The work leading to this thesis focuses on assessing and extending quantum theories in order to explore and test their implications across various regimes -- including thermodynamics, semiclassical and quantum gravity scenarios, and the in principle detectable predictions of such theories. The general motivation stems from a basic desire to understand the world form its very foundations. For instance, how can we bridge the gap between what we observe or `perceive' and the fundamental quantum nature in our theories. In particular, this work originated from the search to a better understanding of the nature of time according to our physical theories and the common perception that it invariably `flows' to the future, or, in other words, why do we observe distinct natural processes evolving asymmetrically in time? These motivations led to three distinct, yet interconnected and successful, lines of research, presented here in three separate parts: I. On Possible Detections within Physical Theories; II. On Explaining the Approach to Thermodynamic Equilibrium; and III. Relativistic Collapse Theories and a Self-Consistent Model of Semiclassical Gravity.
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Doctoral thesis submitted to the Universidad Nacional Autónoma de México (UNAM). Front page and acknowledgments are in Spanish, the main body is in English. Introductions are in both languages. This version contains typographical and editorial improvements made after the institutional submission: https://hdl.handle.net/20.500.14330/TES01000881995 .(112 pages, 5 figures)
The work leading to this thesis focuses on assessing and extending quantum theories in order to explore and test their implications across various regimes -- including thermodynamics, semiclassical and quantum gravity scenarios, and the in principle detectable predictions of such theories. The general motivation stems from a basic desire to understand the world form its very foundations. For instance, how can we bridge the gap between what we observe or perceive' and the fundamental quantum nature in our theories. In particular, this work originated from the search to a better understanding of the nature of time according to our physical theories and the common perception that it invariably flows' to the future, or, in other words, why do we observe distinct natural processes evolving asymmetrically in time? These motivations led to three distinct, yet interconnected and successful, lines of research, presented here in three separate parts: I. On Possible Detections within Physical Theories; II. On Explaining the Approach to Thermodynamic Equilibrium; and III. Relativistic Collapse Theories and a Self-Consistent Model of Semiclassical Gravity.
This book presents a comprehensive treatment of quantum theory using theoretical variables as its foundation. The work progresses from fundamental quantum mechanical principles to sophisticated applications in quantum field theory and gravity. It provides rigorous mathematical frameworks while establishing connections between theoretical physics and practical engineering applications. Particularly suitable for graduate students and researchers in physics, mathematics, and engineering disciplines seeking to understand quantum phenomena across theoretical and applied domains. Presents an advanced treatment of quantum theory across two interconnected parts Examines Bell experiments, quantum probability, information theory, and connections to relativity Extends these foundations to explore quantum field theory, gravity, and noise theory Bridges the gap between theoretical physics and engineering applications Covers topics from Yang-Mills fields, quantum gravity to neural networks and biomedical applications of quantum electrodynamics Addresses quantum measurement of gravitons, Hawking radiation, classical and quantum neural networks with practical examples Provides mathematical frameworks for understanding quantum phenomena in applied contexts This title has been co-published with Manakin Press. Taylor & Francis does not sell or distribute the print edition in India, Pakistan, Nepal, Bhutan, Sri Lanka and Bangladesh.
This book examines the foundational consistency of quantum mechanics incorporated within relativistic frameworks. Quantum physics remains a perplexing formalism that, although very successful in explaining physical phenomena, poses many philosophical and interpretational questions. Several of the subtleties of quantum physics become more manifest when quantum processes are described using relativistic dynamics. For instance, the successful connection of spin to quantum statistics is a consequence of the consistent incorporation of special relativity into the quantum formalism. There should be similar profound explanations awaiting discovery as gravitating phenomena are successfully incorporated into quantum formulations. The common theme of this manuscript is the examination of the incorporation of relativistic behaviors upon the foundations of quantum physics. The approach is to keep all formulations as close to observed phenomena as possible, rather than to present a set of speculative models whose primary motivations are internal aesthetics. In the search for the most elegant models of physical phenomena, one must recognize that at its core, physics is an experimental science. The dimensional analysis of fundamental units, taught at the very beginning of introductory physics classes, demonstrates that phenomenology lies at the foundations of physics. Fundamental ideas such as correspondence, the principle of relativity, and complementarity provide direct contact with the physics used to guide this exploration. This manuscript is an elaboration and expansion on previously published work, but also contains some new material.
