Original Paper

Delayed Choice Quantum Eraser Experiment Explained

1999

http://arxiv.org/pdf/quant-ph/9903047v1.pdf

2014

http://journals.aps.org/prl/pdf/10.1103/PhysRevLett.112.180401

A delayed choice quantum eraser, first performed by Yoon-Ho Kim, R. Yu, S.P. Kulik, Y.H. Shih and Marlan O. Scully,^[1] and reported in early 1999, is an elaboration on thequantum eraser experiment that incorporates concepts considered in Wheeler's delayed choice experiment. The experiment was designed to investigate peculiar consequences of the well-known double slit experiment in quantum mechanics as well as the consequences of quantum entanglement.

The delayed choice quantum eraser experiment investigates a paradox. If a photon manifests itself as though it had come by a single path to the detector, then "common sense" (which Wheeler and others challenge) says it must have entered the double-slit device as a particle. If a photon manifests itself as though it had come by two indistinguishable paths, then it must have entered the double-slit device as a wave. If the experimental apparatus is changed while the photon is in mid‑flight, then the photon should reverse its original "decision" as to whether to be a wave or a particle. Wheeler pointed out that when these assumptions are applied to a device of interstellar dimensions, a last-minute decision made on earth on how to observe a photon could alter a decision made millions or even billions of years ago.

Delayed choice experiments have uniformly confirmed the seeming ability of measurements made on photons in the present to alter events occurring in the past. On the other hand, if a photon in flight is interpreted as being in a so-called "superposition of states," i.e. if it is interpreted as something that has the potentiality to manifest as a particle or wave, but during its time in flight is neither, then there is no time paradox. Recent experiments have supported the latter view.^[2][3]

Contents

  [hide] 

• 1 Introduction
  • 1.1 A simple quantum eraser experiment
  • 1.2 Delayed choice
• 2 The experiment of Kim et al. (2000)
  • 2.1 Significance
• 3 Implications
  • 3.1 Possibility of retrocausality
  • 3.2 Does delayed choice violate causality?
  • 3.3 Against consensus
• 4 Other delayed choice quantum eraser experiments
• 5 Notes
• 6 References
• 7 External links

Introduction[edit]

In the basic double slit experiment, a beam of light (usually from a laser) is directed perpendicularly towards a wall pierced by two parallel slit apertures. If a detection screen (anything from a sheet of white paper to a CCD) is put on the other side of the double slit wall, a pattern of light and dark fringes will be observed, a pattern that is called an interference pattern. Other atomic-scale entities such as electrons are found to exhibit the same behavior when fired toward a double slit.^[4] By decreasing the brightness of the source sufficiently, individual particles that form the interference pattern are detectable.^[5] The emergence of an interference pattern suggests that each particle passing through the slits interferes with itself, and that therefore in some sense the particles are going through both slits at once.^[6]:110 This is an idea that contradicts our everyday experience of discrete objects.

A well-known thought experiment, which played a vital role in the history of quantum mechanics (for example, see the discussion on Einstein's version of this experiment), demonstrated that if particle detectors are positioned at the slits, showing through which slit a photon goes, the interference pattern will disappear.^[4] This which-way experiment illustrates the complementarityprinciple that photons can behave as either particles or waves, but not both at the same time.^[7][8][9] However, technically feasible realizations of this experiment were not proposed until the 1970s.^[10]

Which-path information and the visibility of interference fringes are hence complementary quantities. In the double-slit experiment, conventional wisdom held that observing the particles inevitably disturbed them enough to destroy the interference pattern as a result of the Heisenberg uncertainty principle.

However, in 1982, Scully and Drühl found a loophole around this interpretation.^[11] They proposed a "quantum eraser" to obtain which-path information without scattering the particles or otherwise introducing uncontrolled phase factors to them. Rather than attempting to observe which photon was entering each slit (thus disturbing them), they proposed to "mark" them with information that, in principle at least, would allow the photons to be distinguished after passing through the slits. Lest there be any misunderstanding, the interference pattern does disappear when the photons are so marked. However, the interference pattern reappears if the which-path information is further manipulated after the marked photons have passed through the double slits to obscure the which-path markings. Since 1982, multiple experiments have demonstrated the validity of the so-called quantum "erasure."^[12][13][14]

A simple quantum eraser experiment[edit]

Figure 1. Experiment that shows delayed determination of photon path

A simple version of the quantum eraser can be described as follows: Rather than splitting one photon or its probability wave between two slits, the photon is subjected to a beam splitter. If one thinks in terms of a stream of photons being randomly directed by such a beam splitter to go down two paths that are kept from interaction, it would seem that no photon can then interfere with any other or with itself.

However, if the rate of photon production is reduced so that only one photon is entering the apparatus at any one time, it becomes impossible to understand the photon as only moving through one path, because when the path outputs are redirected so that they coincide on a common detector or detectors, interference phenomena appear.

In the two diagrams in Fig. 1, photons are emitted one at a time from a laser symbolized by a yellow star. They pass through a 50% beam splitter (green block) that reflects or transmits 1/2 of the photons. The reflected or transmitted photons travel along two possible paths depicted by the red or blue lines.

In the top diagram, the trajectories of the photons are clearly known: If a photon emerges from the top of the apparatus, it had to have come by way of the blue path, and if it emerges from the side of the apparatus, it had to have come by way of the red path.

In the bottom diagram, a second beam splitter is introduced at the top right. It can direct either beam toward either path. Thus, photons emerging from each exit port may have come by way of either path.

By introducing the second beam splitter, the path information has been "erased". Erasing the path information results in interference phenomena at detection screens positioned just beyond each exit port. What issues to the right side displays reinforcement, and what issues toward the top displays cancellation.^[15]

Delayed choice[edit]

Elementary precursors to current quantum eraser experiments such as the "simple quantum eraser" described above have straightforward classical-wave explanations. Indeed, it could be argued that there is nothing particularly quantum about this experiment.^[16] Nevertheless, Jordan has argued on the basis of the correspondence principle, that despite the existence of classical explanations, first-order interference experiments such as the above can be interpreted as true quantum erasers.^[17]

These precursors use single-photon interference. Versions of the quantum eraser using entangled photons, however, are intrinsically non-classical. Because of that, in order to avoid any possible ambiguity concerning the quantum versus classical interpretation, most experimenters have opted to use nonclassical entangled-photon light sources to demonstrate quantum erasers with no classical analog.

Furthermore, use of entangled photons enables the design and implementation of versions of the quantum eraser that are impossible to achieve with single-photon interference, such as the delayed choice quantum eraser which is the topic of this article.

The experiment of Kim et al. (2000)[edit]

Figure 2. Setup of the delayed choice quantum eraser experiment of Kim et al.

The experimental setup, described in detail in Kim et al.,^[1]is illustrated in Fig 2. An argon laser generates individual 351.1 nm photons that pass through a double slit apparatus (vertical black line in the upper left hand corner of the diagram).

An individual photon goes through one (or both) of the two slits. In the illustration, the photon paths are color-coded as red or light blue lines to indicate which slit the photon came through (red indicates slit A, light blue indicates slit B).

So far, the experiment is like a conventional two-slit experiment. However, after the slits, spontaneous parametric down conversion (SPDC) is used to prepare an entangled two-photon state. This is done by a nonlinear optical crystal BBO (beta barium borate) that converts the photon (from either slit) into two identical, orthogonally polarized entangled photons with 1/2 the frequency of the original photon. The paths followed by these orthogonally polarized photons are caused to diverge by the Glan-Thompson Prism.

One of these 702.2 nm photons, referred to as the "signal" photon (look at the red and light-blue lines going upwards from the Glan-Thompson prism) continues to the target detector called D₀. During an experiment, detector D₀ is scanned along its x-axis, its motions controlled by a step motor. A plot of "signal" photon counts detected by D₀ versus x can be examined to discover whether the cumulative signal forms an interference pattern.

The other entangled photon, referred to as the "idler" photon (look at the red and light-blue lines going downwards from the Glan-Thompson prism), is deflected by prism PS that sends it along divergent paths depending on whether it came from slit A or slit B.

Somewhat beyond the path split, the idler photons encounter beam splitters BS_a, BS_b, and BS_c that each have a 50% chance of allowing the idler photon to pass through and a 50% chance of causing it to be reflected. M_a and M_b are mirrors.

Figure 3. Plots of joint detection rates between D₀ and D₁, D₂, D₃, D₄ (R₀₁, R₀₂, R₀₃,R₀₄) R₀₄ has no counterpart in the Kim article, and is supplied according to their verbal description.

Figure 4. Simulated recordings of photons jointly detected between D₀ and D₁, D₂, D₃, D₄(R₀₁, R₀₂, R₀₃, R₀₄)

The beam splitters and mirrors direct the idler photons towards detectors labeled D₁, D₂, D₃ and D₄. Note that:

• If an idler photon is recorded at detector D₃, it can only have come from slit B.
• If an idler photon is recorded at detector D₄, it can only have come from slit A.
• If an idler photon is detected at detector D₁ or D₂, it might have come from slit A or slit B.
• The optical path length measured from slit to D₁, D₂, D₃, and D₄ is 2.5 m longer than the optical path length from slit to D₀. This means that any information that one can learn from an idler photon must be approximately 8 ns later than what one can learn from its entangled signal photon.

Detection of the idler photon by D₃ or D₄ provides delayed "which-path information" indicating whether the signal photon with which it is entangled had gone through slit A or B. On the other hand, detection of the idler photon by D₁or D₂ provides a delayed indication that such information is not available for its entangled signal photon. Insofar as which-path information had earlier potentially been available from the idler photon, it is said that the information has been subjected to a "delayed erasure".

By using a coincidence counter, the experimenters were able to isolate the entangled signal from photo-noise, recording only events where both signal and idler photons were detected (after compensating for the 8 ns delay). Refer to Figs 3 and 4.

• When the experimenters looked at the signal photons whose entangled idlers were detected at D₁ or D₂, they detected interference patterns.
• However, when they looked at the signal photons whose entangled idlers were detected at D₃ or D₄, they detected simple diffraction patterns with no interference.

Significance[edit]

This result is similar to that of the double-slit experiment since interference is observed when it is not known which slit the photon went through, while no interference is observed when the path is known.

Figure 5. Raw results at D₀ (with ambient illumination removed) will not reveal interference, which has important implications in regards to the possibility of using delayed choice quantum eraser results to violate causality.

However, what makes this experiment possibly astonishing is that, unlike in the classic double-slit experiment, the choice of whether to preserve or erase the which-path information of the idler was not made until 8 ns after the position of the signal photon had already been measured by D₀.

