Abstract
This thesis explores instances of the intersection of quantum mechanics and gravity using tools from quantum information theory to apply it to gravity. The first chapter investigates the consistency of Hawking’s black hole evaporation process with fundamental physical principles such as unitarity, no-signaling, entanglement monogamy, and the equivalence principle. The analysis suggests that standard quantum theory and general relativity can account for the entire state consisting of matter and radiation, which remains pure at any stage of the evaporation process. Furthermore, the final state after the full black hole evaporation is pure and in one-to-one correspondence with the initial state forming the black hole, indicating no information loss. The second chapter derives the gravitational field and the spacetime metric generated by sources in quantum superposition of different locations. We there adopt two independent constructions, the first of which promotes the potential to an operator and lets it act on the matter state, then covariant uplifting its expectation value to derive the radial and temporal components of the metric. The second approach writes down a joint superposition state for the matter and the corresponding spacetime metric, assuming that in each branch of the matter’s superposition the spacetime metric’s state follows GR and is fully determined by the Einstein equations. The The resulting quantum corrected effective metric is consistent with that obtain with the first method, but augments it in that it derives the angular components as well. In both cases we work out the example of the matter is in a Gaussian state, with a certain width that we call R. The physical system that the metric represents will differ depending on the width of the Gaussian relative to its would-be Schwarzschild horizon. The solutions will be categorized in three families according to whether the width is larger than rs, equal, or smaller. This study provides a detailed exploration of the geometric and thermodynamic properties of the spacetime structure for the three families of models, including nonsingular black holes, one-way wormholes with a critical null throat, and traversable wormholes. The third chapter investigates the degradation in the performance of quantum communication protocols that take place near a black hole due to quantum information being lost beyond the event horizon. However, the study shows that when the quantum nature of a black hole and its spacetime are taken into account, their quantum properties can be used as resources to limit the amount of quantum information loss, improving the performance of quantum communication protocols. This project extends the scope of recent results in quantum foundations on the coherent control of quantum channels, indicating that quantum features of spacetime could serve as resources for quantum information processing. Finally, the fourth article highlights the necessity of resolving the apparent gauge dependence in the quantum corrections of cosmological observables for Higgs-like inflation models. We propose a practical shortcut to gauge-independent inflationary observables by using effective potential obtained from a polar-like background current choice. This study demonstrates this shortcut for several explicit examples and presents a gauge-independent prediction of inflationary observables in the Abelian Higgs model. Furthermore, the authors show that for any theory to all orders, the use of a gauge-invariant current term gives a gauge-independent effective potential and thus gauge-invariant inflationary observables. Together, these projects contribute to a better understanding of the interaction between quantum mechanics and gravity and provide insights into the quantum nature of black holes, spacetime, and inflationary observables. The thesis often uses tools from quantum information theory to explore the intersection of quantum mechanics and gravity, taking a conservative approach to combining quantum mechanics and gravity, exploring the possibility of further quantum corrections or more accurately adding more quantumness to classical and semi-classical scenarios.