Entanglement has long been the subject of discussion by philosophers of quantum theory, and has recently come to play an essential role for physicists in their development of quantum information theory. In this paper we show how the formalism of algebraic quantum "eld theory (AQFT) provides a rigorous framework within which to analyse entanglement in the context of a fully relativistic formulation of quantum theory. What emerges from the analysis are new practical and theoretical limitations on an experimenter's ability to perform operations on a "eld in one spacetime region that can disentangle its state from the state of the "eld in other spacelike-separated regions. These limitations show just how deeply entrenched entanglement is in relativistic quantum "eld theory, and yield a fresh perspective on the ways in which the theory di!ers conceptually from both standard non-relativistic quantum theory and classical relativistic "eld theory. � 2001 Elsevier

This paper is a commentary on the foundational significance of the Clifton-Bub-Halvorson theorem characterizing quantum theory in terms of three information-theoretic constraints. I argue that: (1) a quantum theory is best understood as a theory about the possibilities and impossibilities of information transfer, as opposed to a theory about the mechanics of nonclassical waves or particles, (2) given the information-theoretic constraints, any mechanical theory of quantum phenomena that includes an account of the measuring instruments that reveal these phenomena must be empirically equivalent to a quantum theory, and (3) assuming the information-theoretic constraints are in fact satisfied in our world, no mechanical theory of quantum phenomena that includes an account of measurement interactions can be acceptable, and the appropriate aim of physics at the fundamental level then becomes the representation and manipulation of information.

We first introduce and discuss density operator interpretations of quantum theory as a special case of a more general class of interpretations, giving special attention to a version that we call the ‘atomic version’. We then review some crucial parts of the theory of stochastic processes (the proper context in which to discuss dynamics), and develop a general framework for specifying a dynamics for density operator interpretations. This framework admits infinitely many empirically equivalent dynamics. We give some examples, and discuss some of the properties of one of them. Dynamics for Density Operator Interpretations of Quantum Theory We first introduce and discuss density operator interpretations of quantum theory as a special case of a more general class of interpretations, giving special attention to a version that we call the ‘atomic version’. We then review some crucial parts of the theory of stochastic processes (the proper context in which to discuss dynamics), and develop a general framework for specifying a dynamics for density operator interpretations. This framework admits infinitely many empirically equivalent dynamics. We give some examples, and discuss some of the properties of one of them.

Hidden-variable models of quantum mechanics (QM) are complete descriptions of quantum phenomena. These models have been analyzed under conditions such as locality (Bell [1, 1964]) and non-contextuality (Kochen-Specker [20, 1967]). We give a uniform presentation of six underlying properties that can be asked of hidden-variable models and show all the relationships among them (as depicted in Figure 1.1). Two positive existence theorems are given which show that hidden-variable models of certain types always exist. We follow this with a unified treatment of the “no-go ” theorems of Einstein-Podolsky-Rosen [15, 1935], Bell [1, 1964], and Kochen-Specker [20, 1967]. Within our six-property classification scheme, we are able to give a complete picture of hidden-variable models.

We formulate a paradox in relation to the description of a joint entity consisting of two subentities by standard quantum mechanics. We put forward a proposal for a possible solution, entailing the interpretation of `density states' as `pure states'. We explain where the inspiration for this proposal comes from and how its validity can be tested experimentally.

This Chapter deals with theoretical developments in the subject of quantum information and quantum computation, and includes an overview of classical information and some relevant quantum mechanics. The discussion covers topics in quantum communication, quantum cryptography, and quantum computation, and concludes by considering whether a perspective in terms of quantum information

L ’ École Polytechnique n’entend donner aucune approbation, ni improbation aux opinions émises dans les thèses. Ces opinions doivent être considérées comme propres à leur auteur. Résumé Les dérivations théorético-informationnelles du formalisme de la théorie quantique soulèvent un intérêt croissant depuis le début des années 1990, grâce à l’émergence de la discipline connue sous le nom d’information quantique et au retour des questions épistémologiques dans les programmes de recherche de nombreux physiciensthéoriciens. Nous proposons une axiomatique informationnelle dont nous dérivons le formalisme de la théorie quantique. La première partie de la thèse est consacrée aux fondements philosophiques de l’approche informationnelle. Cette approche s’insère dans un cadre épistémologique que nous présentons sous la forme d’une boucle entre descriptions théoriques, ce qui nous permet de proposer une méthode nouvelle d’analyse de la frontière entre toute théorie et sa méta-théorie.

Even if we are able to decide on a canonical formulation of our theory, there is the further problem of metaphysical underdetermination with respect to, for example, whether the entities postulated by a theory are individuals or not... We need to recognise the failure of our best theories to determine even the most fundamental ontological characteristic of the purported entities they feature... What is required is a shift to a different ontological basis altogether, one for which questions of individuality simply do not arise. Perhaps we should view the individuals and nonindividuals packages, like particle and field pictures, as different representations of the same structure. There is an analogy here with the debate about substantivalism in general relativity. (Ladyman, 1998) In his paper “What is Structural Realism? ” (1998) James Ladyman drew a distinction between epistemological structural realism (ESR) and metaphysical (or ontic) structural realism (OSR). In recent years this distinction has set much of the agenda for philosophers of science interested in scientific realism. It has also led to the emergence of a related discussion in the philosophy of physics that concerns the alleged difficulties of interpreting general relativity that revolve around the question of the ontological status of spacetime points. Ladyman drew a suggestive analogy between the perennial debate between substantivalist and relationalist interpretations of spacetime on the one hand, and the debate about whether quantum mechanics treats identical particles as individuals or as ‘non-individuals ’ on the other. In both cases, Ladyman’s suggestion is that a structural realist interpretation of the physics—in particular, an ontic structural realism—might

Abstract The concept of classical indistinguishability is analyzed and defended against a number of well-known criticisms, with particular attention to the Gibbs ’ paradox. Granted that it is as much at home in classical as in quantum statistical mechanics, the question arises as to why indistinguishability, in quantum mechanics but not in classical mechanics, forces a change in statistics. The answer, illustrated with simple examples, is that the equilibrium measure on classical phase space is continuous, whilst on Hilbert space it is discrete. The relevance of names, or equivalently, properties stable in time that can be used as names, is also discussed.

I will discuss only one of the several entwined strands of the philosophy of