[Martin Taylor 2005.03.28.11.30]
(Taking a littel break from real work -- i.e. visit from grandchildren 
Bill mentioned the role of the observer in quantum effects. Rick wanted to know about the kind of experiment.
The article "Quantum Erasure" by Walborn et al., in American Scientist July-August 2003 (v91-4,p336) should be relevant to both enquiries. The experimental situation is the Young's two-slit experiment, in which fringes appear on a screen that is illuminated by light that passes through two closely-spaced slits in a card. The astounding experimental result is that whether fromges appear depends on whether the experimenter could, in principle, observe which slit each particular photon went through. If yes, no fringes. If no, fringes appear.
The other thing to note is that the result is not explicable by the uncertainty principle (which, by the way, puts no limits on the precision with which any single parameter can be measured (or produced). The uncertainty principle affects only the precisions of the joint measurements of two parameters, and even that limitation seems to be getting a little unsure in the presence of entanglement. We shall see what the future brings in that regard.
Here's a few paragraphs from the article. I hope it's not more than copyright laws allow. You can get the whole article on-line at www.americanscientist.org if you are a Sigma Xi member or subscribe to the journal. Otherwise, I expect you could find it in your nearby University library.
----------Am.Sci article--------------
With his "slip of card," Young set off a revolution in physics whose ripples are still being felt today. His experiment is now a staple of freshman physics laboratories, though it is now typically performed with two slits etched into a piece of opaque microfilm. (Thus the name, "Young's double-slit experiment.") The phenomenon he had observed, called interference, can be demonstrated easily enough with waves in a tank of water. Thus, by analogy, Young's experiment seemed to prove that light is made up of waves, as Dutch physicist Christiaan Huygens had advocated. But that was not the end of the story.
...
In 1909, Cambridge physicist Geoffrey Taylor repeated an experiment similar to Young's, which showed that individual photons suffer another interference phenomenon, called diffraction. By dimming the light until only one photon at a time reached the screen, he eliminated any possibility that the photons could interfere with one other. Yet after recording the results of many photons, Taylor found the same pattern of diffraction fringes. Apparently, then, an individual photon could "interfere with itself."
...
Recently, physicists have started to shed some light on this mystery through the demonstration of quantum erasers-in which one can actually choose to turn the interference fringes on or off. Our group has constructed a quantum eraser using a more elaborate version of Young's experiment and used it to demonstrate, in principle, the idea of "delayed choice," in which the experimenter can make the decision after the particle has been detected.
...
The appearance of interference fringes in the classical double-slit experiment is well understood. According to the wave theory of light, when two beams of coherent light with the same wavelength encounter each other, they combine. The most extreme situations are constructive interference, in which the waves reinforce each other, or destructive interference, in which they cancel each other out completely.
...
To understand why quantum interference is unexpected, it may help to draw an analogy to a coin toss. If it is a fair coin, the probability of getting heads is 50 percent and the probability of getting tails is also 50 percent. The probability of getting heads or tails is the sum of the individual probabilities:
Prob (heads or tails) = Prob (heads) + Prob (tails) = 100 percent
Now consider a quantum "coin toss" based on Young's experiment. We send a beam of light at the double-slit apparatus, and put a photodetector a certain distance away on the other side. To dramatize the paradox, we place it in the middle of a dark interference fringe. Now, we turn down the light so that only one photon at a time passes through the slits. First we cover up slit 2, and we find, say, that 5 percent of the photons pass through slit 1 and trigger the detector. So Prob (slit 1) = 5 percent. Next we block slit 1 and find that 5 percent of the photons pass through slit 2 and trigger the detector: Prob (slit 2) = 5 percent. Now when we uncover both slits, creating two possible routes, we would expect to detect 10 percent of the photons. But no! Because we placed the detector in a dark fringe, we can run the experiment for hours and not see a single photon. That is,
Prob (1 or 2) = 0 percent � Prob (1) + Prob (2)
The mind-bending explanation that quantum physicists have found for this behavior is the principle of superposition, which says that wavelike events combine according to a probability amplitude rather than a probability. Mathematically, a probability amplitude is a complex number (that is, a number like 0.1 + 0.2i, where i denotes the square root of -1), not a positive real number. Thus two nonzero probability amplitudes (say, 0.1 + 0.2i and -0.1 - 0.2i) can add to zero, which is never true of classical probabilities.
