1,720,961 research outputs found
Optical encryption using phase-shifting interferometry in a joint transform correlator
Full text linkWe propose an optical encryption technique where the encrypted field and the decrypting key are obtained by three-step phase-shifting interferometry and registered as digital Fresnel holograms in a joint transform correlator architecture. Decryption can be achieved by digital or optical means. The technique allows the complete process to be achieved at high speed and data to be transfered via digital communication channels. Experimental implementation is performed in a system based on a programmable liquid-crystal TV display working in pure phase mode to represent the input data and to introduce the required phase shifts. A CCD is used to register the output data.Fil: La Mela, Cecilia. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física; ArgentinaFil: Iemmi, Claudio Cesar. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin
Binary polarization pupil filter: Theoretical analysis and experimental realization with a liquid crystal display
Full text linkThe Jones matrix formalism has been applied to evaluate the response of an optical system when a non-uniform polarizing pupil is introduced. With this formalism we analyze and experimentally demonstrate the properties of a binary polarization pupil filter having two regions with two orthogonal linear polarization orientations. We first study the case when no analyzer is placed behind the pupil filter, and both, the transversal and the axial behavior are described in terms of the intensity and the local state of polarization. Then it is shown how the response of the optical system can be easily changed through the orientation of an analyzer placed behind the pupil. We experimentally verified the theory using a twisted nematic liquid crystal display, which produces two orthogonal linear polarization states for two different addressed voltages.Fil: Moreno, Ignacio. Universidad de Miguel Hernandez; EspañaFil: Iemmi, Claudio Cesar. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Campos, Juan. Universitat Autonoma de Barcelona; EspañaFil: Yzuel, María J.. Universitat Autonoma de Barcelona; Españ
Depth of focus increase by multiplexing programmable diffractive lenses
Full text linkA combination of several diffractive lenses written onto a single programmable liquid crystal display (LCD) is proposed for increasing the Depth of Focus (DOF) of the imaging system as a whole. The lenses are spatially multiplexed in a random scheme onto the LCD. The axial irradiance distribution produced by each lens overlaps with the next one producing an extended focal depth. To compare the image quality of the multiplexed lenses, the Modulation Transfer Function (MTF) is calculated. Finally we obtain the experimental Point Spread Functions (PSF) for these multiplexed lenses and experimental results in which an extended object is illuminated under spatially incoherent monochromatic light. We compare the images obtained in the focal plane and in some defocused planes with the single lens and with three multiplexed lenses. The experimental results confirm that the multiplexed lenses produce a high increase in the depth of focus.Fil: Iemmi, Claudio Cesar. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Campos, Juan. Universitat Autonoma de Barcelona; EspañaFil: Escalera, J. C.. Universitat Autonoma de Barcelona; EspañaFil: López Coronado, O. Universitat Autonoma de Barcelona; EspañaFil: Gimeno, R.. Universitat Autonoma de Barcelona; EspañaFil: Yzuel, María J.. Universitat Autonoma de Barcelona; Españ
The minimum Euclidean distance principle applied to improve the modulation diffraction efficiency in digitally controlled spatial light modulators
Full text linkDigital addressing of the electrical signal in spatial light modulators, as it is the case in present liquid crystal on silicon (LCoS) displays, may lead to temporal phase fluctuations in the optical beam. In diffractive optics applications a reduction in the modulation diffraction efficiency may be expected. Experimental work is done characterizing the fluctuations amplitude and phase depth for three different digital addressing sequences. We propose a diffractive model to evaluate the modulation diffraction efficiency of phase diffractive optical elements (DOEs) in the presence of phase fluctuations. Best results are obtained for the most stable electrical sequence even though its phase depth is as small as 280°. The results show good agreement with the numerical calculation given by the model.Fil: Lizana, Ángel. Universitat Autonoma de Barcelona; EspañaFil: Márquez, A.. Universidad de Alicante; EspañaFil: Lobato, L.. Universitat Autonoma de Barcelona; EspañaFil: Rodange, Y.. Universitat Autonoma de Barcelona; EspañaFil: Moreno, I.. Universidad de Miguel Hernandez; EspañaFil: Iemmi, Claudio Cesar. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Campos, Juan. Universitat Autonoma de Barcelona; Españ
