From: anon.281@pax.tpa.com.au
Newsgroups: sci.physics
Subject: Chiao's Quantum Eraser Experiment (repost)
Date: 5 Jan 1993 11:28:03 +1030
Organization: PAX - Public Access Unix (Adelaide,South Australia) - Anonymous Service
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Here is a repost of a recent posting I made.  The other posting was cut
off after a series of dashes I had in the posting due to the anonymous
posting software. 
=================

I have a question regarding the Quantum Eraser experiment described in
Scientific American July 1992 (see below) performed by Raymond Y. Chiao of the
University of California at Berkeley.  First the relevent passages from the
article below (I think copyright laws allow me to do this for the purposes of
discussion) then my question about the experiment.

>From Scientific American, July 1992, "Quantum Philosophy," p.76
[Quote begins here; any spelling mistakes are my own.  I have placed words that
I have inserted myself, to better describe the diagram below, in square
brackets.]
=============================================================================

Earlier this year Chiao's group passed a beam of light through a
down-conversion crystal, generating two identical photons.  After being
directed by mirrors along separate paths, the two photons crossed paths again
at a half-silvered mirror [Beam Splitter] and then 
entered two detectors.  Because it was
impossible to know which photon ended up in which detector, each photon seemed
to go both ways.  As in Mandel's experiment, the intereference pattern was
revealed by lengthening one arm of the interferometer; a device called a
coincidence counter showed the simultaneous firings of the two photon detectors
rising and falling as the two wavelets entering each detector went in and out
of phase.

Then the workers added a device to the interferometer that shifted the
polarization of one set of photons by 90 degrees [Polarization Shifter].  
If one things of a ray of
light as an arrow, polarization is the orientation of the plane of the
arrowhead.  One of the peculiarities of polarization is that it is a strictly
binary property; photons are always polarized either vertically or
horizontally.  The altered polarization served as a tag; by putting
polarization detectors in front of the simple light detectors at the end of the
routes, one could determine which route each photon had taken.  The two paths
were no longer indistinguishable, and so the interference pattern disappeared.

Finally, Chiao's group inserted two devices that admitted only light polarized
in one direction just in front of the detectors [Polarizing Filters].  
The paths were indistinguishable again, and the interference pattern 
reappeared.  Unlike Humpty-Dumpty, a collapsed wave function can be put back 
together again.
===========================================================================
[Quote ends here]

Here's a diagram of the apparatus:

                                     D
                 ____M____       |_______|
                    / \             /
          S ==> \\/     \       \\/
           __   /\\       \     /\\ <== F
Incident  |  \/             \ /
=======>==| C >           ======== <== B 
Photon    |__/\             / \
                \         /     \// <== F
                  \     /       //\
                 ___\_/___        __\___
                     M           |      |
                                     D

C = Down Converter
S = Polarization Shifter
M = Mirrors
B = Beam Splitter
F = Polarizing Filters
D = Detectors

(Sorry if the diagram isn't very clear - it's the best I can do with ASCII
characters.)

I assume that you can know the polarization of the incident photon on the
down-conversion crystal (otherwise you could not know which path each photon
took with the polarization shifter there anyway).  After one photon
has passed through the polarization shifter, we then have two photons polarized
at 90 degrees to each other.  Let's define the axis parallel to the direction
of polarization of one photon the x-axis, and the axis parallel to the direction
of polarization of the other photon the y-axis.  Now, my initial problem was
with understanding how the polarizing filters "erased" the information -
initially I had visualised the polarization of each filter being parallel to
each of the photons (which wouldn't work).  However, I think it *would* work in
erasing the information if the filters were each placed at 45 degrees to the x
and y axes, as then each filter would have a 50% chance of passing each
photon.  The orientation of the filters isn't described anywhere in the
article.  So is my understanding correct?

Thanks in advance,

		    F.


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