Detection of signal photons at D₀ does not directly yield any which-path information. Detection of idler photons at D₃ or D₄, which provide which-path information, means that no interference pattern can be observed in the jointly detected subset of signal photons at D₀. Likewise, detection of idler photons at D₁ or D₂, which do not provide which-path information, means that interference patterns can be observed in the jointly detected subset of signal photons at D₀.

In other words, even though an idler photon is not observed until long after its entangled signal photon arrives at D₀ due to the shorter optical path for the latter, interference atD₀ is determined by whether a signal photon's entangled idler photon is detected at a detector that preserves its which-path information (D₃ or D₄), or at a detector that erases its which-path information (D₁ or D₂).

Some have interpreted this result to mean that the delayed choice to observe or not observe the path of the idler photon changes the outcome of an event in the past. However, the consensus contemporary position is that retrocausality is not necessary to explain the phenomenon of delayed choice.^[18] Note in particular that an interference pattern may only be pulled out for observation after the idlers have been detected (i.e., at D₁ or D₂).

The total pattern of all signal photons at D₀, whose entangled idlers went to multiple different detectors, will never show interference regardless of what happens to the idler photons.^[19] One can get an idea of how this works by looking at the graphs of R₀₁, R₀₂, R₀₃, and R₀₄, and observing that the peaks of R₀₁ line up with the troughs of R₀₂ (i.e. a π phase shift exists between the two interference fringes). R₀₃ shows a single maximum, and R₀₄, which is experimentally identical to R₀₃ will show equivalent results. The entangled photons, as filtered with the help of the coincidence counter, are simulated in Fig. 5 to give a visual impression of the evidence available from the experiment. In D₀, the sum of all the correlated counts will not show interference. If all the photons that arrive at D₀ were to be plotted on one graph, one would see only a bright central band.

Implications[edit]

Possibility of retrocausality[edit]

Delayed choice experiments raise questions about time and time sequences, and thereby bring our usual ideas of time and causal sequence into question.^{[note 1]} If events at D₁, D₂, D₃, D₄ determine outcomes at D₀, then effect seems to precede cause. If the idler light paths were greatly extended so that a year goes by before a photon shows up at D₁, D₂, D₃, or D₄, then when a photon shows up in one of these detectors, it would cause a signal photon to have shown up in a certain mode a year earlier. Alternatively, knowledge of the future fate of the idler photon would determine the activity of the signal photon in its own present. Neither of these ideas conforms to the usual human expectation of causality. Retrocausality has not won over more than a handful of partisans as a rational explanation of the findings of delayed choice experiments.

Does delayed choice violate causality?[edit]

Experiments that involve entanglement exhibit phenomena that may make some people doubt their ordinary ideas about causal sequence. In the delayed choice quantum eraser, an interference pattern will form on D₀ even if which-path data pertinent to photons that form it are only erased later in time than the signal photons hit that primary detector. Not only that feature of the experiment is puzzling; D₀ can, in principle at least, be on one side of the universe, and the other four detectors can be "on the other side of the universe" to each other.^[20]:197f

However, the interference pattern can only be seen retroactively once the idler photons have been detected and the experimenter has had information about them available, with the interference pattern being seen when the experimenter looks at particular subsets of signal photons that were matched with idlers that went to particular detectors.^[20]:197

The total pattern of signal photons at the primary detector never shows interference (see Fig. 5), so it is not possible to deduce what will happen to the idler photons by observing the signal photons alone. The delayed choice quantum eraser does not communicate information in a retro-causal manner because it takes another signal, one which must arrive via a process that can go no faster than the speed of light, to sort the superimposed data in the signal photons into four streams that reflect the states of the idler photons at their four distinct detection screens.^{[note 2][note 3]}

In fact, a theorem proved by Phillippe Eberhard shows that if the accepted equations of relativistic quantum field theory are correct, it should never be possible to experimentally violate causality using quantum effects.^[21] (See reference^[22] for a treatment emphasizing the role of conditional probabilities.)

In addition to challenging our common sense ideas of temporal sequence in cause and effect relationships, this experiment is among those that strongly attack our ideas about locality, the idea that things cannot interact unless they are in contact, if not by being in direct physical contact then at least by interaction through magnetic or other such field phenomena.^[20]:199

Against consensus[edit]

Despite Eberhard's proof, some physicists have speculated that these experiments might be changed in a way that would be consistent with previous experiments, yet which could allow for experimental causality violations.^[23][24][25]

Other delayed choice quantum eraser experiments[edit]

This section requires expansion.(February 2014)

Many refinements and extensions of Kim et al.'s delayed choice quantum eraser have been performed or proposed. Only a small sampling of reports and proposals are given here:

Scarcelli et al. (2007) reported on a delayed-choice quantum eraser experiment based on a two-photon imaging scheme. After detecting a photon which passed through a double-slit, a random delayed choice was made to erase or not erase the which-path information by the measurement of its distant entangled twin; the particle-like and wave-like behavior of the photon were then recorded simultaneously and respectively by only one set of joint detectors.^[26]

Peruzzo et al. (2012) have reported on a quantum delayed choice experiment, based on a quantum controlled beam-splitter, in which particle and wave behaviors were investigated simultaneously. The quantum nature of the photon's behavior was tested via a Bell inequality, which replaced the delayed choice of the observer.^[27]

The construction of solid state electronic Mach-Zehnder interferometers (MZI) has led to proposals to use them in electronic versions of quantum eraser experiments. This would be achieved by Coulomb coupling to a second electronic MZI acting as a detector.^[28]

Entangled pairs of neutral kaons have also been examined and found suitable for investigations using quantum marking and quantum erasure techniques.^[29]

Notes[edit]

1. Jump up^ Stanford Encyclopedia of Philosophy, "More recently, the Bell type experiments have been interpreted by some as if quantum events could be connected in such a way that the past light cone might be accessible under non-local interaction; not only in the sense of action at a distance but as backward causation. One of the most enticing experiments of this kind is the Delayed Choice Quantum Eraser designed by Yoon-Ho Kim et. al (2000). It is a rather complicated construction. It is set up to measure correlated pairs of photons, which are in an entangled state, so that one of the two photons is detected 8 nanoseconds before its partner. The results of the experiment are quite amazing. They seem to indicate that the behavior of the photons detected these 8 nanoseconds before their partners is determined by how the partners will be detected. Indeed it might be tempting to interpret these results as an example of the future causing the past. The result is, however, in accordance with the predictions of quantum mechanics."http://plato.stanford.edu/entries/causation-backwards/
2. Jump up^ "... the future measurements do not in any way change the data you collected today. But the future measurements do influence the kinds of details you can invoke when you subsequently describe what happened today. Before you have the results of the idler photon measurements, you really can't say anything at all about the which-path history of any given signal photon. However, once you have the results, you conclude that signal photons whose idler partners were successfully used to ascertain which-path information can be described as having ... traveled either left or right. You also conclude that signal photons whose idler partners had their which-path information erased cannot be described as having ... definitely gone one way or the other (a conclusion you can convincingly confirm by using the newly acquired idler photon data to expose the previously hidden interference pattern among this latter class of signal photons). We thus see that the future helps shape the story you tell of the past." — Brian Greene, The Fabric of the Cosmos, pp 198–199
3. Jump up^ The Kim paper says:
   P. 1f: The experiment is designed in such a way that L0, the optical distance between atoms A, B and detector D₀, is much shorter than Li, which is the optical distance between atoms A, B and detectors D₁, D₂, D₃, and D₄, respectively. So that D₀ will be triggered much earlier by photon 1. After the registration of photon 1, we look at these "delayed" detection events of D₁, D₂, D₃, and D₄ which have constant time delays, i ≃ (Li − L0)/c, relative to the triggering time of D₀. P.2: In this experiment the optical delay (Li − L0) is chosen to be ≃ 2.5m, where L0 is the optical distance between the output surface of BBO and detector D₀, and Li is the optical distance between the output surface of the BBO and detectors D₁, D₂, D₃, and D₄, respectively. This means that any information one can learn from photon 2 must be at least 8ns later than what one has learned from the registration of photon 1. Compared to the 1ns response time of the detectors, 2.5m delay is good enough for a "delayed erasure". P. 3: The which-path or both-path information of a quantum can be erased or marked by its entangled twin even after the registration of the quantum. P. 2:After the registration of photon 1, we look at these "delayed" detection events of D₁, D₂, D₃, and D₄ which have constant time delays, i ≃ (Li − L0)/c, relative to the triggering time of D₀. It is easy to see these "joint detection" events must have resulted from the same photon pair. (Emphasis added. This is the point at which what is going on at D₀ can be figured out.)

References[edit]

