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De Gruyter eBooks

8 Delayed-Choice Experiments and the Metaphysics of Entanglement

Published Jul 28, 2014• 0 citations
Publisher

Abstract

Delayed-choice experiments in quantum mechanics are often taken to undermine a realistic interpretation of the quantum state.More specifically, Healey has recently argued that the phenomenon of delayed-choice entanglement swapping is incompatible with the view that entanglement is a physical relation between quantum systems.This paper argues against these claims.It first reviews two paradigmatic delayed-choice experiments and analyzes their metaphysical implications.It then applies the results of this analysis to the case of entanglement swapping, showing that such experiments pose no threat to realism about entanglement.

DOI
10.1515/9783110354409.137

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Delayed-choice experiments in quantum mechanics are often taken to undermine a realistic interpretation of the quantum state.More specifically, Healey has recently argued that the phenomenon of delayed-choice entanglement swapping is incompatible with the view that entanglement is a physical relation between quantum systems.This paper argues against these claims.It first reviews two paradigmatic delayed-choice experiments and analyzes their metaphysical implications.It then applies the results of this analysis to the case of entanglement swapping, showing that such experiments pose no threat to realism about entanglement.
Experiments demonstrating entanglement swapping have been alleged to challenge realism about entanglement. Seevinck (2006) claims that entangle- ment “cannot be considered ontologically robust” while Healey (2012) claims that entanglement swapping “undermines the idea that ascribing an entangled state to quantum systems is a way of representing some new, non-classical, physical relation between them.” My aim in this paper is to show that realism is not threatened by the possibility of entanglement swapping, but rather, it should be informed by the phenomenon. I argue—expanding the argument of Timpson and Brown (2010)—that ordinary entanglement swapping cases present no new challenges for the realist. With respect to the delayed-choice variant discussed by Healey, I claim that there are two options available to the realist: (a) deny these are cases of genuine swapping (following Egg (2013)) or (b) allow for existence of entanglement between timelike separated regions. This latter option, while radical, is not incoherent and has been suggested in quite different contexts. While I stop short of claiming that the realist must take this option, doing so allows one to avoid certain costs associated with Egg’s “orthodox” account. I conclude by noting several important implication of entanglement swapping for how one thinks of entanglement generally.
In recent years, there has been a growing interest in the possibility of temporal nonlocality, mirroring the spatial nonlocality supposedly evidenced by the Bell correlations. In this context, Glick (2019) has argued that timelike entanglement and temporal nonlocality is demonstrated in delayed-choice entanglement swapping (DCES) experiments, like that of Ma et al. (2012), Megidish et al. (2013) and Hensen et al. (2015). I will argue that a careful analysis of these experiments shows that they in fact display nothing more than "ordinary" spacelike entanglement, and that any purported timelike entanglement is an artefact of selection bias. Regardless any other reason one may have for challenging the assumption of temporal locality, timelike entanglement as evidenced by these experiments is not among them. I conclude by discussing what lessons on the nature of entanglement might be drawn from an examination of DCES experiments.
Wave-particle duality lies at the root of quantum mechanics and is central in the description of interferences observed with elementary objects. In a delayed-choice experiment, the decision to observe the particle or wave character of a quantum system is delayed with respect to the time at which the system enters the interferometer. This paper reviews the history of the delayed-choice idea, introduced as a challenge to a realistic explanation of the wave-particle duality. It also describes recent experimental realizations of this idea and discusses intriguing extensions, such as the duality between separability and entanglement in multiple quantum systems.
It is shown that quantum teleportation and entanglement swapping can be easily derived from Einstein, Podolsky and Rosen correlations and special relativity (SR). A recent proposal of entanglement swapping via delayed-choice experiment is discussed by means of a concrete example. Experiments in decoherence in high-Q cavities are briefly examined from the point of view of SR. It is argued that although decoherence can explain why we never see a Schrödinger ‘cat’ which is dead and alive at the same time, this approach apparently does little to explain the cat's ‘real’ state.
As the previous chapter made clear, in the present interpretation quantum mechanics gives a rather specific account of EPR-type correlations and what underlies them. In the present chapter I argue that this account is genuinely explanatory (rather than merely an idle overlay on a simple description of the observed correlations), and that this constitutes a significant advantage of the present interpretation over certain rival interpretations, on which quantum mechanics is unable to offer an adequate explanation of the observed correlations. In order to make this argument it will prove necessary to examine the character of causal explanation in theoretical science, and in particular to develop a coherent metaphysics of a kind of holistic, nonseparable, causal explanation. I take the development of such a coherent metaphysics to constitute a significant task independent of the details of the present interpretation of quantum mechanics. Quantum mechanics is a mine of metaphysical insights, and I hope here at least to have exposed a rich seam. The first task is to defend the adequacy of the present account of EPR-type correlations as at least a coherent description of what may underlie the observed correlations. I foresee three main challenges to its descriptive adequacy. The first is to claim that the account is incompatible with the relativistic requirement that there can be no direct causal connection between spacelike-separated events, and must therefore be rejected. A weaker version of this challenge is to claim that although the incompatibility just mentioned may not by itself entail rejection of the account, it does seriously undermine its claim to be genuinely explanatory.
Experimental realization of Gedanken experiments on Quantum Mechanics continue to suggest inferences of action at a distance, action into the future or past as reported in Delayed choice Quantum entanglement experiments recently by Ma et al (2012). Is it possible to read through this and investigate through simple logic and experiments whether control of the past or future is possible? A set of experiments are suggested here to do the same whose results may be interesting. Gedanken experiments on Quantum Mechanics principles Einstein, Podolsky and Rosen (Einstein et al, 1935) were the first to suggest a Gedanken experiment which would suggest instant communication over a distance between particles which was against the principle of Special