Academic literature on the topic 'Quantum Astronomy'

Based on a number of experimentally verified physical observations, it is argued that the standard principles of quantum mechanics should be applied to the Universe as a whole. Thus, a paradigm is proposed in which the entire Universe is represented by a pure state wavefunction contained in a factorisable Hilbert space of enormous dimension, and where this statevector is developed by successive applications of operators that correspond to unitary rotations and Hermitian tests. Moreover, because by definition the Universe contains everything, it is argued that these operators must be chosen self-referentially; the overall dynamics of the system is envisaged to be analogous to a gigantic, self-governing, quantum computation. The issue of how the Universe could choose these operators without requiring or referring to a fictitious external observer is addressed, and this in turn rephrases and removes the traditional Measurement Problem inherent in the Copenhagen interpretation of quantum mechanics. The processes by which conventional physics might be recovered from this fundamental, mathematical and global description of reality are particularly investigated. Specifically, it is demonstrated that by considering the changing properties, separabilities and factorisations of both the state and the operators as the Universe proceeds though a sequence of discrete computations, familiar notions such as classical distinguishability, particle physics, space, time, special relativity and endo-physical experiments can all begin to emerge from the proposed picture. A pregeometric vision of cosmology is therefore discussed, with all of physics ultimately arising from the relationships occurring between the elements of the underlying mathematical structure. The possible origins of observable physics, including physical objects positioned at definite locations in an arena of apparently continuous space and time, are consequently investigated for a Universe that incorporates quantum theory as a fundamental feature. Overall, a framework for quantum cosmology is introduced and explored which attempts to account for the existence of time, space, matter and, eventually, everything else in the Universe, from a physically consistent perspective.

In this thesis we study the dynamics of cosmological scenarios inspired by quantum gravity. Part I investigates novel features of the semi-classical regime of homogeneous and isotropic loop quantum cosmology. Dynamics in this regime becomes modified by nonperturbative quantum effects, subject to a number of ambiguities. For a flat universe the quantum effects accelerate a scalar field along its self-interaction potential during a period of super-inflation. We study how this behaviour can in principle set the initial conditions for subsequent slow-roll inflation. We also calculate a first approximation for the spectrum of perturbations produced during the super-inflationary phase. For the positively-curved case we investigate how a bounce from a contracting to an expanding phase can occur, and show that this can lead to oscillations of the universe. During the oscillations the inflaton field can roll monotonically up its potential. Once the potential energy becomes sufficiently large, however, the cycles end and inflation commences. For a constant potential the oscillations occur about a centre fixed point allowing the construction of `new emergent universe' scenarios where the universe is past-eternally an Einstein static universe, but subsequently evolves into inflation. Part II considers positively-curved braneworld models in which the dynamical equations become modified in such a way as to permit a bounce. It is conjectured that models of this type can exhibit similar behaviour to the positively-curved LQC scenario. General conditions for this behaviour are determined in braneworld settings and we investigate an explicit example - the baneworld of Shtanov and Sanhi - in detail