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For many years it was thought that Heisenberg's principle was the mechanism responsible for enforcing complementarity. However, it was recently hypothesized that complementarity is more fundamental-that it should be possible to "mark" a particle's position in a way that does not alter its momentum. This leads to a class of experiments known as quantum erasers.
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Roughly 20 years ago, physicists Marlan O. Scully and Kai Dr�hl, then of the Max Planck Institute for Quantum Optics in Garching, Germany, and the University of New Mexico, shook the physics community with the idea of quantum erasure. Their logic was as follows: If the information providing the object's trajectory can be determined without significantly perturbing it, then the interference should disappear (in accordance with complementarity). But if that information is subsequently "erased," then the interference should return. One might even say that "interference equals ignorance" (of the particle's path).
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Our experiment uses polarization as a path marker. ... Now imagine that we repeat Young's experiment with many horizontally polarized photons. Behind the slits we insert two quarter-wave plates, one that turns the horizontally polarized photons into right-circularly polarized photons, and the other that makes them left-circularly polarized. Remarkably, the interference fringes will disappear and be replaced with a single swath of light, most intense in the middle. If we plot the distribution of photons on a graph we get a bell-shaped curve.
What happened to the interference? The photons no longer seem to behave like quantum coins but instead like boring, classical ones. The wave plates have now unambiguously correlated each slit with a particular polarization. Using a circular polarizer, we could measure the polarization and discover which slit each photon passed through. Note that we don't actually have to measure the polarization to destroy the interference pattern. It is enough that the which-path information is available to us; playing dumb will not restore the interference.
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To demonstrate quantum erasure, one must do more than find a way to mark which path the photon took; one must also show how to "erase" that information. We do this by inserting a linear horizontal polarizer between the quarter-wave plates and the detector. When we put the polarizer into place and repeat the experiment, instead of the bell-shaped curve of photon detections, we see an interference pattern.
But how can that be? We have already said that simply playing dumb does not bring back interference. Why does a horizontal polarizer bring it back? The answer is that it erases the which-path information. Remember that our horizontal polarizer filters either a right-circular or left-circular polarized photon into a horizontally polarized one, so that there is no longer any way to tell the difference between them. So once a photon has passed through the polarizer, it cannot be determined whether it came from slit 1 or slit 2. With the particle-like information removed, the photons are free to start acting like waves again
Similarly, if we place a linear vertical polarizer between the quarter-wave plates and the detector, we again erase the which-path information. However, in this case we observe a fringe pattern-commonly called anti-fringes-that is exactly out of phase with the pattern we saw through the horizontal polarizer. Anti-fringes exhibit a central minimum (dark stripe).
....
Does the uncertainty principle say anything about this experiment? No. Polarization and position are not complementary variables, so, as in the Scully-Englert-Walther proposal, Heisenberg's uncertainty principle does not apply here. So what is enforcing the complementarity principle?
The answer is quantum entanglement. ... (much more follows)
--------End of selected quotes--------
The article says a lot more than this. The main idea I get from it is that experimentally, it's _observability_ rather than _observation_ that changes the situation from entangled quantum to particular classical. This isn't a philosophical position, nor is it an explanation of the quantum mystery. All it does is to demonstrate that one cannot rely on classical meso-scale intuition when dealing with the real world of the very small.
I'm sure this doesn't clear up any mysteries, but it should go a little way to answering Bill's and Rick's questions.
Please don't ask me to expand on this. If you want to know more about it, please go to the original article or search for related material in the literature or on the Web.
Martin
