Two-Photon Blockade in an Atom-Driven Cavity QED System

Photon blockade is a dynamical quantum-nonlinear effect that occurs in driven systems with an anharmonic excitation ladder. For a single atom strongly coupled to an optical cavity, we show that driving the atom gives a decisively larger optical nonlinearity than driving the cavity. This enhances single-photon blockade and allows for the implementation of two-photon blockade where the absorption of two photons suppresses the absorption of further photons. As a signature, we report on three-photon antibunching with simultaneous two-photon bunching observed in the light emitted from the cavity. Our experiment constitutes a significant step towards multi-photon quantum-nonlinear optics.Comment: paper (6 pages, 5 figures) + supplement (6 pages, 5 figures

GEANT: detector description and simulation tool

As the scale and complexity of High Energy Physics experiments increase, simulation studies require more and more care and become essential to design and optimise the detectors, develop and test the reconstruction and analysis programs, and interpret the experimental data. GEANT is a system of detector description and simulation tools that help physicists in such studies

Faster data structures and graphics hardware techniques for high performance rendering

Computer generated imagery is used in a wide range of disciplines, each with different requirements. As an example, real-time applications such as computer games have completely different restrictions and demands than offline rendering of feature films. A game has to render quickly using only limited resources, yet present visually adequate images. Film and visual effects rendering may not have strict time requirements but are still required to render efficiently utilizing huge render systems with hundreds or even thousands of CPU cores. In real-time rendering, with limited time and hardware resources, it is always important to produce as high rendering quality as possible given the constraints available. The first paper in this thesis presents an analytical hardware model together with a feed-back system that guarantees the highest level of image quality subject to a limited time budget. As graphics processing units grow more powerful, power consumption becomes a critical issue. Smaller handheld devices have only a limited source of energy, their battery, and both small devices and high-end hardware are required to minimize energy consumption not to overheat. The second paper presents experiments and analysis which consider power usage across a range of real-time rendering algorithms and shadow algorithms executed on high-end, integrated and handheld hardware. Computing accurate reflections and refractions effects has long been considered available only in offline rendering where time isn’t a constraint. The third paper presents a hybrid approach, utilizing the speed of real-time rendering algorithms and hardware with the quality of offline methods to render high quality reflections and refractions in real-time. The fourth and fifth paper present improvements in construction time and quality of Bounding Volume Hierarchies (BVH). Building BVHs faster reduces rendering time in offline rendering and brings ray tracing a step closer towards a feasible real-time approach. Bonsai, presented in the fourth paper, constructs BVHs on CPUs faster than contemporary competing algorithms and produces BVHs of a very high quality. Following Bonsai, the fifth paper presents an algorithm that refines BVH construction by allowing triangles to be split. Although splitting triangles increases construction time, it generally allows for higher quality BVHs. The fifth paper introduces a triangle splitting BVH construction approach that builds BVHs with quality on a par with an earlier high quality splitting algorithm. However, the method presented in paper five is several times faster in construction time

Contributions to the improvement of image quality in CBCT and CBμCT and application in the development of a CBμCT system

