Wheeler's delayed choice experiment
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Wheeler's delayed choice experiment is a thought experiment proposed by John Archibald Wheeler in 1978[1]. Wheeler proposed a variation of the famous Double-slit experiment of quantum physics, one in which the method of detection can be changed after the photon passes the double slit, so as to delay the choice of whether to detect the path of the particle, or detect its interference with itself. Since the measurement itself seems to determine how the particle passes through the double slits, and thus its state as a wave or particle, Wheeler's thought experiment has been useful in trying to understand the strange properties of quantum particles.
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[edit] Introduction
Wheeler's experiment consisted of a standard double-slit experiment, except that the detector screen could be removed at the last moment, thereby directing light into two more remote telescopes, each one focused on one of the slits. This allowed a "delayed choice" of the observer, i.e. a choice made after the presumed photon would have cleared the midstream barrier containing two parallel slits. The two telescopes, behind the (removed) screen could presumably "see" a flash of light from one of the slits, and would detect by which path the photon traveled.
According to the results of the double slit experiment, if experimenters do something to learn which slit the photon goes through, they change the outcome of the experiment and the behavior of the photon. If the experimenters know which slit it goes through, the photon will behave as a particle. If they do not know which slit it goes through, the photon will behave as if it were a wave when it is given an opportunity to interfere with itself. The double-slit experiment is meant to observe phenomena that indicate whether light has a particle nature or a wave nature. The fundamental lesson of Wheeler's delayed choice experiment is that the result depends on whether the experiment is set up to detect waves or particles.
[edit] Working out implementation of the experiment
The conventional double-slit experiment shows that determining which path a particle takes destroys the interference pattern. To avoid the notion that the photon somehow "knows" when the "other" slit is open or closed (or is being watched), Wheeler suggested detecting which slit the photon used only long after it passed through the slits. Wheeler asked what happens when a single photon, presumably already determined to get detected as part of a two-slit interference pattern, suddenly gets detected in a path coming from only one slit. Does the interference pattern then disappear?
In terms of the traditional double-slit apparatus, the Wheeler delayed choice experiment is to put telescopes that are pointed directly at each of the two slits behind the removable detector wall. If the photon goes through telescope A it is argued that it must have come by way of slit A, and if it goes through telescope B it is argued that it must have come by way of slit B. Compare that assertion with Young's diagram of the propagation of light through double slits:
Wheeler planned a thought experiment in which two ways of observing an incoming photon could be used, and the decision of which one to use could be made after the photon had cleared the double-slit part of the apparatus. At that point a detection screen could either be raised or lowered. If the detection screen were to be put in place, Wheeler fully expected that the photon would interfere with itself and (if many more photons were permitted to follow it to the screen) would form part of a series of fringes due to interference. If, on the other hand, the detection screen were to be removed, then:
Sufficiently far beyond the region of the plate, the beams from upper and lower slits cease to overlap and become well separated. There place photodetectors. Let each have an opening such that it records with essentially 100 percent probability a quantum of energy arriving it its own beam, and with essentially zero probability a quantum arriving arriving in the other beam.
In that case, he argues, "one of the two counters will go off and signal in which beam — and therefore from which slit — the photon has arrived."[2]
[edit] Wheeler's astronomical experiment
In a response to the argument that at short distances interactions at the screen with slits in it might be compromised by "knowledge" of events that occur at the location of the detector screen, Wheeler is reported to have come up with a more elaborate thought experiment.[3] Wheeler suggests that one may imagine a more extraordinary scenario wherein the scale of the experiment is magnified to astronomical dimensions: a photon has originated from a star or even a distant galaxy, and its path is bent by an intervening galaxy, black hole, or other massive object, so that it could arrive at a detector on earth by either of two different paths.
The thought experiment assumes that the emitter of the photon is so positioned that the two paths are equal. If experimenters observe the single photon with a detector screen, e.g., a photographic plate or other imaging device (as in the original experiment), they should see it as part of an interference pattern (to be filled out by additional incoming photons), but if they instead use two telescopes focused to either side of the black hole they may expect to observe the photon only in one of them.
