, high -temperature superconductors and transition metal dichalcogenides. From the theoretical perspective, the Extended Fermi-Hubbard model is one of the simplest models that exhibit the physics of this interplay. Recent simulations and experiments suggest that by introducing a moderate amount of disorder in the local parameters of the Hamiltonian, it is possible to strengthen SC. Here, we explored the interplay of SC and CDW by disordering the coupling parameters of the EFH Hamiltonian in a way that closely resembles the variations generated by chemical substitution in actual experiments. We perform DMRG simulations and additional calculations to characterize the ground states. We observed that the effect of introducing disorder towards an attractive neighbor interaction induces domain-walls formation where CDW seems to have stronger correlations.
  • PublicaciónAcceso abierto
    Cavity-spin-orbit competition in quantum dot systems under a magnetic field
    (Universidad de los Andes, 2023-01-27) Beltrán Romero, Santiago Steven
    In this work we accomplish a exhaustive study about few particle and many-body systems under a perpendicular magnetic field and spin-orbit coupling inside a quantum electrodynamics cavity, whose analysis has so far not been treated in such detail. In fact, the interactions arising from these phenomena are interpreted by means of detecting rotating and antirotating terms between radiation and matter. In a single-electron quantum dot, we identify spin transitions that can be modified by cavity coupling strength with circularly polarized light using numerical calculations. As a result of spin-radiation relations, the Zeeman effect is countered by forbidden transitions that prefer spin "up" or "down". Therefore, the cavity-spin-orbit competition results in spin-field transitions in the ground state. Such transitions, which are connected to shifts in spin dominance, are predicted by a quasianalytic computation of an effective Landé factor that depends on the degree of coupling between the cavity and the magnetic field. In the system with two interacting electrons in quantum dot, we demonstrate that the cavity only couples, in dipolar approximation, to the center of mass coordinates. However, this coupling leads to slight variation in the single-electron density and spin fields cause by the interaction with radiation. In addition, a variation in the density of Bell spin states is perceived. Based on the above results, we suggest applications such as the construction of switches in spintronics and quantum information processing systems, as well as measurements by nuclear magnetic resonance. Finally, some insides in the future work are introduced, with the aim of extend our research to many body cases and go beyond to the dipolar approximation
  • PublicaciónAcceso abierto
    Meta-materiales basados en puntos cuánticos : geometría y acoplamiento
    (Universidad de los Andes, 2007-12) Nossa Márquez, Javier Francisco
    "Es interesante conocer las razones por las cuales algunos materiales manifiestan comportamientos diferentes a otros. Cómo se quema la madera, liberando gran cantidad de energía?; cómo permite el vidrio el paso de luz pero no de aire?; cómo atraviesa un trozo de metal una lámina de aceite, pero no un trozo de acero?. Definitivamente es posible encontrar gran variedad de diferencias en el comportamiento de los materiales, pero, qué hay detrás de estas distinciones?. Estas diferencias hacen o no de algunos materiales, mejores candidatos para aplicaciones tecnológicas. Al buscar un conductor es mejor un metal que un cerámico; si se desea el paso de luz un vidrio es el adecuado; ahora, si se necesita aislar térmicamente una sustancia, un recipiente de icopor es buen candidato. La respuesta a estas preguntas se encuentra en la estructura de la materia, estudiada hasta ahora, a nivel fundamental, por el estado sólido. A pesar que dentro de los materiales los electrones definen gran cantidad de sus propiedades, éstos no están solos. Ellos son influenciados por los núcleos, formando así, una colección de átomos que en su interacción generan propiedades interesantes. Los átomos a esta escala menor que microscópica, son los responsables de las diferencias entre un material y otro. Un átomo de cobre contiene 29 electrones, a diferencia de un átomo de nitrógeno el cual está compuesto por solo 7 electrones. Cuando se reúnen dos átomos, las propiedades individuales se pierden para dar lugar a propiedades diferentes, estos entes son llamados moléculas. Ahora, al reunir no solo dos átomos sino varios de éstos, por ejemplo de cobre y bajo algunas condiciones especiales, es posible generar un trozo de cobre para transportar electricidad, a diferencia de átomos de nitrógeno reunidos los cuales generan un gas de nitrógeno..."