The project concerns the interplay among quantum mechanics, statistical mechanics and thermodynamics, in isolated quantum systems. The underlying goal is to improve our understanding of the concept of thermal equilibrium in quantum systems. First, I investigated the role played by observables and measurements in the emergence of thermal behaviour. This led to a new notion of thermal equilibrium which is specific for a given observable, rather than for the whole state of the system. The equilibrium picture that emerges is a generalization of statistical mechanics in which we are not interested in the state of the system but only in the outcome of the measurement process. I investigated how this picture relates to one of the most promising approaches for the emergence of thermal behaviour in isolated quantum systems: the Eigenstate Thermalization Hypothesis. Then, I applied the results to study some equilibrium properties of many-body localised systems. Despite the localization phenomenon, which prevents thermalization of subsystems, I was able to show that we can still use the predictions of statistical mechanics to describe the equilibrium of some observables. Moreover, the intuition developed in the process led me to propose an experimentally accessible way to unravel the interacting nature of many-body localised systems. Second, I exploited the "Concentration of Measure" phenomenon to study the macroscopic properties of the basis states of Loop Quantum Gravity. These techniques were previously used to explain why the thermal behaviour in quantum systems is such an ubiquitous phenomenon, at the macroscopic scale. I focused on the local properties, their thermodynamic behaviour and interplay with the semiclassical limit. This was motivated by the necessity to understand, from a quantum gravity perspective, how and why a classical horizon exhibits thermal properties.
A key aim of quantum foundations is to characterize the sense in which nature goes beyond classical physics. Understanding nonclassicality is one of our best avenues towards finding a satisfactory interpretation of quantum theory. By determining which classical principles cannot be satisfied in any empirically adequate physical theory, we begin to see which principles can be preserved, which in turn gives us insight into the ontology of the world. These insights then guide us in determining which questions to ask and which experiments to perform next. Furthermore, it is these nonclassical aspects of nature that give rise to new technologies such as the speed-ups of quantum computation or the security of quantum key distribution. \n \nThe gold standard for establishing that a phenomenon is truly nonclassical is to prove that it violates the principle of local causality or the principle of noncontextuality. Much of this thesis reports on my research relating to these two principles. This research primarily involves (i) finding new justifications for our notions of nonclassicality; (ii) refining their fundamental definitions; (iii) quantifying and characterizing their various manifestations; and (iv) finding applications where nonclassical phenomena act as resources for information processing. \n \nUltimately, all of this work on nonclassicality is woven together into a novel framework for physical theories introduced by myself, John Selby, and Rob Spekkens. Its main advantage over preexisting frameworks is that it maintains a clear distinction between which elements of a given physical theory directly describe causal processes, and which refer only to one’s inferences about causal processes. This clarifies a number of confusions in the literature which arose precisely because previous frameworks scrambled causal and inferential concepts. Furthermore, local causality and noncontextuality emerge in this framework as the assumptions that the causal and inferential structures (respectively) that are operationally observed must be respected in the underlying ontology. This work constitutes a first step in developing a new interpretation of quantum theory—the first interpretation designed to satisfy the spirit of both local causality and noncontextuality.