1. ^ Jump up to:^a b Kim, Yoon-Ho; R. Yu, S.P. Kulik, Y.H. Shih and Marlan Scully (2000). "A Delayed Choice Quantum Eraser". Physical Review Letters 84: 1–5. arXiv:quant-ph/9903047. Bibcode:2000PhRvL..84....1K. doi:10.1103/PhysRevLett.84.1.
2. Jump up^ Ma, Zeilinger, et al., "Quantum erasure with causally disconnected choice." See: http://www.pnas.org/content/110/4/1221 "Our results demonstrate that the viewpoint that the system photon behaves either definitely as a wave or definitely as a particle would require faster-than-light communication. Because this would be in strong tension with the special theory of relativity, we believe that such a viewpoint should be given up entirely. "
3. Jump up^ Peruzzo, et al., "A quantum delayed choice experiment," arXiv:1205.4926v2 [quant-ph] 28 Jun 2012. This experiment uses Bell inequalities to replace the delayed choice devices, but it achieves the same experimental purpose in an elegant and convincing way.
4. ^ Jump up to:^a b Feynman, Richard P.; Robert B. Leighton; Matthew Sands (1965). The Feynman Lectures on Physics, Vol. 3. US: Addison-Wesley. pp. 1.1–1.8. ISBN 0-201-02118-8.
5. Jump up^ Donati, O, Missiroli, G F, Pozzi, G (1973). An Experiment on Electron Interference. American Journal of Physics 41:639–644doi:10.1119/1.1987321
6. Jump up^ Greene, Brian (2003). The Elegant Universe. Random House, Inc. ISBN 0-375-70811-1.
7. Jump up^ Harrison, David (2002). "Complementarity and the Copenhagen Interpretation of Quantum Mechanics". UPSCALE. Dept. of Physics, U. of Toronto. Retrieved 2008-06-21.
8. Jump up^ Cassidy, David (2008). "Quantum Mechanics 1925–1927: Triumph of the Copenhagen Interpretation". Werner Heisenberg. American Institute of Physics. Retrieved 2008-06-21.
9. Jump up^ Boscá Díaz-Pintado, María C. (29–31 March 2007). "Updating the wave-particle duality". 15th UK and European Meeting on the Foundations of Physics. Leeds, UK. Retrieved 2008-06-21.
10. Jump up^ Bartell, L. (1980). "Complementarity in the double-slit experiment: On simple realizable systems for observing intermediate particle-wave behavior". Physical Review D 21 (6): 1698. doi:10.1103/PhysRevD.21.1698.
11. Jump up^ Scully, Marlan O.; Kai Drühl (1982). "Quantum eraser: A proposed photon correlation experiment concerning observation and "delayed choice" in quantum mechanics". Physical Review A 25 (4): 2208–2213. Bibcode:1982PhRvA..25.2208S.doi:10.1103/PhysRevA.25.2208.
12. Jump up^ Zajonc, A. G.; Wang, L. J.; Zou, X. Y.; Mandel, L. (1991). "Quantum eraser". Nature 353: 507–508.Bibcode:1991Natur.353..507Z. doi:10.1038/353507b0.
13. Jump up^ Herzog, T. J.; Kwiat, P. G.; Weinfurter, H.; Zeilinger, A. (1995). "Complementarity and the quantum eraser". Physical Review Letters 75 (17): 3034–3037. Bibcode:1995PhRvL..75.3034H. doi:10.1103/PhysRevLett.75.3034. Retrieved 13 February 2014.
14. Jump up^ Walborn, S. P.; et al. (2002). "Double-Slit Quantum Eraser". Phys. Rev. A 65 (3): 033818. arXiv:quant-ph/0106078.Bibcode:2002PhRvA..65c3818W. doi:10.1103/PhysRevA.65.033818.
15. Jump up^ Jacques, Vincent; Wu, E; Grosshans, Frédéric; Treussart, François; Grangier, Philippe; Aspect, Alain; Rochl, Jean-François (2007). "Experimental Realization of Wheeler's Delayed-Choice Gedanken Experiment". Science 315 (5814): pp. 966–968.arXiv:quant-ph/0610241. Bibcode:2007Sci...315..966J. doi:10.1126/science.1136303. PMID 17303748.
16. Jump up^ Chiao, R. Y.; P. G. Kwiat; Steinberg, A. M. (1995). "Quantum non-locality in two-photon experiments at Berkeley". Quantum and Semiclassical Optics: Journal of the European Optical Society Part B 7 (3): 259. Retrieved 13 February 2014.
17. Jump up^ Jordan, T. F. (1993). "Disppearance and reappearance of macroscopic quantum interference". Physical Review A 48 (3): 2449–2450. Bibcode:1993PhRvA..48.2449J. doi:10.1103/PhysRevA.48.2449.
18. Jump up^ Ionicioiu, R.; Terno, D. R. (2011). "Proposal for a quantum delayed-choice experiment". Phys. Rev. Lett. 107: 230406.
19. Jump up^ Greene, Brian (2004). The Fabric of the Cosmos. Alfred A. Knopf. p. 198. ISBN 0-375-41288-3.
20. ^ Jump up to:^a b c Greene, Brian (2004). The Fabric of the Cosmos. Alfred A. Knopf. ISBN 0-375-41288-3.
21. Jump up^ Eberhard, Phillippe H.; Ronald R. Ross (1989). "Quantum field theory cannot provide faster-than-light communication".Foundations of Physics Letters 2 (2): p. 127–149. Bibcode:1989FoPhL...2..127E. doi:10.1007/BF00696109.
22. Jump up^ Bram Gaasbeek. Demystifying the Delayed Choice Experiments. arXiv preprint, 22 July 2010.
23. Jump up^ John G. Cramer. NASA Goes FTL - Part 2: Cracks in Nature's FTL Armor. "Alternate View" column, Analog Science Fiction and Fact, February 1995.
24. Jump up^ Paul J. Werbos, Ludmila Dolmatova. The Backwards-Time Interpretation of Quantum Mechanics - Revisited With Experiment.arXiv preprint, 7 August 2000.
25. Jump up^ John Cramer, "An Experimental Test of Signaling using Quantum Nonlocality" has links to several reports from the University of Washington researchers in his group. See: http://faculty.washington.edu/jcramer/NLS/NL_signal.htm
26. Jump up^ Scarcelli, G.; Zhou, Y.; Shih, Y. (2007). "Random delayed-choice quantum eraser via two-photon imaging". The European Physical Journal D 44 (1): 167–173. Bibcode:2007EPJD...44..167S. doi:10.1140/epjd/e2007-00164-y.
27. Jump up^ Peruzzo, A.; Shadbolt, P.; Brunner, N.; Popescu, S.; O'Brien, J. L. (2012). "A quantum delayed-choice experiment". Science338: 634–637. arXiv:1205.4926. Bibcode:2012Sci...338..634P. doi:10.1126/science.1226719. Retrieved 14 February 2014.
28. Jump up^ Dressel, J.; Choi, Y.; Jordan, A. N. (2012). "Measuring which-path information with coupled electronic Mach-Zehnder interferometers". Physical Review B 85 (4): 045320. Retrieved 14 February 2014.
29. Jump up^ Bramon, A.; Garbarino, G.; Hiesmayr, B. C. (2004). "Quantum marking and quantum erasure for neutral kaons". Physical review letters 92 (2): 020405. Retrieved 14 February 2014.

External links[edit]

• presentation of the experiment
• basic delayed choice experiment
• delayed choice quantum eraser
• the notebook of philosophy and physics
• Comprehensive experimental test of quantum erasure, Alexei Trifonov, Gunnar Bjork, Jonas Soderholm, and Tedros Tsegaye (doi:10.1140/epjd/e20020030)
• A non-local quantum eraser (June 2012; 12 authors, including Anton Zeilinger)

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In quantum mechanics, the quantum eraser experiment is an interferometer experiment that demonstrates several fundamental aspects of quantum mechanics, including quantum entanglement and complementarity.

The double-slit quantum eraser experiment described in this article has three stages:^[1]

1. First, the experimenter reproduces the interference pattern of Young's double-slit experiment by shining photons at the double-slit interferometer and checking for an interference pattern at the detection screen.
2. Next, the experimenter marks through which slit each photon went, without disturbing its wavefunction, and demonstrates that thereafter the interference pattern is destroyed. This stage indicates that it is the existence of the "which-path" information that causes the destruction of the interference pattern.
3. Third, the "which-path" information is "erased," whereupon the interference pattern is recovered. (Rather than removing or reversing any changes introduced into the photon or its path, these experiments typically produce another change that obscures the markings earlier produced.)

A key result is that it does not matter whether the erasure procedure is done before or after the detection of the photons.^[1][2]

Quantum erasure technology can be used to increase the resolution of advanced microscope.^[3]

Contents

  [hide] 

• 1 Introduction
• 2 The experiment
• 3 See also
• 4 References
• 5 External links

Introduction[edit]

The quantum eraser experiment described in this article is a variation of Thomas Young's classic double-slit experiment. It establishes that when action is taken to determine which slit a photon has passed through, the photon cannot interfere with itself. When a stream of photons is marked in this way, then the interference fringes characteristic of the Young experiment will not be seen. The experiment described in this article is capable of creating situations in which a photon that has been "marked" to reveal through which slit it has passed can later be "unmarked." A photon that has been "marked" cannot interfere with itself and will not produce fringe patterns, but a photon that has been "marked" and then "unmarked" can thereafter interfere with itself and will cooperate in producing the fringes characteristic of Young's experiment.^[1]

This experiment involves an apparatus with two main sections. After two entangled photons are created, each is directed into its own section of the apparatus. It then becomes clear that anything done to learn the path of the entangled partner of the photon being examined in the double-slit part of the apparatus will influence the second photon, and vice-versa. The advantage of manipulating the entangled partners of the photons in the double-slit part of the experimental apparatus is that experimenters can destroy or restore the interference pattern in the latter without changing anything in that part of the apparatus. Experimenters do so by manipulating the entangled photon, and they can do so before or after its partner has passed through the slits and other elements of experimental apparatus between the photon emitter and the detection screen. So, under conditions where the double-slit part of the experiment has been set up to prevent the appearance of interference phenomena (because there is definitive "which path" information present), the quantum eraser can be used to effectively erase that information. In doing so, the experimenter restores interference without altering the double-slit part of the experimental apparatus.^[1]

A variation of this experiment, delayed choice quantum eraser, allows the decision whether to measure or destroy the "which path" information to be delayed until after the entangled particle partner (the one going through the slits) has either interfered with itself or not.^[4] Doing so appears to have the bizarre effect of causing the outcome of an event after the event has already occurred. In other words, something that happens at time t apparently reaches back to some time t - 1 and acts as a determining causal factor at that earlier time.

The experiment[edit]

Figure 1. Crossed polarizations prevent interference fringes

First, a photon is shot through a specialized nonlinear optical device: a beta barium borate (BBO) crystal. This crystal converts the single photon into two entangled photons of lower frequency, a process known as spontaneous parametric down-conversion (SPDC). These entangled photons follow separate paths. One photon goes directly to a detector, while the second photon passes through the double-slit mask to a second detector. Both detectors are connected to a coincidence circuit, ensuring that only entangled photon pairs are counted. A stepper motor moves the second detector to scan across the target area, producing an intensity map. This configuration yields the familiar interference pattern.

Figure 2. Introduction of polarizer in upper path restores interference fringes below

Next, a circular polarizer is placed in front of each slit in the double-slit mask, producing clockwise circular polarization in light passing through one slit, and counter-clockwise circular polarization in the other slit (see Figure 1). This polarization is measured at the detector, thus "marking" the photons and destroying the interference pattern (see Fresnel–Arago laws).

Finally, a linear polarizer is introduced in the path of the first photon of the entangled pair, giving this photon a diagonal polarization (see Figure 2). Entanglement ensures a complementary diagonal polarization in its partner, which passes through the double-slit mask. This alters the effect of the circular polarizers: each will produce a mix of clockwise and counter-clockwise polarized light. Thus the second detector can no longer determine which path was taken, and the interference fringes are restored.

A double slit with rotating polarizers can also be accounted for by considering the light to be a classical wave.^[5] However this experiment uses entangled photons, which are not compatible with classical mechanics.

