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- From: aephraim@physics3 (Aephraim M. Steinberg)
- Newsgroups: sci.physics
- Subject: Re: Chiao's Quantum Eraser Experiment (repost)
- Date: 13 Jan 1993 00:44:02 GMT
- Organization: /etc/organization
- Lines: 82
- Message-ID: <1ivoki$eq6@agate.berkeley.edu>
- References: <1iamerINNkuq@flash.pax.tpa.com.au>
- NNTP-Posting-Host: physics3.berkeley.edu
-
- In article <1iamerINNkuq@flash.pax.tpa.com.au> anon.281@pax.tpa.com.au writes:
- %
- %
- %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.
-
- %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
- %
- %
- %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?
-
- Your understanding is correct. The two photons are produced horizontally
- polarized, and the waveplate (when positioned at 45 degrees to the hor-
- izontal) rotates one of them to vertical polarization. We showed that
- as you change the waveplate angle from 0 to 45, the visibility drops from
- close to 100% down to 0.
-
- In order to "erase" the information, the polarizers must be placed at
- 45 degrees to the horizontal, as you conclude. We also presented data
- showing how as you turned them towards 45, the visibility increased again.
-
- Actually, you can also place one at 45 and the other at -45; in this
- case, you get an "antifringe." Instead of a dip, you observe a peak.
-
- All of these phenomena can also be understood as a manifestation of the
- effective spin-singlet state produced by the interferometer, which is
- the same kind used in studies of Bell Inequalities. Individually, the
- photons are unpolarized, but whenever you detect a photon at one port
- after a polarizer, the other photon immediately becomes orthogonally
- polarized. Once you change the path lengths to move off the dip, however,
- this changes, and you simply have two non-interfering photons, one
- horizontal and one vertical. In that case, if you put both polarizers
- at 0 degrees, you get no counts at all even away from the dip. On the
- other hand, if you put one at 0 and the other at 90 so you get a lot of
- counts far from the "dip," you also get a lot of counts at the "dip."
- --
- Aephraim M. Steinberg | "WHY must I treat the measuring
- UCB Physics | device classically?? What will
- aephraim@physics.berkeley.edu | happen to me if I don't??"
- | -- Eugene Wigner
-