From:
"Caroline Thompson" <ch.thompson1@virgin.net>
Newsgroups:
sci.physics.research
Subject:
Re: A possible loophole in the Delayed
Choice Quantum Eraser
Date: Fri,
26 Nov 2004 07:22:26 +0000 (UTC)
"Schneibster"
<wbishop@mbay.net> wrote in message news:Schneibster.1g91h2@physicsforums.com...
> Hi,
I've been honing this idea for about 3 or 4 months, and
> now I'd
like to bring it here and discuss it. I'll start out by stating
> that I
am not a professional physicist; I have been studying
> physics
on an amateur basis for most of my life, so most of my
>
knowledge is from various general-reader-level books. However,
> I do
have a degree in electronics engineering, so hopefully I will
> not
make a complete fool of myself here.
Hi, I too am
not a professional physicist, though I became interested in physics 10 years
ago, starting with trying to find out what really happened in the Bell test
experiments. I too have looked Kim et
al's experiment. It was discussed in
sci.physics in 1999, and I wrote to the author to ask for more information. I'm afraid I never did feel I had quite
enough facts to be able to say definitely what happened, though I am convinced
that this experiment, together with all others currently classified as
"quantum optics" ones, can be explained by ordinary wave theory once
you take account of the clever ways in which phase, frequency, polarisation and
exact direction are all tied together in the outputs from a pumped nonlinear crystal.
> As a
review, the Delayed Choice quantum eraser was proposed by
> Scully
and Drühl in 'this'
>
(http://prola.aps.org/abstract/PRA/v25/i4/p2208_1)
>
article, and then performed by Kim, Kulik, Shih, and Scully in 1999 as
>
reported in 'this'
>
(http://xxx.lanl.gov/PS_cache/quant-ph/pdf/9903/9903047.pdf) article.
>
Basically, photons are led through a dual-slit apparatus, and then
> split
with a β-BBO crystal spontaneous parametric down-converter
> (SPDC)
into orthogonally-polarized pairs that are separated with a
>
Glan-Thompson prism and sent to two separate measurement areas.
Ah, thanks
for reminding me! The prism is not
shown on their diagram. Perhaps I'd
better go away and re-read the paper before commenting too much.
> In the
first area, the photons are allowed to fall on a movable
>
detector that checks various positions for photon count. This is
>
essentially looking for interference fringes ...
I'm not so
sure this is the right way to think of it.
The actual interference in the experiment takes place not here but when
the beams from the two parts are combined at the beamsplitter. It is not measured directly as an
interference pattern but detected by means of variations in coincidence
counts. [This technique for identifying
interference, incidentally, occurs in many quantum optics applications,
including the one I'm currently interested in: Grangier's team's new proposal
for a "loophole-free" Bell test, http://arxiv.org/abs/quant-ph/0403191
]
> ...
which would show up as high incidences at some locations and
> low or
zero incidences at others. These photons are labeled "signal"
> photons
in the 1999 paper.
>
> In the
second measurement area, the photons are either captured
> by one
of two detectors that determine their "which-path" data,
> which
is valid for determination of the which-path data for their
>
associated signal photon, or are sent to a "scrambler" that makes
> it
impossible to reclaim the which-path data from them.
Yes, but I
think there is a completely different explanation for the whole experiment and
it does not assume that we know which path the light went by since it always
goes both ways. We are dealing here
with coherent light. The signal and
idler beams pretty certainly have a definite phase relationship. I haven't checked the details but the
potential for interference is there, so long as the light is passed through,
say, a polarising cube or equivalent that is set at 45 deg to the polarisation directions. Another feature of the light that may be
relevant (it is very important in certain experiments, though I haven't yet
checked here) is that it is likely to be in a mixture of two phase relationships
to the pump laser. [See http://arxiv.org/abs/quant-ph/9912082]
> These
are labeled as "idler" photons in the 1999 paper. The
>
scrambler is labeled as a "quantum eraser" in the 1982 paper.
>
> The
"Delayed Choice" is introduced by making the path to the idler
>
measurement area longer than the path to the signal measurement
> area.
If my kind
of explanation is right, this path length difference is not relevant. There is some real interference at the
beamsplitter, and path lengths to there look equal. The actual measurement is a coincidence one, so presumably
signals are counted that arrive at the same time and must have been emitted at
the same time.
> The
experimenters report that, after correlation, the collection of
> signal
photons that are correlated with idler photons that went to the
> quantum
eraser show interference, whereas the collection of signal
> photons
that are correlated with idler photons that went to the
>
which-path determination detectors do not show interference.
The part
that does not show interference is the part in which there could not have been
any since the beams did not mix at the beamsplitter.