During the last years cone-beam x-ray CT (CBCT) has been established as a widespread imaging technique and a feasible alternative to conventional CT for dedicated imaging tasks for which the limited flexibility offered by conventional CT advises the development of dedicated designs. CBCT systems are starting to be routinely used in image guided radiotherapy; image guided surgery using C-arms; scan of body parts such as the sinuses, the breast or extremities; and, especially, in preclinical small-animal imaging, often coupled to molecular imaging systems. Despite the research efforts advocated to the advance of CBCT, the challenges introduced by the use of large cone angles and two-dimensional detectors are a field of vigorous research towards the improvement of CBCT image quality. Moreover, systems for small-animal imaging add to the challenges posed by clinical CBCT the need of higher resolution to obtain equivalent image quality in much smaller subjects. This thesis contributes to the progress of CBCT imaging by addressing a variety of issues affecting image quality in CBCT in general and in CBCT for small-animal imaging (CBμCT). As part of this work we have assessed and optimized the performance of CBμCT systems for different imaging tasks. To this end, we have developed a new CBμCT system with variable geometry and all the required software tools for acquisition, calibration and reconstruction. The system served as a tool for the optimization of the imaging process and for the study of image degradation effects in CBμCT, as well as a platform for biological research using small animals. The set of tools for the accurate study of CBCT was completed by developing a fast Monte Carlo simulation engine based on GPUs, specifically devoted to the realistic estimation of scatter and its effects on image quality in arbitrary CBCT configurations, with arbitrary spectra, detector response, and antiscatter grids. This new Monte Carlo engine outperformed current simulation platforms by more than an order of magnitude. Due to the limited options for simulation of spectra in microfocus x-ray sources used in CBμCT, we contributed in this thesis a new spectra generation model based on an empirical model for conventional radiology and mammography sources modified in accordance to experimental data. The new spectral model showed good agreement with experimental exposure and attenuation data for different materials. The developed tools for CBμCT research were used for the study of detector performance in terms of dynamic range. The dynamic range of the detector was characterized together with its effect on image quality. As a result, a new simple method for the extension of the dynamic range of flat-panel detectors was proposed and evaluated. The method is based on a modified acquisition process and a mathematical treatment of the acquired data. Scatter is usually identified as one of the major causes of image quality degradation in CBCT. For this reason the developed Monte Carlo engine was applied to the in-depth study of the effects of scatter for a representative range of CBCT embodiments used in the clinical and preclinical practice. We estimated the amount and spatial distribution of the total scatter fluence and the individual components within. The effect of antiscatter grids in improving image quality and in noise was also evaluated. We found a close relation between scatter and the air gap of the system, in line with previous results in the literature. We also observed a non-negligible contribution of forward-directed scatter that is responsible to a great extent for streak artifacts in CBCT. The spatial distribution of scatter was significantly affected by forward scatter, somewhat challenging the usual assumption that the scatter distribution mostly contains low-frequencies. Antiscatter grids showed to be effective for the reduction of cupping, but they showed a much lower performance when dealing with streaks and a shift toward high frequencies of the scatter distributions. --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------A lo largo de los últimos años, el TAC de rayos X de haz cónico (CBCT, de “conebeam” CT) se ha posicionado como una de las técnicas de imagen más ampliamente usadas. El CBCT se ha convertido en una alternativa factible al TAC convencional en tareas de imagen específicas para las que la flexibilidad limitada ofrecida por este hace recomendable el desarrollo de sistemas de imagen dedicados. De esta forma, el CBCT está empezando a usarse de forma rutinaria en varios campos entre los que se incluyen la radioterapia guiada por imagen, la cirugía guiada por imagen usando arcos en C, imagen de partes de la anatomía en las que el TAC convencional no es apropiado, como los senos nasales, las extremidades o la mama, y, especialmente el campo de imagen preclínica con pequeño animal. Los sistemas CBCT usados en este último campo se encuentran habitualmente combinados con sistemas de imagen molecular. A pesar del trabajo de investigación dedicado al avance de la técnica CBCT en los últimos años, los retos introducidos por el uso de haces cónicos y de detectores bidimensionales son un campo candente para la investigación médica, con el objetivo de obtener una calidad de imagen equivalente o superior a la proporcionada por el TAC convencional. En el caso de imagen preclínica, a los retos generados por el uso de CBCT se une la necesidad de una mayor resolución de imagen que permita observar estructuras anatómicas con el mismo nivel de detalle obtenido para humanos. Esta tesis contribuye al progreso del CBCT mediante el estudio de usa serie de efectos que afectan a la calidad de imagen de CBCT en general y en el ámbito preclínico en particular. Como parte de este trabajo, hemos evaluado y optimizado el rendimiento de sistemas CBCT preclínicos en función de la tarea de imagen concreta. Con este fin se ha desarrollado un sistema CBCT para pequeños animales con geometría variable y todas las herramientas necesarias para la adquisición, calibración y reconstrucción de imagen. El sistema sirve como base para la optimización de protocolos de adquisición y para el estudio de fuentes de degradación de imagen además de constituir una plataforma para la investigación biológica en pequeño animal. El conjunto de herramientas para el estudio del CBCT se completó con el desarrollo de una plataforma acelerada de simulación Monte Carlo basada en GPUs, optimizada para la estimación de radiación dispersa en CBCT y sus efectos en la calidad de imagen. La plataforma desarrollada supera el rendimiento de las actuales en más de un orden de magnitud y permite la inclusión de espectros policromáticos de rayos X, de la respuesta realista del detector y de rejillas antiscatter. Debido a las escasas opciones ofrecidas por la literatura para la estimación de espectros de rayos X para fuentes microfoco usadas en imagen preclínica, en esta tesis se incluye el desarrollo de un nuevo modelo de generación de espectros, basado en un modelo existente para fuentes usadas en radiología y mamografía. El modelo fue modificado a partir de datos experimentales. La precisión del modelo presentado se comprobó mediante datos experimentales de exposición y atenuación para varios materiales. Las herramientas desarrolladas se usaron para estudiar el rendimiento de detectores de rayos tipo flat-panel en términos de rango dinámico, explorando los límites impuestos por el mismo en la calidad de imagen. Como resultado se propuso y evaluó un método para la extensión del rango dinámico de este tipo de detectores. El método se basa en la modificación del proceso de adquisición de imagen y en una etapa de postproceso de los datos adquiridos. El simulador Monte Carlo se empleó para el estudio detallado de la naturaleza, distribución espacial y efectos de la radiación dispersa en un rango de sistemas CBCT que cubre el espectro de aplicaciones propuestas en el entorno clínico y preclínico. Durante el estudio se inspeccionó la cantidad y distribución espacial de radiación dispersa y de sus componentes individuales y el efecto causado por la inclusión de rejillas antiscatter en términos de mejora de calidad de imagen y de ruido en la imagen. La distribución de radiación dispersa mostró una acentuada relación con la distancia entre muestra y detector en el equipo, en línea con resultados publicados previamente por otros autores. También se encontró una influencia no despreciable de componentes de radiación dispersa con bajos ángulos de desviación, poniendo en tela de juicio la tradicional asunción que considera que la distribución espacial de la radiación dispersa está formada casi exclusivamente por componentes de muy baja frecuencia. Las rejillas antiscatter demostraron ser efectivas para la reducción del artefacto de cupping, pero su efectividad para tratar artefactos en forma de línea (principalmente formados por radiación dispersa con bajo ángulo de desviación) resultó mucho menor. La inclusión de estas rejillas también enfatiza las componentes de alta frecuencia de la distribución espacial de la radiación dispersa