Optimization and performance criteria of a Stokes polarimeter based on two variable retarders
Full text linkIn this paper we present the analysis, optimization and implementation of several Stokes polarimeter configurations based on a setup including two variable retarders. The polarimeter analysis is based on the Mueller-Stokes formalism, and as a consequence, it is suitable to deal with depolarized light. Complete Stokes polarimeters are optimized by minimizing the amplification of simulated errors into the final solution. Different indicators useful to achieve this aim, as the condition number or the equally weighted variance, are compared in this paper. Moreover, some of the optimized polarimeters are experimentally implemented and it is studied the influence of small deviations from the theoretical ones on the amplification of the Stokes component error. In addition, the benefit of using incomplete polarimeters, when detecting specific ranges of states of polarization, is discussedFil: Peinado, Alba. Universitat Autonoma de Barcelona; EspañaFil: Lizana, Ángel. Universitat Autonoma de Barcelona; EspañaFil: Vidal, Josep. Universitat Autonoma de Barcelona; España. ALBA Synchrotron Light Source Facility; EspañaFil: Iemmi, Claudio Cesar. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Campos, Juan. Universitat Autonoma de Barcelona; Españ
Characterization of photonic quantum states with a minimum number of measurements
No full textEl procesado de información cuántica es un campo de investigación que desde las últimas décadas se encuentra en constante crecimiento. Ha sido crucial tanto en aspectos fundamentales de la mecánica cuántica como en el desarrollo de una gran cantidad de aplicaciones tecnológicas. Un problema fundamental del área es el de determinar el estado desconocido de un sistema cuántico. En este sentido, el proceso de reconstrucción de un estado general de dimensión d, conocida como tomografía cuántica, consiste en encontrar su matriz densidad. Para efectuar dicha reconstrucción típicamente deben realizarse d² mediciones lo cual dificulta el tratamiento para sistemas cuánticos de alta dimensión. Trabajar en espacios de Hilbert de alta dimensión posibilita una gran tasa de transmisión de datos en información cuántica. Si bien hasta hace unos años era muy difícil generar sistemas cuánticos de dimensión mayor que 2 (qudits) el uso de fotones individuales, en combinación con moduladores espaciales de luz, que permiten el control dinámico de distintas características de la luz, permitieron la manipulación de sistemas cuánticos de alta dimensión. Los moduladores espaciales de luz son elementos que permiten controlar determinadas magnitudes de la radiación emitida por una fuente luminosa, tales como su amplitud, su fase y su polarización. En esta Tesis se estudió la implementación y caracterización de estados y procesos cuánticos mediante el uso de pantallas de cristal líquido trabajando en modo de fase. En una primera etapa se desarrollaron métodos para caracterizar estados fotónicos espaciales, realizando mediciones interferométricas en el plano imagen del frente de onda que los describe. Por un lado presentamos un método para caracterizar cualquier qudit espacial puro, de dimensión arbitraria, que se basa en la técnica clásica de interferometría por corrimiento de fase. En el esquema propuesto un total de solo 4d mediciones son necesarias, lo que implica una reducción significativa con respecto a los esquemas estándar. Al usar esta técnica, hemos reconstruido experimentalmente una gran cantidad de estados que van desde dimensión d = 2 hasta 14 con valores de fidelidad medios superiores a 0, 97. Para ello, los qudits se codificaron en la posición discreta de momento transversal de fotones individuales, lo que se logró enviando los fotones a través de una abertura con d hendiduras. Se llevó a cabo una implementación experimental del método basado en un interferómetro Mach-Zehnder, que permite reducir el número de configuraciones de medición a cuatro, ya que las d ranuras se pueden medir simultáneamente. Por otro lado, presentamos un método para reconstruir qudits espaciales puros de dimensión arbitraria d, pero ahora basado en un interferómetro por difracción de punto. En el esquema propuesto, los estados cuánticos también están codificado en el momento transversal discretizado del frente de ondas del fotón cuando este atraviesa d rendijas, pero con el agregado de una región que provee la fase de referencia. Para hacer la caracterización del estado fotónico, la fase completa del frente de ondas se reconstruye mediante una técnica de corrimiento de fase. Combinado con un detector multipixel, la adquisición se puede paralelizar, y solo cuatro interferogramas se requieren para reconstruir cualquier estado puro de qudit, independientemente de la dimensión d. Probamos el método