See also[edit]

• Delayed choice quantum eraser
• Wheeler's delayed choice experiment
• Marlan Scully

References[edit]

1. ^ Jump up to:^a b c d Walborn, S. P.; et al. (2002). "Double-Slit Quantum Eraser". Phys. Rev. A 65 (3): 033818. arXiv:quant-ph/0106078.Bibcode:2002PhRvA..65c3818W. doi:10.1103/PhysRevA.65.033818.
2. Jump up^ Englert, Berthold-Georg (1999). "REMARKS ON SOME BASIC ISSUES IN QUANTUM MECHANICS". Zeitschrift für Naturforschung 54 (1): 11–32.
3. Jump up^ Aharonov, Yakir; Zubairy, M. Suhail (2005). "Time and the Quantum: Erasing the Past and Impacting the Future". Science 307(5711): pp. 875–879. Bibcode:2005Sci...307..875A. doi:10.1126/science.1107787. PMID 15705840.
4. Jump up^ Kim, Yoon-Ho; R. Yu, S.P. Kulik, Y.H. Shih and Marlan Scully (2000). "A Delayed Choice Quantum Eraser". Physical Review Letters 84: 1–5. arXiv:quant-ph/9903047. Bibcode:2000PhRvL..84....1K. doi:10.1103/PhysRevLett.84.1.
5. Jump up^ Chiao, R. Y.; P. G. Kwiat; Steinberg, A. M. (1995). "Quantum non-locality in two-photon experiments at Berkeley". Quantum and Semiclassical Optics: Journal of the European Optical Society Part B 7 (3): 6. Retrieved 13 February 2014.

External links[edit]

• A more technical analysis of the quantum eraser experiment
• A Scientific American article: A Do-It-Yourself Quantum Eraser - note: SciAm online subscribers only^{[dead link]}
  • but here they let you see the images for free.
• Demystifying the Delayed Choice Experiments

-------------------------------------

Wheeler's delayed choice experiment is actually several thought experiments in quantum physics, proposed by John Archibald Wheeler, with the most prominent among them appearing in 1978 and 1984.^[1]These experiments are attempts to decide whether light somehow "senses" the experimental apparatus in thedouble-slit experiment it will travel through and adjusts its behavior to fit by assuming the appropriate determinate state for it, or whether light remains in an indeterminate state, neither wave nor particle, and responds to the "questions" asked of it by responding in either a wave-consistent manner or a particle-consistent manner depending on the experimental arrangements that ask these "questions."^[2]

The common intention of these several types of experiments is to first do something that some interpretations of theory say would make each photon "decide" whether it was going to behave as a particle or behave as a wave, and then, before the photon had time to reach the detection device, create another change in the system that would make it seem that the photon had "chosen" to behave in the opposite way. Some interpreters of these experiments contend that a photon either is a wave or is a particle, and that it cannot be both at the same time. Wheeler's intent was to investigate the time-related conditions under which a photon makes this transition between alleged states of being. His work has been productive of many revealing experiments. He may not have anticipated the possibility that other researchers would tend toward the conclusion that a photon retains both its "wave nature" and "particle nature" until the time it ends its life, e.g., by being absorbed by an electron which acquires its energy and therefore rises to a higher orbit in its atom. However, he himself seems to be very clear on this point. He says:

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."^[3]

This line of experimentation proved very difficult to carry out when it was first conceived. Nevertheless, it has proven very valuable over the years since it has led researchers to provide "increasingly sophisticated demonstrations of the wave–particle duality of single quanta."^[4] [5] As one experimenter explains, "Wave and particle behavior can coexist simultaneously." ^[6]

Contents

  [hide] 

• 1 Introduction
• 2 Simple interferometer
• 3 Cosmic interferometer
• 4 Double-slit version
• 5 Experimental details
  • 5.1 Interferometer in the lab
  • 5.2 Interferometer in the cosmos
  • 5.3 Double-slits in lab and cosmos
  • 5.4 Current experiments of interest
• 6 Conclusions
• 7 Bibliography
• 8 References
• 9 External links

Introduction[edit]

"Wheeler's delayed choice experiment" refers to a series of thought experiments in quantum physics, the first being proposed by him in 1978. Another prominent version was proposed in 1983. All of these experiments try to get at the same fundamental issues in quantum physics. Many of them are discussed in Wheeler's 1978 article, "The 'Past' and the 'Delayed-Choice' Double-Slit Experiment", which has been reproduced in A. R. Marlow's Mathematical Foundations of Quantum Theory, pp. 9–48.

According to the complementarity principle, a photon can manifest properties of a particle or of a wave, but not both at the same time. What characteristic is manifested depends on whether experimenters use a device intended to observe particles or to observe waves.^[7] When this statement is applied very strictly, one could argue that by determining the detector type one could force the photon to become manifest only as a particle or only as a wave. Detection of a photon is a destructive process because a photon can never be seen in flight. When a photon is detected it "appears" in the consequences of its demise, e.g., by being absorbed by an electron in a photomultiplier that accepts its energy which is then used to trigger the cascade of events that produces a "click" from that device. A photon always appears at some highly localized point in space and time. In the apparatuses that detect photons, the locations on its detection screen that indicate reception of the photon give an indication of whether or not it was manifesting its wave nature during its flight from photon source to the detection device. Therefore it is commonly said that in a double-slit experiment a photon exhibits its wave nature as it is passing through both of the slits and manifests its particle nature when it appears on the detection screen as a highly localized scintillation rather than a dim wash of illumination across the screen.

Given the interpretation of quantum physics that says a photon is either in its guise as a wave or in its guise as a particle, the question arises: When does the photon decide whether it is going to travel as a wave or as a particle? Suppose that a traditional double-slit experiment is prepared so that either of the slits can be blocked. If both slits are open and a series of photons are emitted by the laser then an interference pattern will quickly emerge on the detection screen. The interference pattern can only be explained as a consequence of wave phenomena, so experimenters can conclude that each photon "decides" to travel as a wave as soon as it is emitted. If only one slit is available then there will be no interference pattern, so experimenters may conclude that each photon "decides" to travel as a particle as soon as it is emitted.

Simple interferometer[edit]

One way to investigate the question of when a photon decides whether to act as a wave or a particle in an experiment is to use the interferometer method. Here is a simple schematic diagram of an interferometer in two configurations:

Open and Closed

If a single photon is emitted into the entry port of the apparatus at the lower-left corner, it immediately encounters a beam-splitter. Because of the equal probabilities for transmission or reflection the photon will continue straight ahead, be reflected by the mirror at the lower-right corner, and be detected by the detector at the top of the apparatus, or it will be reflected by the beam-splitter, strike the mirror in the upper-left corner, and emerge into the detector at the right edge of the apparatus. Observing that photons show up in equal numbers at the two detectors, experimenters generally say that each photon has behaved as a particle from the time of its emission to the time of its detection, has traveled by either one path or the other, and further affirm that its wave nature has not been exhibited.

If the apparatus is changed so that a second beam splitter is placed in the upper-right corner, then the two detectors will exhibit interference effects. Experimenters must explain these phenomena as consequences of the wave nature of light. They may affirm that each photon must have traveled by both paths as a wave else that photon could not have interfered with itself.

Since nothing else has changed from experimental configuration to experimental configuration, and since in the first case the photon is said to "decide" to travel as a particle and in the second case it is said to "decide" to travel as a wave, Wheeler wanted to know whether, experimentally, a time could be determined at which the photon made its "decision." Would it be possible to let a photon pass through the region of the first beam-splitter while there was no beam-splitter in the second position, thus causing it to "decide" to travel, and then quickly let the second beam-splitter pop up into its path? Having presumably traveled as a particle up to that moment, would the beam splitter let it pass through and manifest itself as would a particle were that second beam splitter not to be there? Or, would it behave as though the second beam-splitter had always been there? Would it manifest interference effects? And if it did manifest interference effects then to have done so it must have gone back in time and changed its decision about traveling as a particle to traveling as a wave. Note that Wheeler wanted to investigate several hypothetical statements by obtaining objective data.

Albert Einstein did not like these possible consequences of quantum mechanics.^[8] However, when experiments were finally devised that permitted both the double-slit version and the interferometer version of the experiment, it was conclusively shown that a photon could begin its life in an experimental configuration that would call for it to demonstrate its particle nature, end up in an experimental configuration that would call for it to demonstrate its wave nature, and that in these experiments it would always show its wave characteristics by interfering with itself. Furthermore, if the experiment was begun with the second beam-splitter in place but it was removed while the photon was in flight, then the photon would inevitably show up in a detector and not show any sign of interference effects. So the presence or absence of the second beam-splitter would always determine "wave or particle" manifestation. Many experimenters reached an interpretation of the experimental results that said that the change in final conditions wouldretroactively determine what the photon had "decided" to be as it was entering the first beam-splitter. As mentioned above, Wheeler rejected this interpretation.

Cosmic interferometer[edit]

Double quasar known as QSO 0957+561, also known as the "Twin Quasar", which lies just under 9 billion light-years from Earth. ^[9]

Wheeler's plan

In an attempt to avoid destroying normal ideas of cause and effect, some theoreticians suggested that information about whether there was or was not a second beam-splitter installed could somehow be transmitted from the end point of the experimental device back to the photon as it was just entering that experimental device, thus permitting it to make the proper "decision." So Wheeler proposed a cosmic version of his experiment. In that thought experiment he asks what would happen if a quasar or other galaxy millions or billions of light years away from earth passes its light around an intervening galaxy or cluster of galaxies that would act as a gravitational lens. A photon on a dead heading toward Earth would encounter the distortion of space in the vicinity of the intervening massive galaxy. At that point it would have to "decide" whether to go by one way around the lensing galaxy, traveling as a particle, or go both ways around by traveling as a wave. When the photon arrived at an astronomical observatory at earth, what would happen? Due to the gravitational lensing, telescopes in the observatory see two images of the same quasar, one to the left of the lensing galaxy and one to the right of it. If the photon has traveled as a particle and comes into the barrel of a telescope aimed at the left quasar image it must have decided to travel as a particle all those millions of years, or so say some experimenters. That telescope is pointing the wrong way to pick up anything from the other quasar image. If the photon traveled as a particle and went the other way around, then it will only be picked up by the telescope pointing at the right "quasar." So millions of years ago the photon decided to travel in its guise of particle and randomly chose the other path. But the experimenters now decide to try something else. They direct the output of the two telescopes into a beam-splitter, as diagrammed, and discover that one output is very bright (indicating positive interference) and that the other output is essentially zero, indicating that the incoming wavefunction pairs have self-cancelled.

Paths separated and paths converged via beam-splitter

Wheeler then plays the devil's advocate and suggests that perhaps for those experimental results to be obtained would mean that at the instant astronomers inserted their beam-splitter, photons that had left the quasar some millions of years ago retroactively decided to travel as waves, and that when the astronomers decided to pull their beam splitter out again that decision was telegraphed back through time to photons that were leaving some millions of years plus some minutes in the past, so that photons retroactively decided to travel as particles.

Several ways of implementing Wheeler's basic idea have been made into real experiments and they support the conclusion that Wheeler anticipated — that what is done at the exit port of the experimental device before the photon is detected will determine whether it displays interference phenomena or not. Retrocausality is a mirage.

