From: cdj@world.std.com (conrad d johnson)
Subject: Re: Are there plans to perform a quantum eraser type experiment?
Keywords: Quantum, measurement
Organization: The World Public Access UNIX, Brookline, MA
Date: Tue, 29 Dec 1992 03:24:49 GMT

In article <1992Dec27.010505.29958@colorado.edu> schiffd@tigger.cs.Colorado.EDU (David M. Schiff) writes:
>I was just wondering if anyone has heard of any plans to perform an
>experiment that will test the "quantum eraser" effect.  The last thing I
>read about this said it was still just a thought experiment but implied
>that it might be within experimentalists' capabilities in the near future.

What I know of this comes from the July '92 Scientific American (pp 97-8),
which summarizes recent research at Rochester and UC Berkeley, as well as
Wheeler's delayed-choice experiment.  The upshot confirms Bohr's position
that a quantum system achieves a determinate state -- its wave-function
"collapses" -- only in the context of a physical arrangement that makes it
possible to determine its state.

It seems to me that these experiments sharpen the problem of quantum
measurement by showing that it is not any particular event that brings
about a determinate state.  What counts is the informational structure of
the arrangement as a whole.
_____

These experiments are all versions of the two-slit experiment, in which
particles can take either of two separate paths that later reconverge at
a detector.  If the arrangement makes it impossible to tell which path
a particle takes, each particle in effect takes both paths at once,
producing a wave-interference pattern at the detector.  If it is in any
way possible to tell which path a particle takes, the interference
pattern disappears.

The arrangements set up at Rochester and UCB include components that do
determine which route a particle takes through the apparatus.  That
should bring about the "collapse" and prevent interference.  But they
also include other components that subsequently block or erase the path-
distinguishing information.  As it then again becomes impossible to tell
which path the particle took, the interference-pattern reappears.
_____

The conclusion seems to be that what makes the difference between a
superposition of virtual states and a single definite state is not any
particular thing that happens in the course of a measurement.  The idea
of the "collapse" as something that happens physically to a particle at
a certain time and place has to be wrong.  

If the "collapse" were an actual event, it would have happened in the
course of the Rochester/UCB experiments.  The path taken by each
particle was in fact determined by the apparatus.  But since this
information was subsequently erased from the system, the "collapse"
retroactively never happened.

This is much like Wheeler's delayed-choice experiment, which showed that
a change to the experimental arrangement at a certain moment can result
in a "collapse" having taken place at an earlier moment.  A kind of
"backward causality" seems to be implied.

This should also be seen in connection with the Aspect/Bell experiments
verifying Quantum non-locality.  There the point is that the "collapse,"
if considered as a physical event, has to involve instantaneous action-
at-a-distance.
_____


If the "collapse" is not a physical event, what's going on?  The "many-
worlds" interpretation doesn't help here.  The same difficulty applies
to the "splitting up" of the universe as to the "collapse" of the wave-
function -- if it's a real event, it has to be able to happen (and un-
happen) retroactively, and it has to be able to happen instantaneously
at distant points in space.

I think the best interpretation is the one that sticks with Bohr's
original idea.  It is not the particle "in itself" that is in a certain
definite state, or "has" certain "properties."  Rather, determinate
existence can only belong to systems that are big enough, complex enough
and sufficiently inter-connected with the rest of the world so that the
particular states they're in and the particular paths they take MAKE A
DIFFERENCE to other things.

The new experiments sharpen the problem also by showing that it's the
structure of the physical apparatus that counts, NOT the consciousness
of a human observer.  The issue has to do with the existence or non-
existence of information in the physical arrangement itself.
_____

I think then that the mystery of Quantum measurement probably comes down
to this question:

      What is the relationship between information and existence? 
      Or, how does information physically exist in the world?

More to come on this in later messages.
_____

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