experimentalmente, para la reconstrucción de estados de dimensión d = 6 elegidos al azar, y obtuvimos una fidelidad media de 0, 95. Adicionalmente, desarrollamos un esquema experimental que permite estimar las aberraciones de fase que afectan al frente de ondas durante la propagación, y por lo tanto permiten mejorar la estimación del estado cuántico. En ese sentido, presentamos una prueba de principio que muestra la posibilidad de corregir la influencia de la turbulencia en procesos de comunicación en el espacio libre, en nuestro caso simulada por máscaras de fase con la distribución estadística de Kolmogorov, en donde logramos obtener fidelidades de reconstrucción medias comparables con el caso sin turbulencia. En una segunda etapa, trabajamos en esquemas de reconstrucción basados en medidas proyectivas. Para estados qudits fotónicos codificados en el momento transversal discretizado del fotón, estas medidas se logran haciendo imagen del estado incógnita sobre un elemento de la base sobre la cual se proyecta, y midiendo en el centro de la transformada de Fourier del frente de ondas emergente. Esto se implementó experimentalmente mediante una arquitectura de procesador óptico 4 − f, que utilizando pantallas de cristal líquido en modo de fase, tanto para la preparación de estados rendija (con fase y amplitud arbitraria), como para representar los estados arbitrarios sobre los cuales se realiza la proyección. Por un lado generalizamos el primer método interferométrico presentado en esta Tesis, el basado en un dispositivo tipo Mach-Zehnder, de tal manera de que la reconstrucción del estado desconocido pueda realizarse a partir del resultado de 4d – 3 proyectores independientemente de la codificación empleada. Es decir que este nuevo método es aplicable a cualquier implementación física de qudits. Otra ventaja de este método, es que las mismas 4d − 3 mediciones permiten verificar la hipótesis a priori de que el estado reconstruido es puro. Se obtuvieron excelentes fidelidades de reconstrucción, comparables con métodos que requieren un número mucho mayor de mediciones, como ser la tomografía cuántica de estados basada en bases mutuamente no sesgadas. Por otro lado, implementamos experimentalmente un esquema de tomografía de procesos cuánticos selectivo y eficiente en dimensión arbitraria. Obtuvimos excelentes fidelidades de reconstrucción y verificamos experimentalmente la eficiencia del método. Los desarrollos en esta Tesis son ideales para ser incorporados en las etapas de calibración y validación de sistemas de computación cuántica y de comunicación cuántica reales.Quantum information processing is a field of research that has been constantly growing in recent decades. It has been crucial both in fundamental aspects of quantum mechanics and in the development of a large number of technological applications. A fundamental problem in the area is that of determining the unknown state of a quantum system. In this sense, the process of reconstruction of a general state of dimension d, known as quantum tomography, consists of finding its density matrix. To carry out this reconstruction, typically d² measurements must be made, which makes processing difficult for high-dimensional quantum systems. Working in high-dimensional Hilbert spaces enables a high data transmission rate in quantum information. Although until a few years ago it was very difficult to generate quantum systems with a dimension greater than 2 (qudits), the use of individual photons, in combination with spatial light modulators, which allow the dynamic control of different characteristics of light, allowed the manipulation of high-dimensional quantum systems. The spatial light modulators are elements that allow controlling certain magnitudes of the radiation emitted by a light source, such as its amplitude, its phase and its polarization. In this Thesis, the implementation and characterization of quantum states and processes was studied through the use of liquid crystal displays working in phase mode. In a first stage, methods were developed to characterize spatial photonic states, performing interferometric measurements in the image plane of the wavefront that describes them. On the one hand, we present a method to characterize any pure spatial qudit, of arbitrary dimension, which is based on the technique of phase shift interferometry. In the proposed scheme a total of only 4d measurements are necessary, which implies a significant reduction with respect to the standard schemes. Using this technique, we have experimentally reconstructed a large number of states ranging from dimension d = 2 to 14 with mean fidelity values greater than 0,97. To do this, the qudits were encoded at the discrete transverse moment position of individual photons, which was achieved by sending the photons through an aperture with d slits. An experimental implementation of the method based on a Mach-Zehnder interferometer was