Double-slit version[edit]

Wheeler's double-slit apparatus.^[10]

A second kind of experiment resembles the ordinary double-slit experiment. The schematic diagram of this experiment shows that a lens on the far side of the double slits makes the path from each slit diverge slightly from the other after they cross each other fairly near to that lens. The result is that at the two wavefunctions for each photon will be in superposition within a fairly short distance from the double slits, and if a detection screen is provided within the region wherein the wavefunctions are in superposition then interference patterns will be seen. There is no way by which any given photon could have been determined to have arrived from one or the other of the double slits. However, if the detection screen is removed the wavefunctions on each path will superimpose on regions of lower and lower amplitudes, and their combined probability values will be much less than the unreinforced probability values at the center of each path. When telescopes are aimed to intercept the center of the two paths, there will be equal probabilities of nearly 50% that a photon will show up in one of them. When a photon is detected by telescope 1, researchers may associate that photon with the wavefunction that emerged from the lower slit. When one is detected in telescope 2, researchers may associate that photon with the wavefunction that emerged from the upper slit. The explanation that supports this interpretation of experimental results is that a photon has emerged from one of the slits, and that is the end of the matter. A photon must have started at the laser, passed through one of the slits, and arrived by a single straight-line path at the corresponding telescope.

The retrocausal explanation, which Wheeler does not accept, says that with the detection screen in place, interference must be manifested. For interference to be manifested, a light wave must have emerged from each of the two slits. Therefore a single photon upon coming into the double-slit diaphragm must have "decided" that it needs to go through both slits to be able to interfere with itself on the detection screen. For no interference to be manifested, a single photon coming into the double-slit diaphragm must have "decided" to go by only one slit because that would make it show up at the camera in the appropriate single telescope.

In this thought experiment the telescopes are always present, but the experiment can start with the detection screen being present but then being removed just after the photon leaves the double-slit diaphragm, or the experiment can start with the detection screen being absent and then being inserted just after the photon leaves the diaphragm. Some theorists aver that inserting or removing the screen in the midst of the experiment can force a photon to retroactively decide to go through the double-slits as a particle when it had previously transited it as a wave, or vice-versa. Wheeler does not accept this interpretation.

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, and it depends on whichever type of detection device 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."^[11]

Experimental details[edit]

John Wheeler's original discussion of the possibility of a delayed choice quantum appeared in an essay entitled "Law Without Law," which was published in a book he and Wojciech Hubert Zurek edited called Quantum Theory and Measurement, pp 182–213. He introduced his remarks by reprising the argument between Albert Einstein, who wanted a comprehensible reality, and Niels Bohr, who thought that Einstein's concept of reality was too restricted. Wheeler indicates that Einstein and Bohr explored the consequences of the laboratory experiment that will be discussed below, one in which light can find its way from one corner of a rectangular array of semi-silvered and fully silvered mirrors to the other corner, and then can be made to reveal itself not only as having gone half way around the perimeter by a single path and then exited, but also as having gone both ways around the perimeter and then to have "made a choice" as to whether to exit by one port or the other. Not only does this result hold for beams of light, but also for single photons of light. Wheeler remarked:

The experiment in the form an interferometer, discussed by Einstein and Bohr, could theoretically be used to investigate whether a photon sometimes sets off along a single path, always follows two paths but sometimes only makes use of one, or whether something else would turn up. However, it was easier to say, "We will, during random runs of the experiment, insert the second half-silvered mirror just before the photon is timed to get there," than it was to figure out a way to make such a rapid substitution. The speed of light is just too fast to permit a mechanical device to do this job, at least within the confines of a laboratory. Much ingenuity was needed to get around this problem.

After several supporting experiments were published, Jacques et al. claimed that an experiment of theirs follows fully the original scheme proposed by Wheeler.^[12][13] Their complicated experiment is based on the Mach-Zender interferometer, involving a triggered diamond N-V colour centre photon generator, polarization, and an electro-optical modulator acting as a switchable beam splitter. Measuring in a closed configuration showed interference, while measuring in an open configuration allowed the path of the particle to be determined, which made interference impossible.

In such experiments, Einstein originally argued, it is unreasonable for a single photon to travel simultaneously two routes. Remove the half-silvered mirror at the [upper right], and one will find that the one counter goes off, or the other. Thus the photon has traveled onlyone route. It travels only one route. but it travels both routes: it travels both routes, but it travels only one route. What nonsense! How obvious it is that quantum theory is inconsistent!

Interferometer in the lab[edit]

The Wheeler version of the interferometer experiment could not be performed in a laboratory until recently because of the practical difficulty of inserting or removing the second beam-splitter in the brief time interval between its entering the first beam-splitter and its arrival at the location provided for the second beam-splitter. This realization of the experiment is done by extending the lengths of both paths by inserting long lengths of fiber optic cable. So doing makes the time interval involved with transits through the apparatus much longer. A high-speed switchable device on one path, composed of a high-voltage switch, a Pockel cell, and a Glan–Thompson prism, makes it possible to divert that path away from its ordinary destination so that path effectively comes to a dead end. With the detour in operation, nothing can reach either detector by way of that path, so there can be no interference. With it switched off the path resumes its ordinary mode of action and passes through the second beam-splitter, making interference reappear. This arrangement does not actually insert and remove the second beam-splitter, but it does make it possible to switch from a state in which interference appears to a state in which interference cannot appear, and do so in the interval between light entering leaving the first beam-splitter and light exiting the second beam-splitter. If photons had "decided" to enter the first beam-splitter as either waves or a particles, they must have been directed to undo that decision and to go through the system in their other guise, and they must have done so without any physical process being relayed to the entering photons or the first beam-splitter because that kind of transmission would be too slow even at the speed of light. Wheeler's interpretation of the physical results would be that in one configuration of the two experiments a single copy of the wavefunction of an entering photon is received, with 50% probability, at one or the other detectors, and that under the other configuration two copies of the wave function, traveling over different paths, arrive at both detectors, are out of phase with each other, and therefore exhibit interference. In one detector the wave functions will be in phase with each other, and the result will be that the photon has 100% probability of showing up in that detector. In the other detector the wave functions will be 180° out of phase, will cancel each other exactly, and there will be a 0% probability of their related photons showing up in that detector.^[14]

Interferometer in the cosmos[edit]

The cosmic experiment envisioned by Wheeler could be described either as analogous to the interferometer experiment or as analogous to a double-slit experiment. The important thing is that by a third kind of device, a massive stellar object acting as a gravitational lens, photons from a source can arrive by two pathways. Depending on how phase differences between wavefunction pairs are arranged, correspondingly different kinds of interference phenomena can be observed. Whether to merge the incoming wavefunctions or not, and how to merge the incoming wavefunctions can be controlled by experimenters. There are none of the phase differences introduced into the wavefunctions by the experimental apparatus as there are in the laboratory interferometer experiments, so despite there being no double-slit device near the light source, the cosmic experiment is closer to the double-slit experiment. However, Wheeler planned for the experiment to merge the incoming wavefunctions by use of a beam splitter.^[15]

The main difficulty in performing this experiment is that the experimenter has no control over or knowledge of when each photon began its trip toward earth, and the experimenter does not know the lengths of each of the two paths between the distant quasar. Therefore it is possible that the two copies of one wavefunction might well arrive at different times. Matching them in time so that they could interact would require using some kind of delay device on the first to arrive. Before that task could be done, it would be necessary to find a way to calculate the time delay.

One suggestion for synchronizing inputs from the two ends of this cosmic experimental apparatus lies in the characteristics of quasars and the possibility of identifying identical events of some signal characteristic. Information from the Twin Quasars that Wheeler used as the basis of his speculation reach earth approximately 14 months apart.^[16] Finding a way to keep a quantum of light in some kind of loop for over a year would not be easy.

Double-slits in lab and cosmos[edit]

Replace beam splitter by registering projected telescope images on a common detection screen.

Wheeler's version of the double-slit experiment is arranged so that the same photon that emerges from two slits can be detected in two ways. The first way lets the two paths come together, lets the two copies of the wavefunction overlap, and shows interference. The second way moves farther away from the photon source to a position where the distance between the two copies of the wavefunction is too great to show interference effects. The technical problem in the laboratory is how to insert a detector screen at a point appropriate to observe interference effects or to remove that screen to reveal the photon detectors that can be restricted to receiving photons from the narrow regions of space where the slits are found. One way to accomplish that task would be to use the recently developed electrically switchable mirrors and simply change directions of the two paths from the slits by switching a mirror on or off. As of early 2014 no such experiment has been announced.

The cosmic experiment described by Wheeler has other problems, but directing wavefunction copies to one place or another long after the photon involve has presumably "decided" whether to be a wave or a particle requires no great speed at all. One has about a billion years to get the job done.

The cosmic version of the interferometer experiment could easily be adapted to function as a cosmic double-slit device as indicated in the illustration. Wheeler appears not to have considered this possibility. It has, however, been discussed by other writers.^[17]

Current experiments of interest[edit]

The first real experiment to follow Wheeler's intention for a double-slit apparatus to be subjected to end-game determination of detection method is the one by Walborn et al.^[18]

An experiment by Ma et al., "Quantum erasure with causally disconnected choice," concludes: "Our results demonstrate that the viewpoint that the system photon behaves either definitely as a wave or definitely as a particle would require faster-than-light communication. Because this would be in strong tension with the special theory of relativity, we believe that such a viewpoint should be given up entirely.^[19]

Researchers with access to radio telescopes originally designed for SETI research have explicated the practical difficulties of conducting the interstellar Wheeler experiment.^[20]

Conclusions[edit]

Ma, Zeilinger, et al. have summarized what can be known as a result of experiments that have arisen from Wheeler's proposals. They say:

Any explanation of what goes on in a specific individual observation of one photon has to take into account the whole experimental apparatus of the complete quantum state consisting of both photons, and it can only make sense after all information concerning complementary variables has been recorded. Our results demonstrate that the viewpoint that the system photon behaves either definitely as a wave or definitely as a particle would require faster-than-light communication. Because this would be in strong tension with the special theory of relativity, we believe that such a viewpoint should be given up entirely.^[21]

Bibliography[edit]

• 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/abs/quant-ph/0610241v1
• On-line bibliography listing all of Wheeler's works
• 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

References[edit]

1. Jump up^ Mathematical Foundations of Quantum Theory, edited by A.R. Marlow, Academic Press, 1978. P, 39 lists seven experiments: double slit, microscope, split beam, tilt-teeth, radiation pattern, one-photon polarization, and polarization of paired photons.
2. Jump up^ George Greenstein and Arthur Zajonc, The Quantum Challenge, p. 37f.
3. Jump up^ Scientific American, July 1992, p. 75
4. Jump up^ Ma, Kofler, Qarry, Tetik, Scheidl, Ursin, Ramelow, Herbst, Ratschbacher, Fedrizzi, Jennewein, and Zeilinger, "Quantum erasure with causally disconnected choice. p. 1 (PNAS, January 22, 2013, vol. 110, no. 4, pp. 1221–1226)
5. Jump up^ Peruzzo, et al., "A quantum delayed choice experiment," arXiv:1205.4926v2 [quant-ph] 28 Jun 2012. This experiment uses Bell inequalities to replace the delayed choice devices, but it achieves the same experimental purpose in an elegant and convincing way.
6. Jump up^ "Entanglement-enabled delayed choice experiment." by Florian Kaiser, Thomas Coudreau, Perola Milman, Daniel B. Ostrowsky, and Sébastien Tanzilli, in arXiv:1206.4348v1
7. Jump up^ Edward G. Steward, Quantum Mechanics: Its Early Development and the Road to Entanglement, p. 145
8. Jump up^ Anil Ananthaswamy, New Scientist, 07 January 2–13, p. 1f says:

   For Niels Bohr... this "central mystery" was ...a principle of the ... complementarity principle. .... Look for a particle and you'll see a particle. Look for a wave and that's what you'll see.

