Chapter 3 explores open quantum systems, emphasizing their interactions with environments, unlike isolated closed systems. It introduces the concept of generalized measurements and mixed quantum states, reflecting the complex scenarios arising from these interactions. The chapter utilizes Positive Operator Valued Measures (POVMs) to describe generalized measurements, broadening the conventional approach to quantum measurements.

A significant focus is on the evolution of open systems through quantum channels, which illustrate the transfer or transformation of quantum information amid noise and external disturbances. This section underpins the dynamics open systems exhibit, critical for understanding quantum computing and information processing in realistic settings.

Through practical examples, the chapter elucidates how environmental factors influence quantum information, vital for applications in quantum technologies. It aims to equip readers with foundational knowledge of open quantum systems, highlighting their importance in the broader context of quantum mechanics.

Many deep results in mathematical general relativity concern the interplay between globally conserved quantities, such as the center of mass and angular momentum, and the geometric structure of initial data sets, using analysis of the scalar curvature, or more generally the full constraint equations. The chapter focuses on constant mean curvature foliations and the geometric center of mass of asymptotically flat initial data sets, a research program initiated by Huisken and Yau. We begin with a partial survey of the classical results of constant mean curvature surfaces and introduce the concept of stability, and then discuss some recent progress on the constant mean curvature surfaces in asymptotically flat initial data sets and the geometric center of mass. In the final section, we adopt a more analytic approach to study the center of mass and angular momentum from the Einstein constraint equations.

Chapter 15 extensively examines the resource theory of asymmetry, focusing on the significance of asymmetry as a quantum resource, particularly in situations lacking a shared reference frame. The chapter begins by identifying the foundational elements, such as free states and operations within this theory, emphasizing their role in alignment of quantum reference frames. A significant part of the discussion revolves around the quantification of asymmetry, utilizing measures like the Fisher information and Wigner–Yanase–Dyson skew information to assess the degree of asymmetry in quantum states. The concept of G-twirling is introduced as a method to achieve symmetric states, serving as a key technique in analyzing and understanding asymmetry. Moreover, the chapter explores how asymmetry can enhance tasks like parameter estimation, leveraging the maximum likelihood method to improve precision.

Chapter 6 builds upon the foundation of divergences from Chapter 5, advancing into entropies and relative entropies with an axiomatic approach, and the inclusion of the additivity axiom. The chapter delves into the classical and quantum relative entropies, establishing their core properties and revealing the significance of the KL-divergence introduced in Chapter 5, notably characterized by asymptotic continuity. Quantum relative entropies are addressed as generalizations of classical ones, with a focus on the conditions necessary for these measures in the quantum framework. Several variants of relative entropies are discussed, including Renyi relative entropies and their extensions to quantum domain such as the Petz quantum Renyi divergence, minimal quantum Renyi divergence, and the maximal quantum Renyi divergence. This discourse underlines the relevance of continuity and its relation to faithfulness in relative entropies. The concept of entropy is portrayed as a measure with a broad spectrum of interpretations and applications across fields, from thermodynamics and information theory to cosmology and economics.

A classical approach to the problem of effectively parametrizing the space of solutions to the vacuum Einstein constraint equations is the conformal method. We will focus on the constant mean curvature case, which reduces to the analysis of the Lichnerowicz equation to solve the Hamiltonian constraint, and which brings the study of the scalar curvature operator to the fore. We begin the chapter with elements of the theory of elliptic partial differential equations, and then develop the conformal method and present in detail the closed constant mean curvature case. In the last section we discuss more general scalar curvature deformation and an obstruction to positive scalar curvature due to Schoen and Yau.

Chapter 5 delves into divergences and distance measures, which are crucial for comparing quantum states. It begins with classical divergences such as the Kullback–Leibler and Jensen–Shannon, then advances to their quantum counterparts, discussing their optimal characteristics. Influenced by quantum resource theories, these quantum extensions provide foundational insights into the robust tools of resource theories. The chapter concentrates on particular divergences that serve as true metrics, including the trace distance and a variant of the fidelity, and explores the concept of distance between subnormalized states, which is essential in the context of quantum measurements. It emphasizes the purified distance, a useful tool for understanding the entanglement cost of quantum systems, setting the stage for further exploration in later chapters. The chapter offers a mathematically approachable survey of these measures, underscoring their practical importance in quantum information theory.

In 1973 Roger Penrose proposed an inequality which states that, in an isolated gravitational system with nonnegative local energy density, the total mass of the system must be at least as much as that contributed by any black holes contained within. The original motivation for proposing the inequality is the fact that a counterexample of this inequality would produce a counterexample for the cosmic censorship conjecture. While the Penrose inequality in full generality remains an open problem, we present in this chapter the main ideas of three proofs of the Riemannian Penrose inequality, a special case that arises in the time-symmetric setting. In this setting, the Penrose inequality is formulated on an asymptotically flat manifold with nonnegative scalar curvature that contains closed minimal surfaces, and asserts a bound of the total mass in terms of the area of the outermost minimal surface. We begin with Lam's proof for the graphical case, then move to the inverse mean curvature flow approach of Huisken and Ilmanen, and finish with a sketch of Bray's conformal flow technique.

This chapter introduces quantum resource theories (QRTs), tracing their evolution and key principles, starting from physics’ quest to unify distinct phenomena into a single framework. It highlights the unification of electricity and magnetism as a pivotal advancement, setting a precedent for QRTs in quantum information science. Quantum resource theories categorize physical system attributes as “resources,” notably transforming the role of quantum entanglement from mere theoretical interest to a crucial element in quantum communication and computation.

The chapter further describes the book’s layout and educational strategy, designed to offer a comprehensive understanding of QRTs. It explores the application of quantum resources in fields like quantum computing and thermodynamics, presenting a unique viewpoint on subjects such as entropy and nonlocality. Emphasizing on axiomatic beginning followed by practical uses, the book serves as a vital resource for both beginners and experts in quantum information science, preparing readers to navigate the complex terrain of QRTs and highlighting their potential to advance quantum science and technology.

The theory of special relativity incorporates a modification of Newtonian mechanics together with electromagnetism. A natural question to consider is how gravitation fits into the framework of relativity. In this chapter we focus our analysis of this question along two main ideas, that of the equivalence between uniform acceleration and a uniform gravitational field, and that of the gravitational redshift. These will lead us to the Einstein equation, which we then show can be given a variational formulation. We present some solutions of the Einstein equation, with particular attention given to the Schwarzschild spacetime and its Kruskal extension.

This chapter develops the geometry of and analysis on initial data sets that arise in models of isolated gravitational systems. We begin with some detailed discussion and analysis involving the Laplace operator on asymptotically flat manifolds, which we use to develop density and deformation results on scalar curvature, leading to a proof of the Riemannian positive mass theorem. In the last section of the chapter we develop a technique for localized scalar curvature deformation, and we apply it to glue an asymptotically flat end with vanishing scalar curvature to an end of a Riemannian Schwarzschild metric, maintaining zero scalar curvature throughout.