Some interpretations of Wheeler's thought experiment are premised on the belief that interference will indeed occur between the two images, and the crux of the experiment lies in determining whether identifying photons as coming from one referred image or the other will make a difference in experimental outcomes. Experimenters are already gathering light from one referred image (one pathway) by means of one telescope, and they can add light that has come by the other pathway by means of the other telescope.
If experimenters keep the two telescope images separate physically, then they ought not to expect any kind of interference fringes or other "spooky" behavior. And it is known that some photons must have reached earth via each pathway.
On the other hand, if experimenters project the images from the two telescopes onto the same spot on a detection screen and they move the images with respect to each other to change their phase relationship so that they can get cancellation in some areas and reinforcement in another area, they will then get an interference pattern, and will have demonstrated that this experiment is another version of the double-slit experiment.
There appears to be a problem, however. It may be claimed that one knows which photons have come by path A and which photons have come by path B, and that one has that knowledge because the photons have been physically fenced in by the tubes out of which the telescopes are constructed. However, once experimenters merge the two images on the detection screen, one can no longer know that a photon that lights up a certain spot on the detection screen has come through telescope A or through telescope B. So they have abandoned that information by mixing the two streams.
There is one more possibility, as indicated in a diagram above. If interference is actually thwarted, then photons should be found only at the position of the two primary maxima. Suppose that experimenters project the images onto two separate detection screens. That should give them a situation analogous to the one where they were viewing a distant light source with only one slit open. In the physics laboratory there are some diffraction effects due to light's having been put through a narrow opening, but not the broad band that is known as the interference fringe.
If interference between the images brought in via two telescopes does appear, experimenters ought to see dimmer images at the secondary, tertiary, etc. maxima predicted for interference effects. They should not expect to see the same range and clarity of secondary images (if any at all) with one telescope capped off. What occurs in this case is again a matter for empirical study to determine.
The idea behind some interpretations of the Wheeler experiment is that it might be possible to determine which side of a double-slit experiment a photon traveled through without destroying the interference pattern that occurs when the two versions of its probability wave interact on the detector screen of the typical double-slit experiment. Another view is that whether interference fringes are noted or not depends not on anything that happened between the distant star and earth. Instead, it depends entirely on what form the observation or the measurement of the photon or photons takes. Looking for a photon on one path or the other will produce the observation of one photon at a single point by a telescope aimed in a certain direction. Looking for an interference pattern by merging the beams coming through both paths will produce interference fringes.
[edit] Wheeler's own discussion of these problems
The thing that causes people to argue about when and how the photon learns that the experimental apparatus is in a certain configuration and then changes from wave to particle to fit the demands of the experiment's configuration is the assumption that a photon had some physical form before the astronomers observed it. Either it was a wave or a particle; either it went both ways around the galaxy or only one way. Actually, quantum phenomena are neither waves nor particles but are intrinsically undefined until the moment they are measured. In a sense, the British philosopher Bishop Berkeley was right when he asserted two centuries ago ‘to be is to be perceived’.[4]
As Heisenberg pointed out, being "observed" does not actually have to involve a human consciousness. What is actually required is that the "intrinsically undefined" photon, encounters a situation that results in its presence being manifested in the Universe in such a way that it is no longer "undefined." In Wheeler's experiment, the photon is "intrinsically undefined" until it either strikes some kind of photon detector at the end of one tube of a pair of telescopes, or it strikes the target screen and is absorbed or reflected there. It is in either case the same intrinsically undefined something that arrives at the telescope eyepiece or the target wall.
What happens if there is a full detector screen in place is well known. What happens to the interference pattern if the wall is replaced by a wall of telescopes can be easily imagined. What happens if the new wall of telescopes is gradually demolished, leaving fewer and fewer telescopes standing in a perfect vacuum? Will the telescopes that are placed at one of the maxima observed on the old detection screen receive hits? Or will only the centermost telescope(s) receive hits?[5]
If one assumes that something really transformative happens depending on whether "the path is known" or not, then one experimental outcome is to be expected. If, however, one assumes that the essential factor is when and how a photon is made to manifest itself, one will expect another experimental outcome.