   "No reasonable definition of reality could be expected to permit this," [Einstein] huffed in a famous paper ... (Physical Review, vol 47, p 777").

9. Jump up^ "Seeing double". ESA/Hubble Picture of the Week. Retrieved 20 January 2014.
10. Jump up^ Mathematical Foundations of Quantum Theory, edited by A.R. Marlow, p. 13
11. Jump up^ John Archibald Wheeler, ""The'Past" and the 'Delayed Choice' Double-Slit experiment," which appeared in 1978 and has been reprinted is several locations, e.g. Lisa M. Dolling, Arthur F. Gianelli, Glenn N. Statilem, Readings in the Development of Physical Theory, p. 486ff.
12. Jump up^ Jacques, Vincent; et al. (2007). "Experimental Realization of Wheeler's Delayed-Choice Gedanken Experiment". Science 315: 966–968. arXiv:quant-ph/0610241v1. Bibcode:2007Sci...315..966J. doi:10.1126/science.1136303. PMID 17303748.
13. Jump up^ Geons, Black Holes & Quantum Foam: A Life in Physics, by John Archibald Wheeler with Kenneth Ford, W.W. Norton & Co., 1998, p. 337
14. Jump up^ Greenstein and Zajonc, The Quantum Challenge, p. 39f.
15. Jump up^ Greenstein and Zajonc, The Quantum Challenge, p. 41.
16. Jump up^ Kundic, T., Turner, E.L., Colley, W.N., Gott, III, R., and Rhoads, J.E., A robust determination of the time delay in 0957+561A,B and a measurement of the global value of Hubble's constant, Astrophys. J., 482, 75-82, (1997).
17. Jump up^ Epistemology and Probability: Bohr, Heisenberg, Schrödinger, and the Nature ..., by Arkady Plotnitsky, p. 66, footnote.
18. Jump up^ PHYSICAL REVIEW A, Volume 65, 033818, "Double-slit quantum eraser" by S. P. Walborn, M. O. Terra Cunha, S. Pa´dua, and C. H. Monken.
19. Jump up^ Ma et al., op sit., p. 6
20. Jump up^ Quantum Astronomy (IV): Cosmic-Scale Double-Slit Experiment
21. Jump up^ "Quantum erasure with causally disconnected choice," by Xiao-Song Ma, Johannes Koflera, Angie Qarrya, Nuray Tetika, Thomas Scheidla, Rupert Ursina, Sven Ramelowa, Thomas Herbsta, Lothar Ratschbachera, Alessandro Fedrizzia, Thomas Jenneweina, and Anton Zeilingera, PNAS, January 22, 2013, vol. 110, no. 4, pp. 1221–1226. See page 6 of the PDF file. Download from: http://www.pnas.org/cgi/doi/10.1073/pnas.1213201110.

External links[edit]

• Wheeler's Classic Delayed Choice Experiment by Ross Rhodes
• The Quantum Eraser by John G. Cramer
• Demystifying the Delayed Choice Experiments

Delayed-Choice Experiments

This sidebar is part of a package that supplements our story on quantum erasure in the May issue of Scientific American

Apr 16, 2007 |By Rachel Hillmer and Paul Kwiat

Some quantum optics researchers in France very recently reported an experiment in which photons went through the apparatus one at a time. Each photon was directed along two paths, with horizontal polarization in one path and vertical polarization in the other. The experimenters then had the option of making the analysis with horizontal or vertical polarizers, revealing which path the photon went, or with diagonal polarizers, revealing fringes and anti-fringes. Does all this sound familiar? The curious feature of this particular experiment is that the choice of which kind of polarizer to use happened after the photon had already split into the two paths. The results showed that the timing of the decision didn't affect the results at all. Indeed the results agreed with those of the simple experiment described in the main article. Reference for this experiment: V. Jacques, et al. Science 315, 966 (2007).

As we mentioned in the main article, it may seem as though you could use this delayed-choice feature to send signals instantaneously. To see how, consider the set-up described there to explain how a quantum eraser works in general: You send particles through two slits so that they can interfere, but individual photons bounce off them near the slits, destroying the interference. By making a special kind of measurement of the photons, you can erase the which-slit information and restore interference.

To use this kind of set-up to try to transmit a signal to someone who is far away, you arrange for the potentially interfering particles to travel all the way to that recipient, and she can observe whether they form fringes or not when they arrive. Meanwhile, you have to keep the photons at the ready, perhaps by circulating them through long coils of optical fiber. Now when the recipient is about to receive a batch of particles, you can transmit a "0" to her by measuring the corresponding photons to determine which slit each particle went through (so she will see no interference with that batch of particles), or you can send a "1" by doing the special measurement that restores interference. Because of the delayed-choice effect, seemingly your choice of measurement will instantaneously determine what the recipient sees no matter how far away she is.

As with the apparent paradox discussed in the main article, this strategy is foiled because to see the fringes formed by the particles, your distant friend must first divide them into two groups to separate ones that form fringes from those that form anti-fringes. To do that division, however, she needs to know the results of your special photon measurements—essentially there are two types of erasing results, those corresponding to particle interference fringes, and those corresponding to particle anti-fringes—and that information is going to crawl its way to her no faster than the speed of light.

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A Double-Slit Quantum Eraser Experiment

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This web-page was created as an assignment for PHY 566, taught by Prof. Luis Orozco at Stony Brook University in the fall semester of 2002. 

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The following describes work done by S. P. Walborn, M. O. Terra Cunha, S. Padua, and C. H. Monken at the Universidade Federal de Minas Gerais in Brazil.  Their work was published last March in Physical Review A, (65, 033818, 2002).  The pdf version of the publication can be found here.

This experiment uses the phenomena of interference, produced by light incident on a double slit, to investigate the quantum mechanical principle of complementarity between the wave and particle characteristics of light.  Using a special state of light, Walborn and his coworkers created an interference pattern, made a "which-way" measurement which destroyed the interference, and then erased the "which-way" marker, bringing the interference back.  This experiment clearly displays the way in which nature is counterintuitive on the quantum scale and makes it clear that our ways of thinking based on our everyday experiences in the classical world are often completely inadequate to understand the quantum world.  

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 A Peculiarity about Quantum Mechanics  

Interference

Any wave in nature is capable of producing interference.  Mathematically, a wave is described by an amplitude which can be positive or negative.  When two waves overlap spatially the amplitudes can add and subtract at different locations, creating a pattern of crests and troughs.  This can be seen in water waves, and heard in the phenomenon of beats caused by sound waves.  Light is also a wave, and when incident upon a double slit will produce a pattern of bright and darks spots.  

Interference and photons

Quantum mechanics governs all phenomena on the atomic scale.  The smallest constituent of light is the indivisible photon.  What happens when a single photon is incident upon a double slit?  

Mathematically the quantum description is not any different from the classical wave interference description.  Quantum mechanics does not predict exact trajectories for particles.  Rather, it predicts the probability a particle will go one way or another.   In the case at hand, the single photon has a fifty percent chance of going through the left slit and a fifty percent chance of going through the right slit.  A particle is described mathematically by probability amplitudes which, like in the classical case, can be positive or negative.  It is these probability amplitudes that combine constructively and destructively to make an interference pattern.  Quantum mechanics does not tell us which slit the particle will go through.  

A single photon cannot of course make a whole interference pattern on a screen by itself.  If single photons are allowed to go through the slits one at a time, however, and produce a splotch on a special phosphorescent screen, after enough time the interference pattern will emerge.

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Formation of the interference pattern.  It is easy to imagine that each photon must go through one slit or the other, whether this is correct or not.

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Which Way?

It is difficult at this point to not be tempted to ask, which way does the photon really go?  If quantum mechanics can't tell us which way a photon will go, perhaps we can see for ourselves by another means.  It seems reasonable to assume that the photon has to pick one slit to go through.  Quantum mechanics must just be inadequate at providing us with all the available information.  

This is a question that many people have given some serious thought, including Albert Einstein, Richard Feynman, and Werner Heisenberg.  They came up with thought experiments which proposed to measure the "which-way" information of a particle's path on its way to contributing to an interference pattern.  They came to a rather perplexing conclusion, however, namely that it is not possible to observe the "which-way" information and the interference pattern simultaneously.  One can set up a measurement to "watch" which slit a photon goes through.  It can be determined that the photon went through one slit and not the other.  However, once this is kind of measurement is set up, the photons will  no longer collectively produce a nice pattern of bright and dark spots.  Instead they will strike the screen in one big bright spot, as if there were only one slit instead of  two.   

One can wonder then, if this perplexing behavior is just due to a disturbance between the "which-way" detector and the photon.  The detector might be changing something about the photon which causes it to get off course to its position in the interference pattern.  The answer is, as the experiment described in the next section shows, that this is not the case.  A "which-way" detector can be designed that in no way disturbs the photon and the same phenomenon is observed.  It is not possible to observe the which-way information and the interference pattern at the same time.  This is an example of quantum mechanics' principle of complementarity.  There are pairs of quantities which can be measured and obtained individually, but never at the same time.  You can know one precisely, but then you will know nothing about the other and vice versa.  

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Experimental Investigation 

Entangling photons

The light used in this experiment is a special state of light consisting of two photons that are said to be entangled.  These photons are intimately intertwined, with polarizations that are correlated.  

