[Martin Taylor 2010.04.12.15.05]
[From Bill Powers (2010.04.12.1140 MDT)]
–
Martin Taylor 2010.04.12.12.39 –
MT: I’m not allowed to
redistribute the full text of the review article by Aharonov and
Zubairy,
but if you have a AAAS membership or a Science subscription, you can
get
it here
<
http://www.sciencemag.org/cgi/content/full/sci;307/5711/875?maxtoshow=&hits=10&RESULTFORMAT=&fulltext=Aharonov+Zubairy&searchid=1&FIRSTINDEX=0&resourcetype=HWCIT
, but perhaps this part of the caption to figure 2B might answer
your
question:
“In the case where the atoms have three levels
(
Fig. 2B ), the drive field excites the atoms from the ground
state c to the excited state a. The atom in state
a can then emit a
photon and end
up in state b. Here, the photon detected on the screen
leaves behind which-path information; that is, the atom
responsible for contributing the
![{gamma}]()
photon is in level
b, whereas the other atom remains in level c. Thus, a
measurement of the internal states of the atoms provides us
the which-path information and no interference is
observed.”
Later,
"The question, however, is whether we can erase the
which-path
information stored in the atom(s) and thus regain interference.
If the loss of interference was caused by some kind of noise
or uncertainty due to quantum fluctuations, the answer would
be no. We now show that this is not the case, and the interference
can be recovered. The question then is whether it is possible
to wipe out the which-path information and recover the
interference.
3) As shown in
Fig.
2C, this can possibly be done by driving the atom by
another field that takes the atom from level b to b’ and,
after an emission of a 
photon at the b’ – c transition, ends up in level c.
Now the final state of both the atoms is c, and a measurement
of internal states cannot provide us the which-path information. It
would therefore seem that the interference fringes will be
restored, but a careful analysis indicates that the which-path
information is still available through the
![{phi}]()
photon. A
measurement on the ![{phi}]()
photon can tell us which atom contributed the
![{gamma}]()
photon. Can we erase
the which-path information contained in the
![{phi}]()
photon and recover the
interference fringes? Scully and Drühl considered an ingenious
device based on an electrooptic shutter that can absorb the
![{phi}]()
photon in such a way
that the which-path information is erased
(
1)."
BP: What makes this sound to me like a non-explanation (i.e., an
unworkable model) is that the which-path information has only to exist
in
order to destroy the interference. That is magic, because it’s
causation
without mechanism.
It doesn’t sound like magic to me, because in order for the which-path
information to exist, at least one of the pathways must involve some
interaction of the photon with its environment, and hence decoherence.
What does seem like magic is the observed fact that if the information
is later erased, the fringe-no_fringe state switches, and
experimentally, this “later” is very large compared to the time scales
of the interactions. What would be magic would be if it specifically
required HUMAN observation for the fringe state to change.
Talking through my hat here, it sounds to me as though this is one area
where quantum mechanics is in direct conflict with general relativity,
not because there is hyperspeed transmission of information – there
isn’t, in any of these entanglement/teleportation effects – but
because there is no potential observer location or velocity that could
alter the before-after relation of the photon(s) passing the detector
and the pathway information being erased.
In a model, one variable affects another variable with
no function between them. If the state of an atom contains which-path
information, that is all that is required; this stored information
doesn’t have to affect any other atoms or give rise to intervening
processes. The information never has to be used; it merely has to
exist.
It’s a dependent variable in the model, but no link is provided to
allow
it to affect another variable.
There is a link in the mathematics, which to me is equivalent to the
model. These experiments are not created out of thin air. They take a
lot of forethought, time, and money, and are done to test
counterintuitive predictions from the mathematics. It’s not as though
someone said one day “Let’s do a two slit experiment” and then “Let’s
see what happens if we erase the pathway information after the fact”,
and then thought out a model to explain the weird result.
The problem isn’t that there isn’t a mechanical model; the problem is
that there is a model that has proved too accurate to allow for much
fudging. You are happy with 5% mismatch between model and data in a
tracking experiment experiment. Quantum physicists tend to worry about
parts per million mismatch. When such an accurate model predicts things
that just can’t be true, and experiment shows that they are true, I’d
say it increases one’s faith in the model/mathematics.
This does appear to settle the questions about the role of subjective
knowledge in the rules of this game. If the state of the atom
determines
existence of interference, that state never has to be known to any
human
being. Like the conceptual experiment with Schroedinger’s Cat, the
state
of the atom could be used to trigger some other event without human
intervention or knowledge, and that event could give rise to others,
until at the end there is a dead cat which a person will either see or
not see. But the interference fringes will not appear, regardless of
human consciousness of the state of the atom or the cat.
It seems to me that
the Delayed
Choice Quantum Eraser experiment is the most important one ever
performed. Where is that apparatus now?
According to A&Z, the experiment has been done in many places and
in
many forms. The present a quote from a book by Brian Greene “The
Fabric of the Cosmos, Knopf, 2004”.
Notice, too, perhaps the most dazzling result of all: the
three
additional beam splitters and the four idler-photon detectors
can only be on the other side of the laboratory or even on the
other side of the universe, since nothing in our discussion
depended at all on whether they receive a given idler photon
before or after its signal photon partner has hit the screen.
Imagine, then, that these devices are all far away, say ten
light-years away, to be definite, and think about what this
entails. You perform the experiment in fig 7.5b today,
recording–one after another–the impact locations of a huge number
of signal photons and you observe that they show no sign of
interference. If someone asks you to explain the data, you might be
tempted to say that because of the idler photons, which path
information is available and hence each signal photon definitely
went along either the left or the right path, eliminating any
possibility of interference. But, as above, this would be a hasty
conclusion about what happened; it would be a thoroughly premature
description of the past.
Here it is again: a paradox that nobody has actually tried to test.
I don’t know why you say nobody has tried to test. I’m beginning to
read your “nobody has done this experiment…” claims with quite a bit
of scepticism. As I read the A&Z paper, Green was providing a
description of a test that was done, obviously not with devices far
away, but with devices the other side of a room, which in the time
scales involved is just about as good as light years. Here’s the
Medline citation:
"
Phys Rev Lett. 2000 Jan
3;84(1):1-5.
Delayed “Choice” quantum eraser
Kim
YH,
Yu
R,
Kulik
SP,
Shih
Y,
Scully
MO.
Department of Physics, University of Maryland, Baltimore
County, Baltimore, Maryland 21250, USA.
Abstract
We report a delayed “choice” quantum eraser experiment of the type
proposed by Scully and Druhl (where the “choice” is made randomly by a
photon at a beam splitter). The experimental results demonstrate the
possibility of delayed determination of particlelike or wavelike
behavior via quantum entanglement. The which-path or both-path
information of a quantum can be marked or erased by its entangled twin
even after the registration of the quantum."
Perhaps I don’t have the requirements for detection or non-detection of
fringes quite right. But somebody has them right, and whatever they
are,
we can set up an experiment in which the detection event results in
destruction of the fringes at an earlier time, which in turn causes
some
event to happen that prevents the later detection event from happening.
Instead of trying to reason out from a highly malleable and ductile
theory what will happen, why not just set up the experiment and let it
show us what happens? Is the point to predict correctly, or to observe
what actually happens?
As I read it, the first part of it is exactly what Shih’s group did, 10
years ago. Maybe it’s been done, but I don’t know. I can’t get Physics
Review Letter papers but the corresponding page at prl.aps.org says it
has been cited 52 times, so I guess there may have been a few follow-up
experiments among those 52 (I can’t get at the reference list without
paying, either).
Martin
