(Quantum) Erasing the Nature of Reality

"Contrary to classical intuitions," writes Paul Kwiat,"quantum mechanics tells us that how and when something is measured can change the outcome of an experiment. Even stranger, the physical reality of an experiment is affected by the knowledge of the experimenter—or more precisely, by what can in principle be known. This inextricable link between reality and information leads to intriguing and fantastic possibilities. Quantum information, in the end, describes 'not only what can be known, but the subtle effect that knowing has on nature.'"

Moreover, our guest editor, Paul Davies, adds that perhaps "the weirdest aspect of quantum physics concerns what Einstein dubbed 'ghostly action at a distance.' This refers to quantum nonlocality—the fact that a quantum system spread over a region of space may not be separable into well-defined components, each with an independent existence. Roughly put, what happens over here may be intimately linked with the nature of things over there. Quantum nonlocality does not appear to permit faster-than-light signalling, but it does imply that the nature of reality at a remote location may depend upon the choice an experimenter makes here and now. Moreover, that 'reality determination' is instantaneous."

These spooky actions at a distance (or, as the notion has been known both historically and philosophically, as the problem of "actio in distans") provoke a problem: Are they real or are they merely artifacts of our current ignorance? Moreover, they point to a series of dilemmas—can force exist without matter? Is dynamism altogether dead or not? And what is the nature of power? Of activity? Are there uncaused causes? Would something like magnetism, which opened up the scientific world once again to this debate in the early 19th century, fall under the rubric of "actio in distans" at some levels? Is action at a distance then, also, a matter of perspective? Or is it a result of time or sequence, or perchance better said, of timing? To read more about these problems, please consult Quantum Non-Locality and Relativity: Metaphysical Intimations of Modern Physics by Tim Maudlin ( Blackwell, 1994) and Philosophical Consequences of Quantum Theory: Reflections on Bell's Theorem edited by James T. Cushing and Ernan McMullin (University of Notre Dame Press, 1989). For more recent papers, there are two websites of interest, one on the philosophical problems associated with quantum theory and another about the physics aspects.

But to enter the discussion now, please continue reading today's column which is part of a special series in anticipation of The Science & Ultimate Reality Symposium in Princeton, a symposium in honor of the 90th year of John Archibald Wheeler, a great physicist and teacher of physicists.

 —Editor

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Perhaps the weirdest aspect of quantum physics concerns what Einstein dubbed "ghostly action at a distance." This refers to quantum nonlocality—the fact that a quantum system spread over a region of space may not be separable into well-defined components, each with an independent existence. Roughly put, what happens over here may be intimately linked with the nature of things over there. Quantum nonlocality does not appear to permit faster-than-light signalling, but it does imply that the nature of reality at a remote location may depend upon the choice an experimenter makes hereand now. Moreover, that "reality determination" is instantaneous.

The theory of relativity tells us that two events that seem simultaneous in one reference frame will be sequential in another, so "reality determination now" can translate, for a different observer, into "reality determination in the past." That is, the activity of an experimenter on Earth today can help determine the nature of reality that was in some other part of the universe. That is the basis of John Wheeler's celebrated "delayed choice" experiment.

Paul Kwiat is a distinguished expert on quantum optics and quantum information, now working at the University of Illinois at Urbana-Champaign. His paper concerns new experimental realizations of quantum nonlocality, delayed choice, and so-called quantum eraser situations. A summary follows.

—Paul Davies

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Contrary to classical intuitions, quantum mechanics tells us that how and when something is measured can change the outcome of an experiment. Even stranger, the physical reality of an experiment is affected by the knowledge of the experimenter—or more precisely, by what can in principle be known. This inextricable link between reality and information leads to intriguing and fantastic possibilities. Quantum information, in the end, describes "not only what can be known, but the subtle effect that knowing has on nature."

In this paper, I intend to highlight some recent works by ourselves and others on the implementation of so-called "quantum erasers." Such experiments directly deal with the issues alluded to in the above quote, namely, in-principle knowledge of a quantum system can alter the observed behavior of that system. In some of these experiments, however, it seems that there is no such knowledge to be had. Nevertheless, by throwing away information in a particular way, we are then able to alter the quantum mechanical outcomes. As I will explain, the outcomes of such experiments can be understood to be consequences of that most-bizarre quantum mechanical feature: entanglement. And because entanglement has an undeniable non-local character to it, it becomes possible to perform various sorts of "delayed-choice" measurements of the sort proposed by Wheeler, in which the decision of whether or not to erase could seemingly be made even after the particles have been detected! Furthermore, we will see that the understanding of these experiments, especially those involving mixed quantum states, varies widely in the different interpretations of quantum mechanics, and indeed helps to clarify our understanding of these different interpretations. In particular, I will try to address what is the role of mixed states, if any, in several of the most popular interpretations of quantum mechanics.

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