(Polarization is the direction in which the electric field of the light is oscillating.  Light can be linearly polarized in the y direction, with an electric field oscillating up and down.  Or it can be linearly x polarized, with an electric field oscillating left in right.  Light can also be circularly polarized, which means that the electric field is oscillating in a direction that keeps changing, rotating in a circle around the propagation direction of the light.  The tip of the electric field vector traces out a corkscrew pattern.  Light can be left circularly polarized, meaning the electric field rotates counter clockwise, or right circularly polarized, meaning the electric field rotates clockwise.)

The entangled photons are produced so that they have linear polarizations which are orthogonal to each other.  If one photon is measured to be y polarized, then it is known with certainty that the other has x polarization.  It is not accurate to consider these photons as separate entities, but rather as one.  They can travel very far away from each other, but they will not loose their correlation.  This peculiar state is called a Bell state, after John Bell.

The entangled photons are produced by a process called spontaneous parametric down conversion.  This takes place in a special nonlinear crystal called beta-barium borate (BBO).  A photon from an argon ion pump laser (351.1 nm) is converted to two longer wavelength (702.2 nm) photons.  The two photons go off in two different directions.  In this experiment, we call one direction p and the other s.  The photons that go down path p are called  p photons and those that go down s are called sphotons.

  

Double-slit interference

The interference pattern from the double slit is created and measured in the following way.  The s photons are the ones that create the interference pattern.  They travel through the double-slit to detector D_s.  The p photons travel directly to detector D_p.  If  D_p registers a photon, it sends a "click" to the coincidence counter.  The counter waits for the p photon's entangled partner to be registered by D_s.  Once this second "click" is detected, a count is recorded.  The counts are tallied for 400 seconds.  Then detector D_s is moved a millimeter and the number of counts in a 400 second interval is recorded for the new detector position.  This is repeated until D_s has scanned across a region equivalent to the screen in the diagrams above.  

 

The results are displayed by plotting the number of counts as a function of detector D_s position.  The interference pattern is clearly observed.

Which-Way Marker

To make the "which-way" detector, a quarter wave plate (QWP) is put in front of each slit.  This device is a special crystal that can change linearly polarized light into circularly polarized light.  The two wave plates are set  so that given a photon with a particular linear polarization, one wave plate would change it to right circular polarization while the other would change it to left circular polarization. 

With this configuration, it is possible to figure out which slit the s photon went through, without disturbing the s photon in any way.  Because the s and p photons are an entangled pair, if we measure the polarization of p to be x we can be sure that the polarization of s before the quarter wave plates was y.  QWP 1, which precedes slit 1, will change a y polarized photon to a right circularly polarized photon while QWP 2 will change it to a left circularly polarized photon.  Therefore, by measuring the polarization of the s photon at the detector, we could determine which slit it went through.  The same reasoning holds for the case where the p photon is measured to be y.  The following table provides a summary.        

Detected polarization for photon p	Polarization of photon s before the QWP's	Polarization of photon s after going through QWP1 and slit 1	Polarization of photon s after going through QWP2 and slit 2
x	y	R	L
y	x	L	R

The presence of the two quarter wave plates creates the possibility for an observer to gain which-way information about photon s.  When which-way information is available, the interference behavior disappears.  It is not necessary to actually measure the polarization of p and figure out what slit s passed through.  Once the quarter wave plates are there, the s photons are marked, so to speak.  

The coincidence counts were tallied at each detector location, as before, and it was found that indeed the interference pattern was gone.

  

In case you might be suspicious of the quarter wave plates, it is worth noting that given a beam of light incident on a double slit, changing the polarization of the light has no effect whatsoever on the interference pattern.  The pattern will remain the same for an x polarized beam, a y polarized beam, a left or a right circularly polarized beam.

It  is peculiar then, that the presence of the quarter wave plates causes the s photons to so drastically change their behavior.  One can't help but ask, how do these photons know that we could know which slit they went through? 

Quantum Erasure

Increasing the strangeness of this scenario, the next step is to bring back the interference without doing anything to the s beam.   A polarizer is placed in the p beam, oriented so that it will pass light that is a combination of x and y.  It is no longer possible to determine with certainty the polarization of s before the quarter wave plates and therefore we cannot know which slit an s photon has passed through.  The s photons are no longer marked.  The potential to gain which-way information has been erased.

The coincidence measurements were repeated with the polarizer in place.  It can be seen from the data that the interference pattern is back.   

 How does photon s know that we put the polarizer there?  

Photon s and photon p are entangled.  Photon p must be able to communicate to s through some means that is unknown to us.  It must be telling s whether it should be producing a pattern or not.  But as we will see, this does not seem to be the case.  In the next section, things get stranger still.

Delayed Erasure

The experiment up to this point has been performed by detecting photon p before photon s.  The erasure of the which-way information was performed by modifying the path of p and then measuring s.  One could regain a bit of reassurance in commonsense by believing that there must be some form of communication taking place between photon p and s so that s knows whether to interfere or not.  Perhaps photon p encounters the polarizer and sends s an immediate message telling it that it can again go the interference route.  This is not the case, however, as the next and final portion of the experiment shows. 

The path of beam p is lengthened (the polarizer and detector  moved farther away from the BBO crystal), so that photon s can be detected first.  The interference fringes are obtained as before.   Then the quarter wave plates are added to provide the which-way marker.  The interference pattern and lack of interference pattern from these runs are shown here.

Next the erasure measurement is performed.  Before photon p can encounter the polarizer, s will be detected.  Yet it is found that the interference pattern is still restored.  It seems photon s knows the "which-way" marker has been erased and that the interference behavior should be present again, without a secret signal from photon p.  

How this happening?  It wouldn't make sense that photon p could know about the polarizer before it got there.  It can't "sense" the polarizer's presence far away from it, and send photon s a secret signal to let s know about it.  Or can it?  And if  photon p is sensing things from far away, we shouldn't assume that photon s isn't.    

Perhaps the funny business of entanglement plays a more important role than we thought.  The two photons are entangled.  They are connected together in a special way that doesn't break no matter how far apart they are.  It seems that these entangled photons also have some sort of entangled connection with the quarter wave plates and the polarizer.

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Making Sense of the Nonsensical

From this experiment it is apparent that interference is destroyed by a "which-way" marker and that it can be restored through erasure of  the marker, accomplished by making the appropriate measurement on the entangled partner photon p.

In this set up, the "which-way" measurement does not alter the momentum or position of the photons to cause destruction of the interference pattern.  We can think of the loss of interference as being due only to the fact that the photons are entangled and that the presence of the quarter wave plates changes this entanglement.  The interference pattern can be brought back through the erasure measurement because of the entanglement of the photons, and the way that the presence of the quarter wave plates and polarizer changes the entanglement. 

Entanglement is not something we encounter in our everyday world.  The concept of locality does not hold for the entangled state like it does for everything in our  experience.  We encounter things that have a particular location, we can say that a particular thing is here and not there.  We certainly do not encounter things that are in two places at once.  However, this is possible on the quantum level.  Two photons that are in an entangled state can be separated across the universe, but they are still connected together.  In this experiment, with each measurement that was performed, the way the photons were entangled changed.  This caused the very strange results that were observed.  We like to think about photon p as being in one place and photon s as being in another apart from p.  But this is not really the case..  We have to start thinking in ways that aren't consistent with what we experience in our larger scale world.  Entanglement seems to play a very important role on the quantum scale of the world,  so we need to think about it in new ways.

This quantum erasure experiment is one of many experiments being done that provides a way for us to better understand the strange nature of quantum mechanics.  We have encountered strange concepts like entanglement and non-locality.  Perhaps this is just the beginning of a journey to a deeper understanding of  the universe and new discoveries. 

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Please explain the delayed-choice experiment and the quantum eraser self.askscience

submitted 1 year ago by psYberspRe4Dd

The delayed-choice experiment and the quantum eraser are sophisticated variations of the double-slit with particle detectors placed not at the slits but elsewhere in the apparatus. The first demonstrates that extracting "which path" information after a particle passes through the slits can seem to retroactively alter its previous behavior at the slits. The second demonstrates that wave behavior can be restored by erasing or otherwise making permanently unavailable the "which path" information.

~ https://en.wikipedia.org/wiki/Double-slit_experiment#Delayed_choice_and_quantum_eraser_variations

delayed-choice experiment

Quantum eraser experiment

Can anyone explain these in an understandable way without implying the consciousness causes collapse theory which I don't believe in ?

For the delayed choice experiment I could imagine entanglement-like interaction that causes the particle to be either in wave or particle state or some other crazy theory but how in the world can the quantum eraser be explained (without implying some kind of consciousness causes collapse in a simulated universe relative to observation at this level) ?

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[–]Friendly_Fire 2 points 1 year ago

I've taken some college level physics, so no expert by any means, but I might be able to give a little direction. First, you have to realize your everyday intuitions are meaningless. Neither particles nor waves are real descriptions of sub-atomic phenomenon. They are more accurately referred to as wave-packets, but it's still not a particularly helpful analogy.

Entanglement is an even stranger phenomenon. So when you have the quantum eraser, and one particle splits in to two other 'particles' you can't think of it like two normal particles. I assume you read through the experiment you basically have two options as far as I can tell. Either 1.) The entangled particles can send information back in time or 2.)They aren't really separate, or particles at all.

I personally hold strongly that information back in time is silly, almost as silly as consciousness causing collapse. If I was going to give an intuitive explanation, I would lean towards the fact that they are waves and waves can't be localized. The instant two entangled particles are created, they are interacting with everything around them (With regards to distance vs speed of light) via multiple forces, so they can, in the physical sense, observe the path ahead of them.

That might be nonsense though. I read somewhere "If you think you understand quantum mechanics, you don't understand quantum mechanics." Hopefully some PhD's can come in here and do better.

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[–]psYberspRe4Dd[S] 1 point 1 year ago

Thank you!

What I meant in my post is if anything the option 2) or something similiar to it could be explaining this. (These are not the only options though, there are many other crazy ones for example the universe we live in could be one of many and the 'following' or continueing universe just becomes one that is 'correct' by the observing particle).

So if they take both ways when passing through the double slit and observation of any path causes the (other one to be the opposite) collapse via quantum entanglement or something alike (as I've suggested in my post).

How would they 'observe the path ahead of them' ? First you say two particles are created but I thought they'd still be waves until you observe them ?

No PhD's here yet, maybe I should try to rephrase my question a bit clearer and post it again at some time.

However you didn't explain the quantum eraser.

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[–]FormerlyTurnipHuggerQuantum Information | Quantum Computing | Quantum Optics 2 points 1 year ago

You don't need to subscribe to a particular interpretation of QM to understand this, the maths suffices.

The quantum eraser experiment is very simple. Any amount of information about which slit the particle passed through will degrade the interference pattern. This is actually not an "either-or" issue: there is a smooth complementarity between wave-like and particle-like behavior. You can have a little bit of both if the information you extract is not complete.