Local scale structures in earth's thermospheric winds and their consequences for wind driven transport

Thesis (Ph.D.) University of Alaska Fairbanks, 2015In the traditional picture of Earth's upper thermosphere (~190-300 km), it is widely presumed that its convective stability and enormous kinematic viscosity attenuate wind gradients, and hence smooth out any structure present in the wind over scale size of several hundreds of kilometers. However, several independent experimental studies have shown that observed upper thermospheric wind fields at high latitudes contain stronger than expected local-scale spatial structures. The motivation of this dissertation is to investigate how the resulting local-scale gradients would distort neutral air masses and complicate thermospheric wind transport. To achieve this goal, we examined the behavior of a simple parameter that we refer to as the "distortion gradient". It incorporates all of the wind field's departures from uniformity, and is thus capable of representing all resulting contributions to the distortion or mixing of air masses. Climatological analysis of the distortion gradient using 2010, 2011, and 2012 wind data from the All-sky Scanning Doppler Imager (SDI) located at Poker Flat (65.12N, 147.47W) revealed the diurnal and seasonal trends in distortion of thermospheric masses. Distortion was observed to be dependent on geomagnetic activity and orientation of the interplanetary magnetic field. To understand the time-cumulative influence of these local-scale non-uniformities on thermospheric wind driven transport, time-resolved two-dimensional maps of the thermospheric vector wind fields were used to infer forward and backward air parcel trajectories. Tracing air parcel trajectories through a given geographic location indicates where they came from previously, and where they will go in the future. Results show that wind driven transport is very sensitive to small-scale details of the wind field. Any local-scale spatial wind gradients can significantly complicate air parcel trajectories. Transport of thermospheric neutral species in the presence of the local-scale wind gradients that we observed was found to be far more complicated than what current models typically predict. To validate these findings, we cross-compared the upper thermospheric neutral winds inferred from a narrow field of view Fabry-Perot interferometer with winds measured by our all-sky SDI. A high degree of correlation was present between their measurements. This cross-validation study suggests the presence of small-scale short-lived, and previously unobserved wind features in the upper thermosphere, with typical length scales less than ~40 km. The spatially and temporally localized wind features implied by this study represent a new and unexplored regime of dynamics in the thermosphere.Chapter 1: Introduction to Earth's upper atmosphere -- Chapter 2: Dynamics of Earth's upper atmosphere -- Chapter 3: Distortion of thermospheric air masses by horizontal neutralwinds over Poker Flat measured using an all-sky scanning doppler imager -- Chapter 4: Tracing trajectories of air parcels transported through spatially resolved horizontal neutralwind fields observed in the thermosphere above Alaska -- Chapter 5: First ever cross-comparison of thermospheric wind measured by narrow and wide field optical doppler spectroscopy -- Chapter 6: Conclusions -- Appendices

The HERMES recoil photon detector and nuclear pt-broadening at HERMES.

Deze thesis gaat over de bouw en de test van de hermes recoil foton detector. Het tweede deel handelt aangaande verbreding van de transversale impuls van hadronen gevormd in een nucleaire omgeving ter vergelijking met diegene die gevormd worden aan een nuceon (proton of neutron). Dit verschaft informatie over hoe een hadron gevormd wordt uit quarks in functie van ruimte en tijd

Small angle neutron scattering from superconductors

Small angle neutron scattering is a well known technique for studying the properties of solids at small but finite energies and momenta. In this work it has been applied to the study of the low energy properties of superconductors. The low transition temperature superconductors niobium and lead and the high transition temperature superconductor Y(_1)Ba(_2)Cu(_3)O(_7) have been investigated. The results indicate a temperature dependent excess scattering of around 150 mbam/sr/atom from Y(_1)Ba(_2)Cu(_3)O(_7) at wavevectors q of around 0.03 Å and an energy of less than 30 µeV. No such effect was observable, even at much lower wavevectors (q around 0.004 Å (^-1) in the low transition temperature superconductor niobium. However, a drop in transmission of a niobium sample at T(_c) indicates an increase in cross section of 71 ± 15 mbarn/atom. No simple explanation for these effects has been found