  • PublicaciónAcceso abierto
    Applications of Kronecker states in quantum information theory
    (Universidad de los Andes, 2022-10-07) González Olaya, Walther Leonardo
    In quantum information theory, maximally entangled states (MES) are key to implement many well known protocols such as quantum teleportation, quantum error correction, quantum key distribution, among others. The representation theory of the symmetric group provides a mechanism that allows one to generate a wide class of maximally entangled multipartite states. Such states, which we call Kronecker states, belong to the invariant subspace of products of irreducible representations of Sn. The reduced density matrices of such states in each individual subspace are completely mixed, proving that Kronecker states are MES. The purpose of this work is to better understand Kronecker states and their applications in quantum information theory. In particular, we will implement them in four cases: "Superadditivity of classical channel capacity in quantum channels: Hastings, used the invariance of MES in bipartite systems under the action of some elements of composed channels to prove that the minimum output entropy of such channels is subadditive, therefore there is superadditivity of classical channel capacity. We want to build channels where it is possible to exploit the natural invariant property of Kronecker states to achieve superadditivity expanding this behaviour when composing a system with more than two channels. Quantum error correction: In the 5 qubit protocol, codewords can be understood as particular examples of Kronecker states. We want to study the possibility of generalizing the protocol to generic Kronecker states with possible applications to different noise models. Quantum secret sharing: One of the most important characteristics of the codification in this schemes is that different individuals must get no information without help of others; or in density matrix language, reduced density matrices have to be completely mixed, the salient property of Kronecker states. We want to study a possible generalization of the threshold scheme (t,n) that allows one to recover the initial information to selected groups when the codification is done using Kronecker states. Entanglement concentration: Mejía and Botero demonstrated that the entanglement of tripartite states in the W class can be concentrated in Kronecker states of high dimension. We want to better understand the structure of these states and to study how they appear in the entanglement concentration of multipartite states. We expect that the study of these applications will help us to understand in a more general way Kronecker states, pointing to a systematic formalism that establish the generalities behind their applications and draw a path to extend the study to invariant elements in other groups
  • PublicaciónAcceso abierto
    Control coherente de sistemas cuánticos mediante un proceso de landau-zener
    (Universidad de los Andes, 2013) Gómez Ruiz, Fernando Javier
  • PublicaciónAcceso abierto
    Fabrication and characterization of cobalt microbars
    (Universidad de los Andes, 2017) Useche Reyes, Diego Hernando
    Magnetic materials have been widely used in applications such as data storage, magnetic tunnel junctions and spin valve structures. In the present work, we explore the magnetic properties of various millimeter size arrays of magnetic microbars. For this study, we focused on Cobalt microbars fabricated using photolithography, thermal evaporation, sputtering and electron gun techniques. Magnetization measurements as a function of angle were performed on microbars samples which dier slightly in their dimensions. The measurements were carried out using vibrating sample magnetometry (VSM). Also, computer simulations in OOMMF were done to simulate the experimental outcomes. As results, when the applied eld was parallel to the microbars the maximum coercivity was found and decreased slightly as the eld rotated towards the perpendicular direction. It was also found a sheared hysteresis loop in three of the samples.
  • PublicaciónAcceso abierto
    The Wheeler-DeWitt Equation : a schrèodinger equation for the origin of the universe
    (Universidad de los Andes, 2015) Uribe Suárez, Juan David
    The question of the origin of the universe is an ancient one, yet science has had the tools to search for a proper answer for only the last hundred years or so. Over the last fifty years there has been an increasing interest in solving this particular puzzle and it all started with Einstein's General Relativity. In cosmology one uses this theory to study the universe as a physical system. That is, a system to study mathematically with a set of equations, principles and sensible assumptions that agree with observations. Although some models agree to a high degree with current observations, they fail to give a satisfactory answer to the question mentioned. Yet they give several hints that tell us that we should look for the answer within the realm of quantum mechanics. Specifically, we should look for a quantum theory of gravity. The final result of the canonical quantum gravity programme is the Wheeler-DeWitt equation...