In this thesis, we explore different ways in which spacetime exhibits peculiar properties when subjected to the rules of quantum mechanics. These rules are naturally implemented at the level of semiclassical physics, where the dynamical nature of the spacetime metric is neglected. In particular, we explore how quantum effects modify some of the fundamental statements of General Relativity, ranging from different possible solutions, such as traversable wormholes and time machines, to some of the more foundational conjectures, with an emphasis to the one of cosmic censorship. Chapter One takes a deeper look into the connection between geometry and entropy. We revisit the original reasoning leading to their entwinement, and we clarify the different notions of entropy that play a role in it. We emphasize the recurring theme and the pattern in such a relationship: how the union between area and entropy makes sense when put together on the same footing, hinting towards a deeper meaning in a complete theory of quantum gravity. This seemingly simple unification is then shown to lead to incredible results, ranging from improved conjectures about quantum gravity, to illuminating one of the most critical problems of modern theoretical physics - the black hole information paradox. In particular, we mainly focus on one example of semiclassical statements, the (quantum) Penrose inequality, and we show in detail the difficulties one has to overcome for a meaningful conjecture to hold. Furthermore, we revise the basic arguments underlying the recent progress regarding the black hole interior and lay out the possible paths to the interpretation of these striking results. Chapter Two explores different solutions that classical General Relativity forbade, but quantum physics advanced. A number of no-go theorems get circumvented, and configurations previously thought of as impossible become available, and even natural. This is especially clear for solutions such as traversable wormholes and their inherent use in studies of entanglement structures. Indeed, such connections will be relevant in gauge/gravity duality for a fuller understanding of the holographic dictionary. But we can also see the way in which other no-go theorems become easier to infer. In essence, the creation of closed causal curves was understood as a problem of quantum gravity due to the incredibly high energies one seems to need for their demise. However, we show how simple, low-energy arguments are enough to shatter the fiction of time machines. The final Chapter Three perhaps comes closer to the study of quantum gravity than the previous ones. We undertake the problem of naked singularities in gravity, and we see how including quantum effects solidifies some foundational statements while completely fragmenting other ones. In a nutshell, the strong cosmic censorship conjecture is shown to be on much firmer ground than previously thought. Quantum physics is used to destabilize the relevant Cauchy horizon once and for all. However, including quantum effects necessarily means we must abandon our na¨ıve understanding of the weak cosmic censorship and embark on a much stranger path towards a meaningful statement about naked singularities. In doing so, we discuss the purpose of cosmic censorship and its interpretation in the realm of quantum gravity. We finish the dissertation with a summary and a further discussion on the nature of naked sin- gularities, providing a framework in which these questions can be meaningfully posed. After a brief overview of recent developments in this research line, we discuss the possible ways in which we can tackle such a perplexing problem. Namely, the role of critical phenomena in gravitational collapse is emphasized, and a proposal for a future study is outlined.<br>Como es propio de toda teoría clásica, la Relatividad General no puede aspirar a ser más que una teoría efectiva, cuyo campo de estudio se reduce al de fenómenos emergentes de estructuras más elementales. Sin embargo, se trata de una teoría dificil de tratar al poseer propiedades no compartidas por el resto de teorías clásicas: una descripción holográfica. A pesar de no haber proporcionado todas las respuestas que buscábamos acerca de la naturaleza del espacio y del tiempo, la holografía ha jugado un papel fundamental; en especial mostrándonos una conexión entre nociones tan dispares como la información cuántica y la geometría, similar a la conexión que Gibbons y Hawking [1] dieron a conocer entre el área y la entropía. Esta tesis tiene como objetivo el estudio de casos en los que esta relación se vuelve manifiesta, usando el régimen semiclásico de gravedad. El primer capítulo profundiza en la conexión entre área y entropía y algunas de las consecuencias que esta implica: la formulación semiclásica de la Desigualdad de Penrose y las posibles intepretaciones relativas al interior de los agujeros negros. El segundo capítulo se adentra en el estudio de escenarios prohibidos por la Relatividad General pero que resultan accesibles, y naturales, al considerar efectos cuánticos. Se centra en los agujeros de gusano y su relación con el entrelazamiento cuántico (a través de la dualidad “gauge/gravity”), así como en la imposibilidad de transformarse en máquinas del tiempo. El capítulo tercero es el que más avanza hacia el régimen cuántico de la gravedad, explorando el problema de las singularidades desnudas y la Hipótesis de la Censura Cósmica. Se muestra cómo la versión fuerte sale reforzada tras un análisis semiclásico, mientras que la versión débil requiere de nuevas reinterpretaciones para su adaptación a la nueva realidad cuántica. Finalmente se ofrece un resumen junto con una discusión adicional sobre la naturaleza de las singularidades desnudas, con un pequeño repaso sobre los avances en este campo y las posibles rutas que tomar, haciendo hincapié en el papel del colapso crítico gravitatorio y proponiendo una línea de investigación más allá de esta tesis. Bibliografía: [1] G. W. Gibbons and S. W. Hawking, “Action integrals and partition functions in quantum gravity,” Phys. Rev. D 15 (May, 1977) 2752–2756. https://link.aps.org/doi/10.1103/PhysRevD.15.2752.

The main technical problem with background independent approaches to quantum gravity is inapplicability of standard quantum field theory methods. New methods are needed which would be adapted to the basic principles of General Relativity. Topological field theory is a model which provides natural tools for background independent quantum gravity. It is exactly soluble and, at the same time, diffeomorphism invariant. Applications of topological field theory to quantum gravity include description of boundary states of quantum General Relativity, formulation of quantum gravity as a constrained topological field theory, and a new perturbation theory which uses topological field theory as a starting point. The later is the central theme of the thesis. Unlike the traditional perturbation theory it does not require splitting metric into a background and fluctuations, it is exactly diffeomorphism invariant order by order, and the coupling constant of this theory is dimensionless. We describe the basic ideas and techniques of this perturbation theory as well as inclusion of matter particles, boundary states, and other necessary tools for studying scattering problem in background independent quantum gravity.