Wheeler clearly states that the natures of various kinds of experimental apparatus themselves determine whether a trajectory for a photon is detected or whether a diffraction pattern is observed. Depending on the experiment, "we observe the consequences of interference or we find out in which of the two beams the quantum arrives; but, in conformity with the principle of complementarity, we cannot do both kinds of observation on the same photon."[6]
The following passage quotes Wheeler regarding the lessons of delayed-choice experiments:
The double slit experiment, like the other six idealized experiments (microscope, split beam, tilt-teeth, radiation pattern, one-photon polarization, and polarization of paired photons), imposes a choice between complementary modes of observation. In each experiment we have found a way to delay that choice of type of phenomenon to be looked for up to the very final stage of development of the phenomenon, whichever type we then fix upon. That delay makes no difference in the experimental predictions. On this score everything we find was foreshadowed in that solitary and pregnant sentence of Bohr, "...it...can make no difference, as regards observable effects obtainable by a definite experimental arrangement, whether our plans for constructing or handling the instruments are fixed beforehand or whether we prefer to postpone the completion of our planning until a later moment when the particle is already on its way from one instrument to another."
The idea, imported from the macro world, that a photon must have a definite location at any point of time and therefore must go through one or the other slit, is denied by quantum physics.
[edit] Actual Experiments
[edit] Most recent experiment
In 2007, the first "clean" experimental test of Wheeler's ideas was performed in France by the team of Alain Aspect, Philippe Grangier, Jean-François Roch et al.[7][8]
[edit] Earlier experiments
In 2000, Yoon-Ho Kim, et al, reported success in their delayed choice quantum eraser experiment, a variation that combines Wheeler's delayed choice experiment with a quantum eraser experiment, so that the choice to observe the photon or not observe the photon is done after it hits our detector.
Another Quantum eraser experiment was done in 2002 by S. P. Walborn, M. O. Terra Cunha, S. Padua, and C. H. Monken.
[edit] Future experiments
Researchers with access to radio telescopes originally designed for SETI research have pointed to the possibility, and have explicated the practical difficulties, of conducting the Wheeler experiment with actual stellar objects.[9]
[edit] External links
- Wheeler's Classic Delayed Choice Experiment by Ross Rhodes
- The Quantum Eraser by John G. Cramer -- extremely clear elucidation of this experiment and related subjects
[edit] Bibliography
- Vincent Jacques et al., Experimental Realization of Wheeler's Delayed-Choice Gedanken Experiment, Science Vol. 315. no. 5814, pp. 966 - 968 (2007). Preprint available at http://arxiv.org/PS_cache/quant-ph/pdf/0610/0610241v1.pdf
- John Archibald Wheeler, "The 'Past' and the 'Delayed-Choice Double-Slit Experiment'," pp 9-48, in A.R. Marlow, editor, Mathematical Foundations of Quantum Theory, Academic Press (1978)
- John Archibald Wheeler and Wojciech Hubert Zurek , Quantum Theory and Measurement (Princeton Series in Physics)
- John D. Barrow, Paul C. W. Davies, and Jr, Charles L. Harperm Science and Ultimate Reality: Quantum Theory, Cosmology, and Complexity (Cambridge University Press) 2004
[edit] References
- ^ Mathematical Foundations of Quantum Theory, edited by A.R. Marlow, Academic Press, 1978
- ^ John Archibald Wheeler, "The 'Past' and the 'Delayed-Choice' Double-Slit Experiment", in Mathematical Foundations of Quantum Theory, edited by A.R. Marlow, p. 13
- ^ Source for this experiment in Wheeler's own writing has not been traced yet. Dr. John Cramer indicates that Wheeler offered the idea in response to criticism of a proposed experiment on a smaller scale. (Personal communication.)
- ^ Interview reported in Scientific American, July 1992, p. 75
- ^ Wheeler anticipates this question in a slightly different context. See his article, "The 'Past' and the 'Delayed-Choice' Experiment," in Mathematical Foundations of Quantum Theory, edited by A.R. Marlow, p. 30.
- ^ See the article noted above, p. 31.
- ^ V. Jacques et al., Science 315, 966 (2007).
- ^ A thorough discussion by Jacques et al. in pdf form is available at http://fr.arxiv.org/abs/quant-ph/0610241.
- ^ Quantum Astronomy (IV): Cosmic-Scale Double-Slit Experiment