carried out, which allows reducing the number of measurement setups to four, since the d slits can be measured simultaneously. On the other hand, we present a method to reconstruct pure spatial qudits of arbitrary dimension d, but now based on a point diffraction interferometer. In the proposed scheme, the quantum states are also encoded in the discretized transverse moment of the photon’s wavefront when it passes through d slits, but with the addition of a region that provides the reference phase. To characterize the photonic state, the entire phase of the wavefront is reconstructed using a phase shift technique. Combined with a multipixel detector, the acquisition can be parallelized, and only four interferograms are required to reconstruct any pure state of qudit, regardless of the dimension d. We tested the method experimentally, for the reconstruction of states of dimension d = 6 chosen at random, and we obtained a mean fidelity of 0,95. Additionally, we developed an experimental scheme that allows us to estimate the phase aberrations that affect the wavefront during propagation, and therefore allows us to improve the estimation of the quantum state. In this sense, we present a proof of principle that shows the possibility of correcting the influence of turbulence on communication in free space, in our case simulated by phase masks with the Kolmogorov statistical distribution, where we were able to obtain reconstruction fidelities means comparable with the case without turbulence. In a second stage, we work on reconstruction schemes based on projective measures. For photonic qudit states, encoded in the photon’s discretized transverse moment, these measurements are achieved by imaging the unknown state on an element of the base onto which it is projected, and measuring at the center of the Fourier transform of the emerging wavefront. This was implemented experimentally through a 4 − f optical processor architecture in which liquid crystal displays are used in phase mode, both for the preparation of the slit states (with arbitrary phase and amplitude), and to represent the arbitrary states. on which the projection is made. On the one hand, we generalize the first interferometric method presented in this Thesis, the one based on a Mach-Zehnder type device, in such a way that the reconstruction of the unknown state can be carried out from the result of 4d − 3 projectors regardless of the coding used. In other words, this new method is applicable to any physical implementation of qudits. Another advantage of this method is that the same 4d − 3 measurements allow to verify the a priori hypothesis that the reconstructed state is pure. Excellent reconstruction fidelities were obtained, comparable to methods that require a much greater number of measurements, such as quantum state tomography based on mutually unbiased bases. On the other hand, we experimentally implemented a selective and efficient quantum process tomography scheme in arbitrary dimension. We obtained excellent reconstruction fidelities and experimentally verified the efficiency of the method. The developments in this Thesis are ideal to be incorporated into the calibration and validation stages of real quantum computing and quantum communication systems.Fil: Pears Stefano, Quimey Martín. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina
Preparing arbitrary pure states of spatial qudits with a single phase-only spatial light modulator
Full text linkSpatial qudits are D-dimensional (≥2D≥2) quantum systems carrying information encoded in the discretized transverse momentum and position of single photons. We present a proof-of-principle demonstration of a method for preparing arbitrary pure states of such systems by using a single phase-only spatial light modulator (SLM). The method relies on the encoding of the complex transmission function corresponding to a given spatial qudit state onto a preset diffraction order of a phase-only grating function addressed at the SLM. Fidelities of preparation above 94% were obtained with this method, which is simpler, less costly, and more efficient than those that require two SLMs for the same purpose.Fil: Solís Prosser, M. A.. Universidad de Concepción; ChileFil: Arias, A.. Universidad de Concepción; Chile. Instituto Tecnológico Metropolitano; ColombiaFil: Varga, Juan José Miguel. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Rebon, Lorena. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - la Plata. Instituto de Física la Plata. Universidad Nacional de la Plata. Facultad de Ciencias Exactas. Instituto de Física la Plata; ArgentinaFil: Ledesma, Silvia Adriana. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Iemmi, Claudio Cesar. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Neves, L.. Universidade Federal do Minas Gerais; Brasi
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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