A proposal for performing quantum memory schemes with a light matter interface in Hollow Core Fibres is introduced. Various technical aspects of implementing such a scheme in the proposed interface are outlined and the different elements required to realize this scheme are discussed, primarily the detection of atomic levels and the extension of the scheme to magnetically trappable levels. A new method to dispersively measure populations and population difference of alkali atoms prepared in their two clock states is introduced, for future use in the Hollow Core Fibre interface. The method essentially detects the atom numbers based on the influence of the linear birefringence in the ensemble on the detection light beams via polarization homodyning. Sideband detection is performed after dressing the atoms with a radio-frequency field to circumvent low-frequency technical noises. The noise performance of this scheme is discussed along with design modifications aimed at reaching the atomic shot noise limit. Another technical aspect of realizing the quantum memory scheme in the proposed light-matter interface is the extension of the scheme to the trappable states of the atomic system as the atoms will be trapped in an atom chip magnetic field. We achieve this extension by showing the microwave spectroscopy of the ground state ensemble of radio-frequency dressed atoms which proves the existence of pseudo one-photon transitions between the trappable clock states. Finally, the preliminary designs and results of integrating an HCF in an atom chip experiment are discussed.

The connection between light emission and structure of Germanium nanoparticles (3-10 nm) prepared by top-down (etching) and bottom-up (sol-gel and colloidal synthesis) has been investigated using Raman spectroscopy, TEM, x-ray absorption spectroscopy (XAS), x-ray di raction (XRD), and photoluminescence (PL). It was found that TEM, Raman spectroscopy, PL, and XRD techniques all result in di ering values for the nanoparticle size which don't all agree in the limit of experimental error. Several structural models have been proposed and tested by high pressure Raman measurements. It was found that a Raman peak corresponding to diamond-type Ge structure is observed well above the transition pressure of both amorphous ( six GPa) and crystalline ( 11 GPa) Ge. The pressure dependence of the Raman signal peak position was observed to follow an unexpected non-linear shift with a corresponding increase in peak width (FWHM). Possible structural origins of these trends have been investigated by adapting the widely used phonon con nement model to high pressure conditions and comparing experimental data with the model behaviour under assumptions of constant, and size-dependent bulk modulus. Considered collectively with the ambient structural data, the results of the analysis of the high pressure behaviour point to the phenomenon of gradual surface induced amorphisation under pressure in matrix-free Ge nanoparticles. The best structural model to describe this is a core-shell with the small crystalline core and a disordered surface layer. The local structure of samples was investigated using XAS, while opticallydetected XAS, using x-ray excited optical luminescence (XEOL), was used to link structure with optical emission. The emission was found to depend on surface termination; in oxygen terminated nanoparticles the oxide rich regions are responsi- 4 ble for light emission, while in their hydrogen terminated counterparts' pure Ge regions contribute to the luminescence. Furthermore, with the aid of molecular dynamics simulations it was shown that in hydrogen-terminated samples, optical emission is due to a topologically disordered (amorphous) region close to the surface of the nanoparticles. We demonstrated that OD-XAS can potentially provide subnanoparticle resolution due to its sensitivity to the light emitting sites in a sample. We further investigated the microscopic origins of such sensitivity and identi ed possible limitations. This work clearly demonstrates that a combination of methods sensitive to short-range and long-range structure are required for comprehensive characterisation of nanoscale systems.

In this thesis, two methods to model quantum well lasers will be examined. The first model is based on well-known techniques to determine some of the spectral and dynamical properties of the laser. For the spectral properties, an expression for TE and TM modal amplitude gain is derived. For the dynamical properties, the rate equations are shown. The spectral and dynamical properties can be examined separately for specific operating characteristics or used in conjunction with each other for a complete description of the laser. Examples will be shown to demonstrate some of the analysis and results that can be obtained. The second model used is based on Wigner functions and the quantum Boltzmann equation. It is derived from general non-equilibrium Greens functions with the application of the Kadanoff-Baym ansatz. This model is less phenomenological than the previous model and does not require the separation of physical processes such as the former spectral and dynamical properties. It therefore has improved predictive power for the performance of novel laser designs. To the Author's knowledge, this is the first time such a model has been formulated. The quantum Boltzmann equations will be derived and some calculations will be performed for a simplified system in order to illustrate some calculation techniques as well as results that can be obtained.