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American Journal of Physics

When is a quantum measurement?

Asher Peres

Published Aug 1, 1986• 44 citations
Publisher

Abstract

The measurement of a quantum system is not consummated until irreversible processes have destroyed all phase coherence between different possible outcomes of that measurement. It is inconsistent to assume that the wavefunction ‘‘collapses’’ before irreversibility sets in.
DOI
10.1119/1.14505

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Can we be consistent?

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Cambridge University Press eBooks•Mar 26, 2012•0 citations
The measurement problem in quantum mechanics is commonly framed as a conflict between unitary evolution and the emergence of definite outcomes. This paper proposes a structural reinterpretation: quantum measurement is understood as an irreversible selection among physically admissible future possibilities. Rather than invoking wavefunction collapse as a fundamental postulate or branching as a purely formal multiplicity, measurement is defined as the physical process by which alternative futures become irreversibly excluded. This approach integrates quantum measurement into the same structural framework that defines information (DEC-1), time (DEC-2), and causality (DEC-3). Measurement is not a special quantum anomaly, but a concrete instance of irreversible closure acting on quantum possibility space.
The quantum measurement problem persists because quantum mechanics provides no physical criterion for when a measurement occurs or why outcome selection is irreversible. We propose a resolution based on a single principle: a measurement occurs precisely when entropy irreversibly flows from a quantum system into its environment, producing distinguishable environmental states and eliminating interference. Within this framework, wavefunction “collapse” is not an additional axiom but a physical consequence of thermodynamic irreversibility.
Abstract Measurement in quantum mechanics is a long-standing issue, and the measurement problem (as it is known) consists of essentially two parts: 1) no matter what the initial wavefunction of an object, the result of a measurement always results in the observation of a single eigenstate (i.e., “wavefunction collapse”), and 2) once the wavefunction collapses it remains collapsed. Though there have been a number of proposed solutions to the measurement problem, none to date is without debate. Here we propose an additional solution to the measurement problem, in the hope that alternate perspectives on this problem could eventually lead to its final resolution. We assume that for every eigenvector of an operator there is a set of discrete phases, and that the phase sets among eigenvectors are disjoint. With this hypothesis it is possible to show that wavefunction collapse is a consequence of destructive interference between a measurement mapping-operator, exhibiting all of the potential eigenvector phases, and the particular phase of a single realization of the quantum object.
Irreversibility, strengthened by the idea of strong mixing, has been discussed in the last two chapters. We reached the conclusion that, once such processes have been involved in a quantum measurement, it is in principle impossible to perform an interference experiment that would demonstrate the continued existence of a superposition. It is then ‘safe’ to assume that the system has ‘really’ collapsed into a state corresponding to one of the possible measurement outcomes. Does this mean that the measurement problem has been solved? Clearly it has for all practical purposes, as has been pointed out several times in earlier chapters. But it may still not be sufficient to provide a completely satisfactory solution in principle: in particular, we note that we have still not properly addressed the question of actualisation outlined at the end of the first section of Chapter 8. In the present chapter we discuss an interpretation of quantum physics that was developed during the last 15 or so years of the twentieth century and is based on the idea of describing quantum processes in terms of ‘consistent histories’. As we shall see, the resulting theory has much in common with the Copenhagen interpretation discussed in Chapter 4 and when applied to measurement it connects with the viewpoint, discussed in the last chapter, in which irreversible processes are to be taken as the primary reality.