Dear Colleagues,Science & Ultimate Reality
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.
The theory of relativity tells us that two events that seem simultaneous in
one reference fame 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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Title:
Author:Paul Kwiat
Summary: (Quantum) Erasing the Nature of Reality
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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