> By
conventional causality, because
> a) No
matter what we do to the signal photons, the idler photons are
>
unaffected as far as we can tell,
> but
> b)
depending on whether we measure the which-path information we
> do or
do not see interference in the signal photons,
> we
interpret the idler photon measurement or non-measurement of
>
which-path information as the "cause" and non-interference or
>
interference as the "effect." Therefore, by that conventional
>
interpretation, this experiment appears to violate the normal time
>
direction of causes preceding effects.
As you say,
this is the conventional interpretation.
> I
wondered whether it might be possible to make this time reversal
>
explicit. My direction of attack was to attempt to differentiate the
>
interference case from the non-interference case -without resort to
> the
correlation, or in fact to any measurement of the idler photons.-
>
> After
developing this into something like a paper or proposal, I
>
realized that I had forgotten to consider that the superposition of
> the
interference patterns detected in the initial experiment might
> add up
to the non-interference case. After spending some time
> being
frustrated at such an oversight, I evaluated the published
>
experimental results, and found that
> a) they
are not sufficiently detailed to absolutely confirm or deny
> that
there is a difference between the interference and
>
non-interference patterns,
> but
> b) they
tend to indicate that there will be a difference.
[snip
details]
> The
trough-to-peak difference in local minima and maxima of impacts
> appears
to be 17-25% of the maximum for the non-interference case,
> but
23-30% for the interference case. In addition, there are many more
>
apparent "deviations" from the best curve in the interference case
than
> in the
non-interference case ...
Agreed, the
fit to their predicted curves is not that good, but nevertheless I think the
graphs show an interference pattern.
Other similar experiments -- the "ghost interference" ones --
have produced similar results.
> My
proposal is therefore as follows:
> 1.
Replace the stepper-motor-driven detector with a CCD array that can
> read
single lines. Such arrays 'exist'
>
(http://naysika.mi.iasf.cnr.it/epic/pnfocalplane.htm) and if properly
>
oriented would allow the detection of evidence of interference fringes
> very
quickly, if combined with appropriate electronics and fast image
>
processing.
> 2. Use
optical delay lines, preferably variable switched delay lines
> with
external fiber-optic hookups to facilitate the use of measured
> cable
lengths to introduce the desired delay.
> 3.
Replace the beam splitters that randomly direct one half of the
> idler
photons to the eraser, and the other half to the which-path
>
detectors, with LCD optical switches. Such switches are currently in
> use in
optical networking switches available from over fifty companies
>
worldwide.
Yes! That really would test the theory! I would predict that this would destroy the
interference pattern.
> I would
begin by attempting to successfully differentiate the
>
interference pattern from the non-interference pattern. If this were
>
successful, then I would proceed with attempting to apply negative
>
feedback to the experiment, in which the detection of interference
> would
cause the measurement of the idler photons' which-path
>
information, and detection of non-interference would cause the idler
> photons
to be sent to the eraser.
>
> I seek
opinions on whether this experiment is possible (i.e., whether
> the
interference and non-interference cases can be differentiated), and
> on what
the results might be if it -is- possible, and on what might
> happen
if it is possible and negative feedback is applied.
I'm afraid I
have not studied your idea sufficiently to really judge it, but have the
impression that it would be a step in the right direction. How about applying your mind to rather
easier experiments first, though? The Bell
test experiments are not nearly as intricate as this one. They need repeating with slight
modifications, designed to compare the explanatory power of the rival models
(local realism with reliance on the various "loopholes" [see my web
site] versus quantum theory) under a range of conditions, not just
concentrating on the Bell test.
Cheers
Caroline
From:
"Caroline Thompson" <ch.thompson1@virgin.net>
Newsgroups:
sci.physics.research
Subject: Re:
A possible loophole in the Delayed Choice Quantum Eraser
Date: Fri,
26 Nov 2004 09:24:22 -0000
Re Kim,
Kulik, Shih, and Scully in 1999 article,
http://xxx.lanl.gov/PS_cache/quant-ph/pdf/9903/9903047.pdf
Just a small
correction to what I wrote yesterday:
There is
pretty certainly interference at the screen with the movable detector as well
as at the central beamsplitter. In both
places if you look at the total events (i.e. if you look at "singles"
rather than coincidences) you don't easily spot this, the main reason being
that there are two interference patterns superposed, 180 deg out of phase. The patterns at the two locations are the
same, so they show up when you look at coincidences.
That would
be in the perfect case, if all frequencies were monotonic and there were no
extraneous light. In the real
experiment it is not surprising to find the interference pattern rather poor.
Caroline
ch.thompson1@virgin.net
http://freespace.virgin.net/ch.thompson1/