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Science

Time and the Quantum: Erasing the Past and Impacting the Future

Yakir Aharonov, Muhammad Suhail Zubairy

Published Feb 10, 2005• 94 citations
Publisher

Abstract

The quantum eraser effect of Scully and Drühl dramatically underscores the difference between our classical conceptions of time and how quantum processes can unfold in time. Such eyebrow-raising features of time in quantum mechanics have been labeled "the fallacy of delayed choice and quantum eraser" on the one hand and described "as one of the most intriguing effects in quantum mechanics" on the other. In the present paper, we discuss how the availability or erasure of information generated in the past can affect how we interpret data in the present. The quantum eraser concept has been studied and extended in many different experiments and scenarios, for example, the entanglement quantum eraser, the kaon quantum eraser, and the use of quantum eraser entanglement to improve microscopic resolution.

DOI
10.1126/science.1107787

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The quantum eraser effect of Scully and Drühl dramatically underscores the difference between our classical conceptions of time and how quantum processes can unfold in time. Such eyebrow-raising features of time in quantum mechanics have been labeled "the fallacy of delayed choice and quantum eraser" on the one hand and described "as one of the most intriguing effects in quantum mechanics" on the other. In the present paper, we discuss how the availability or erasure of information generated in the past can affect how we interpret data in the present. The quantum eraser concept has been studied and extended in many different experiments and scenarios, for example, the entanglement quantum eraser, the kaon quantum eraser, and the use of quantum eraser entanglement to improve microscopic resolution.
According to Quantum Mechanics, the particles can exhibit either particle properties or wave properties depending on the experimental set up (wave-particle dualism). A special behavior occurs for a system of two entangled particles 1 and 2 that propagate along two different directions in the space. In such a case, the entangled particle 2 should exhibit either the particle behavior or the wave behavior depending on the kind of measurement that is performed on particle 1 whatever is the actual distance between the two particles. The apparently surprising fact is that the "choice" of what kind of measurement is performed on particle 1 can be also made when particle 2 has been already detected (delayed-choice quantum eraser).These theoretical predictions have been confirmed by the experiments and seem to suggest that a future measurement can affect a past event. Recently, both the concepts of "delayed choice" and of "quantum erasure" have been criticized by Ellerman and by Kastner. In this paper we propose an alternative analysis of the delayed-choice quantum eraser with two entangled particles and we show that it does not imply an inversion of the natural cause-effect temporal order of the physical events.
As indicated by the name "quantum erasure," the most common interpretation of certain classes of delayed choice quantum experiments is that they, in some fashion, erase or undo past decisions. Unfortunately, this interpretation cannot be correct since the past decisions were already classically and irreversibly captured as recorded information or datums. A datum is information that, through temporal entanglement, constrains future events. The correct interpretation of such experiments is stranger than erasure: Recordings made early in such quantum experiments predestine choices made later through arbitrarily complex and often human-scale classical choices. Since this process of quantum predestination occurs only within the future light cone of datum creation, another (possibly) less radical way to interpret such experiments is that time is multiscale, granular, and impossible to define outside of the quantum state of the entities involved. The continuum time abstraction is not compatible with this view.
Ever since the proposal of delayed choice quantum erasure and subsequent realization in the experiment by Kim et al., the interpretation and implications of delayed-choice experiments have remained a subject of intense foundational debate. This paper resolves the apparent paradox attached to the experiment using standard quantum mechanics. Using an extended Mach-Zehnder interferometer which captures every operational feature of the original experiment, we show that choosing between which-path and erasure detectors is simply a choice of measurement bases, which does not rewrite a photon's past. Furthermore, by mapping the experiment to a two-way Stern-Gerlach framework, we prove that quantum erasure is an expected result of measuring entangled states, not a physical anomaly. Ultimately, through a pedagogical game, we illustrate that the illusion of retrocausality arises from asking illegitimate questions, and that a forward-in-time description is entirely sufficient to explain the logic.
This paper discusses the well-known delayed choice Quantum Eraser experiment performed by Kim et al. in 2000 and analyzes it from a Classical Physics perspective. I have included a diagram of the setup used in the experiment. I show that the result of the experiment can be explained by Classical Physics and does not require “Spooky action at a distance” due to entangled particles, as Einstein famously once put it, nor events modifying the past due to the delayed choice aspect of the experiment.
This work examines the well-known delayed choice Quantum Eraser experiment carried out by Kim et al in 2000 from the standpoint of Classical Physics. I've drawn a diagram of the experiment's setup. I show that the experiment's outcome can be explained using Classical Physics, and that it does not require "spooky action at a distance" owing to entangled particles, as Einstein famously put it, nor events changing the past due to the experiment's delayed choice feature.