Physics has two main ambitions: to predict and to understand. Indeed, physics aims for the prediction of all natural phenomena. Prediction entails modeling the correlation between an action, the input, and what is subsequently observed, the output.Understanding, on the other hand, involves developing insightful principles and models that can explain the widest possible varietyof correlations present in nature. Remarkably, advances in both prediction and understanding foster our physical intuition and, as a consequence, novel and powerful applications are discovered. Quantum mechanics is a very successful physical theory both in terms of its predictive power as well as in its wide applicability. Nonetheless and despite many decades of development, we do not yet have a proper physical intuition of quantum phenomena. I believe that improvements in our understanding of quantum theory will yield better, and more innovative, protocols and vice versa.This dissertation aims at advancing our understanding and developing novel protocols. This is done through four approaches. The first one is to study quantum theory within a broad family of theories. In particular, we study quantum theory within the family of locally quantum theories. We found out that the principle that singles out quantum theory out of this family, thus connecting quantum local and nonlocal structure, is dynamical reversibility. This implies that the viability of large scale quantum computing can be based on concrete physical principles that can be experimentally tested at a local level without needing to test millions of qubits simultaneously. The second approach is to study quantum correlations from a black box perspective thus making as few assumptions as possible. The strategy is to study the completeness of quantum predictions by benchmarking them against alternative models. Three main results and applications come out of our study. Firstly, we prove that performing complete amplification of randomness starting from a source of arbitrarily weak randomness - a task that is impossible with classical resources - is indeed possible via nonlocality. This establishes in our opinion the strongest evidence for a truly random event in nature so far. Secondly, we prove that there exist finite events where quantum theory gives predictions as complete as any no-signaling theory can give, showing that the completeness of quantum theory is not an asymptotic property. Finally, we prove that maximally nonlocal theories can never be maximally random while quantum theory can, showing a trade-off between the nonlocality of a theory and its randomness capabilities. We also prove that quantum theory is not unique in this respect. The third approach we follow is to study quantum correlations in scenarios where some parties have a restriction on the available quantum degrees of freedom. The future progress of semi-device-independent quantum information depends crucially on our ability to bound the strength of these correlations. Here we provide a full characterization via a complete hierarchy of sets that approximate the target set from the outside. Each set can be in turn characterized using standard numerical techniques. One application of our work is certifying multidimensional entanglement device-independently.The fourth approach is to confront quantum theory with computer science principles. In particular, we establish two interesting implications for quantum theory results of raising the Church-Turing thesis to the level of postulate. Firstly, we show how different preparations of the same mixed state, indistinguishable according to the quantum postulates, become distinguishable when prepared computably. Secondly, we identify a new loophole for Bell-like experiments: if some parties in a Bell-like experiment use private pseudorandomness to choose their measurement inputs, the computational resources of an eavesdropper have to be limited to observe a proper violation of non locality.
This is the fifth update for the quantum gravity framework project. The research work has been split into two parts: Part A deals with unconscious matter, while Part B deals with conscious matter. Each has its own abstract. I introduce retrocausality in both. In Part A, based on the first five papers, I finalize a three-set of fundamental postulates for understanding time, quantum reduction, and consciousness from quantum general relativistic concepts. Then I introduce a new reduction theory that includes retrocausality and spontaneous reduction, and further concepts to include conscious and unconscious influence. Then I discuss three postulates in the quantum general relativistic framework. Toward the end, I discuss sample calculations and experimental verifications. I summarize various calculations done in the previous versions of the Quantum General Relativistic Framework. In Part B of this paper, I propose a set of ideas to explain the phenomena of life and consciousness in the universe and their relation to the interpretation of quantum mechanics using my past papers and other important work. Consciousness is described as a result of specific dynamical configurations of matter and fields aided by fundamental properties of nature proposed in this paper. First, I review the various theories of consciousness in current literature and come up with various hypotheses, definitions, and propositions to build a physical theory of consciousness. I propose three sets of postulates on the physical foundations of consciousness from a quantum general relativistic background, using the proposals in part A, to understand time and reduction. Various new concepts are introduced. The concept of classicum is introduced as an analog of quantum, and its relation to conscious being is discussed. Mathematical models for various ideas are discussed. I discuss theory and experiments to verify these.