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Journal of Physics Conference Series

A few reflections on protective measurements and more.

Marco Genovese

Published Aug 1, 2017• 5 citations
Publisher

Abstract

Quantum measurement represents the most severe and charming problem of foundations of quantum mechanics. Several issues ranging from the collapse of the wave function to the ontic meaning of the wave function remain unclarified. Here we briefly summarize some recent ideas about this problem, with particular attention to protective measurements and their meaning.

DOI
10.1088/1742-6596/880/1/012012

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arXiv:quant-ph/9805012•Physical Review A•Apr 1, 1999•48 citations
Quantum measurement represents the most severe and charming problem of foundations of quantum mechanics. Several issues ranging from the collapse of the wave function to the ontic meaning of the wave function remain unclarified. Here we briefly summarize some recent ideas about this problem, with particular attention to protective measurements and their meaning.
Protective measurements offer an intriguing method for measuring the wave function of a single quantum system. With contributions from leading physicists and philosophers of physics - including two of the original discoverers of this important method - this book explores the concept of protective measurement, investigating its broad applications and deep implications. Addressing both physical and philosophical aspects, it covers a diverse range of topics, including experimental possibility of protective measurements, connections with the PBR theorem, and the implications of protective measurements for understanding the nature of quantum reality. Including a clear and concise introduction to standard quantum mechanics, conventional measurements, and the fundamentals of protective measurements, this is a valuable resource for graduate students and researchers interested in the conceptual foundations of quantum mechanics.
In this article, I shall focus on two of the main problems raising interpre-tational issues in quantum mechanics, namely the notorious measurement problem (discussed together with the theory of measurement in section 4) and the equally important but not quite as widely discussed problem of the classical regime (discussed together with decoherence in section 3). The two problems are distinct, but they are both intimately related to some of the is-sues arising from entanglement and density operators, which are thus briefly reviewed in section 2. A few fundamentals are rehearsed in section 1. The article will aim to be fairly non-technical in language, but modern in out-look and covering the chosen topics in more depth than most introductory treatments. The philosophy and foundations of quantum mechanics offer many more examples of live research issues, and much progress has been achieved re-cently in such traditional approaches as collapse theories, pilot-wave the-ories and Everett interpretations, and in the (time-honoured but recently revived) area of axiomatic reconstructions of the theory. Recent years have
It is a fundamental and widely accepted assumption that a measure-ment result exists universally, and in particular, it exists for every ob-server, independently of whether the observer makes the measurement or knows the result. In this paper, we will argue that, based on an analysis of protective measurements, this assumption is rejected by the many-worlds interpretation of quantum mechanics, and worlds, if they indeed exist ac-cording to the interpretation, can only exist relative to systems which are decoherent with respect to the measurement result. In standard quantum mechanics, it is postulated that when the wave func-tion of a quantum system is measured by a macroscopic device, it no longer follows the linear Schrödinger equation, but instantaneously collapses to one of the wave functions that correspond to definite measurement results. However, this collapse postulate is ad hoc, and the theory does not tell us why and how a definite measurement result appears. This measurement problem is widely
The recently proposed idea of ``protective'' measurement of a quantum state is critically examined, and generalized. Earlier criticisms of the idea are discussed, and their relevance to the proposal assessed. Several constraints on measuring apparatus required by ``protective'' measurements are discussed, with emphasis on how they may restrict their experimental feasibility. Though ``protective'' measurements result in an unchanged system state and a shift of the pointer proportional to the expectation value of the measured observable in the system state, the actual reading of the pointer position gives rise to several subtleties. We propose several schemes for reading the pointer position, both when the apparatus is treated as a classical system as well as when its quantum aspects are taken into account, that address these issues. The tiny entanglement which is always present due to deviation from extreme adiabaticity in realistic situations is argued to be the weakest aspect of the proposal. Because of this, one can never perform a protective measurement on a single quantum system with absolute certainty. This clearly precludes an ontological status for the wave function. Several other conceptual issues are also discussed.