Anyway, in a quantum eraser this 'which-slit' information is available to some degree, but you get rid of it and thus restore the wave picture. Example: an unpolarized photon goes through the two slits, each of which contains a polarizer—one oriented at 0 and one at 90 degrees. The polarization after the slits constitutes the information which will tell you which slit the photon went through. If you now put a polarizers at 45 degrees behind the slit, the photon will randomly go through or not, and shed that information in the process.

And for delayed choice it's important to understand that no results are ever changed after the fact. Whatever you record stays recorded. What does change though is that once you look at the measurement result constituting the delayed choice event, you can sort through your initial list of results and correlate them with the source—thus e.g. separating wave results from particle results. This is also called postselection.

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[–]outerspacepotatoman9 1 point 1 year ago

There are multiple ways of going about it depending on which interpretation of quantum mechanics you subscribe to. Personally, I'm a fan of non-realism which resolves this problem by positing that wave function collapse is not an objective process but rather something that only makes sense with respect to a specific observer (which can be anything at all, not just a conscious being). You can work through all of the bizarre phenomena of quantum mechanics and find that everything makes sense with this view in mind, although it is extremely counterintuitive. I can elaborate further if you are interested.

There are other ideas though. The many-worlds interpretation essentially states that the wavefunction collapse doesn't happen at all, it just looks like it does when a macroscopic system interacts with a microscopic system, which can also fix certain issues. Also, there are so-called "objective collapse" interpretations where the wavefunction collapse is linked to certain dynamical processes (having nothing to do with consciousness) and it just looks like consciousness is causing the collapse to us because the way in which we observe things has the incidental effect of making this dynamical collapse extremely likely. For instance, the GRW interpretation posits that particles have a very small probability of initiating a collapse at any given time which propagates through any quantum system they are entangled with. This way, if you have a few particles entangled in an experiment the chances of a spontaneous collapse are vanishingly small. But, as soon as you perform a measurement and all of the particles that constitute the macroscopic measuring apparatus become entangled the odds of a collapse rapidly approach 1.

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Commentary: Very good one from original Author:

chrome-extension://ecnphlgnajanjnkcmbpancdjoidceilk/https://copy.com/web/companies/company-28845/users/user-5412742/CLs/1.Articles/Delayed%20Choice%20Quantum%20Eraser_Commetary.pdf?download=1

God Vs. The Delayed Choice Quantum Eraser

FEBRUARY 10, 200838 COMMENTS

This is the name of an experiment first proposed in 1982, and is the one that I have had in mind when talking about God and quantum mechanics. I realized that most of the comments I have received seem to be taking me to be talking only about the standard double-slit experiment; this is of course my own fault since I haven’t done a very good job of indicating what I had in mind. So, let me describe these results and then reformulate the argument.

We have to build up to this, so let’s start with the quantum eraser experiment. Here is how Brian Greene describes the experiment in his recent book The Fabric of the Universe.

A simple version of the quantum eraser experiment makes use of the double-slit set up, modified in the following way. A tagging device is placed in front of each slit; it marks any passing photon so that when the photon is examined later, you can tell through which slit it passed…when this double-slit-tagging experiment is run, the photons do not build up an interference pattern.

As he goes on to point out, this is what we would expect. Since we measure the photon’s path, we get the photons acting like particles. But then it gets weirder. As Green continues, the quantum eraser asks,

What if just before the photon hits the detection screen, you eliminate the possibility of determining through which slit it passed by erasing the mark imprinted by the tagging device?

The answer, as it turns out, is that the interference pattern shows up again. Which, is , uh, weird. But again it gets weirder with the delayed-choice quantum eraser. Greene describes it thus,

It begins with [the set-up of the quantum eraser], modified by inserting two so-called down-converters, one on each pathway. Down-converters are devices that take one photon as input and produce two photons as output, each with half the energy (“down converted”) of the signal. One of the photons (called the signal photon) is directed along the path that the original would have followed toward the detector screen. The other photon produced by the down-converter (called the idler photon) is sent in a different direction altogether. On each run of the experiment we can determine which oath a signal photon takes to the screen by observing which down-converter spits out the idler photon partner. And once again, the ability to gleen which-path information about the signal photons– even though it is totally indirect, since we are not interacting with any signal photons at all– has the effect of preventing an interference pattern from forming.

OK, so far so good. This is just a fancier version of what we have already talked about, with the exception that we are now no longer causally interacting with the signal photon. Everything we know about the signal photon we learn by observing the idler photon. But even so, we get the photons acting like particles. But we aren’t done yet. Again Greene

 Now for the weirder part. What if we manipulate the experiment so as to make it impossible to determine from which down-converter a given idler photon emerged? What if, that is, we erase the which-path information embodied by the idler photon? Well, something amazing happens: even though we’ve done nothing directly to the signal photons, by erasing which-path information carried by their idler partners we can recover an interference pattern from the signal photons[!!!!!!]

OK, so what this seems to show is that it is not anything that we do to the photon that determines which way it will behave. Rather what determines this is whether or not we are able to know which path the photon takes to the detector. Nothing changes here except our ability to know which path the photon took.

We can hammer home this point with one further modification of the experiment. Suppose that we set it up so that we could only get which-path information from some of the photons (and further that which ones we get this information about is random). Again Greene.

Does this erasure of some of the which-path information– even though we have done nothing directly to the signal photons– mean that the interference effects are recovered? Indeed it does– but only for those signal photons whose idler photons [had their which-path information erased]…If we hook up equipment so that the screen displays a red dot for the position of each photon whose idler photons [had their which-path information erased] and a green dot for all others, someone who was color-blind would see no interference pattern, but everyone else would see that the red dots we arranded with bright and dark bands– an interference pattern.

So, it is the knowledge of which-path information that determines which way the photons behave. Since God always has which-path information, whether he obtains it in such a way as to effect the physical world or not, He will never see the interference pattern. Or in other words, the wave like nature of reality will be hidden from Him.

Sheez! That took longer than I thought!!  \

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---------------------- Please explain the delayed-choice experiment and the quantum eraser self.askscience submitted 1 year ago by psYberspRe4Dd The delayed-choice experiment and the quantum eraser are sophisticated variations of the double-slit with particle detectors placed not at the slits but elsewhere in the apparatus. The first demonstrates that extracting "which path" information after a particle passes through the slits can seem to retroactively alter its previous behavior at the slits. The second demonstrates that wave behavior can be restored by erasing or otherwise making permanently unavailable the "which path" information. ~ https://en.wikipedia.org/wiki/Double-slit_experiment#Delayed_choice_and_quantum_eraser_variations delayed-choice experiment Quantum eraser experiment Can anyone explain these in an understandable way without implying the consciousness causes collapse theory which I don't believe in ? For the delayed choice experiment I could imagine entanglement-like interaction that causes the particle to be either in wave or particle state or some other crazy theory but how in the world can the quantum eraser be explained (without implying some kind of consciousness causes collapse in a simulated universe relative to observation at this level) ? 4 commentsshare All 4 Comments sorted by: best [–]Friendly_Fire 2 points 1 year ago I've taken some college level physics, so no expert by any means, but I might be able to give a little direction. First, you have to realize your everyday intuitions are meaningless. Neither particles nor waves are real descriptions of sub-atomic phenomenon. They are more accurately referred to as wave-packets, but it's still not a particularly helpful analogy. Entanglement is an even stranger phenomenon. So when you have the quantum eraser, and one particle splits in to two other 'particles' you can't think of it like two normal particles. I assume you read through the experiment you basically have two options as far as I can tell. Either 1.) The entangled particles can send information back in time or 2.)They aren't really separate, or particles at all. I personally hold strongly that information back in time is silly, almost as silly as consciousness causing collapse. If I was going to give an intuitive explanation, I would lean towards the fact that they are waves and waves can't be localized. The instant two entangled particles are created, they are interacting with everything around them (With regards to distance vs speed of light) via multiple forces, so they can, in the physical sense, observe the path ahead of them. That might be nonsense though. I read somewhere "If you think you understand quantum mechanics, you don't understand quantum mechanics." Hopefully some PhD's can come in here and do better. permalink [–]psYberspRe4Dd[S] 1 point 1 year ago Thank you! What I meant in my post is if anything the option 2) or something similiar to it could be explaining this. (These are not the only options though, there are many other crazy ones for example the universe we live in could be one of many and the 'following' or continueing universe just becomes one that is 'correct' by the observing particle). So if they take both ways when passing through the double slit and observation of any path causes the (other one to be the opposite) collapse via quantum entanglement or something alike (as I've suggested in my post). How would they 'observe the path ahead of them' ? First you say two particles are created but I thought they'd still be waves until you observe them ? No PhD's here yet, maybe I should try to rephrase my question a bit clearer and post it again at some time. However you didn't explain the quantum eraser. permalinkparent [–]FormerlyTurnipHuggerQuantum Information | Quantum Computing | Quantum Optics 2 points 1 year ago You don't need to subscribe to a particular interpretation of QM to understand this, the maths suffices. The quantum eraser experiment is very simple. Any amount of information about which slit the particle passed through will degrade the interference pattern. This is actually not an "either-or" issue: there is a smooth complementarity between wave-like and particle-like behavior. You can have a little bit of both if the information you extract is not complete. Anyway, in a quantum eraser this 'which-slit' information is available to some degree, but you get rid of it and thus restore the wave picture. Example: an unpolarized photon goes through the two slits, each of which contains a polarizer—one oriented at 0 and one at 90 degrees. The polarization after the slits constitutes the information which will tell you which slit the photon went through. If you now put a polarizers at 45 degrees behind the slit, the photon will randomly go through or not, and shed that information in the process. And for delayed choice it's important to understand that no results are ever changed after the fact. Whatever you record stays recorded. What does change though is that once you look at the measurement result constituting the delayed choice event, you can sort through your initial list of results and correlate them with the source—thus e.g. separating wave results from particle results. This is also called postselection. permalink [–]outerspacepotatoman9 1 point 1 year ago There are multiple ways of going about it depending on which interpretation of quantum mechanics you subscribe to. Personally, I'm a fan of non-realism which resolves this problem by positing that wave function collapse is not an objective process but rather something that only makes sense with respect to a specific observer (which can be anything at all, not just a conscious being). You can work through all of the bizarre phenomena of quantum mechanics and find that everything makes sense with this view in mind, although it is extremely counterintuitive. I can elaborate further if you are interested. There are other ideas though. The many-worlds interpretation essentially states that the wavefunction collapse doesn't happen at all, it just looks like it does when a macroscopic system interacts with a microscopic system, which can also fix certain issues. Also, there are so-called "objective collapse" interpretations where the wavefunction collapse is linked to certain dynamical processes (having nothing to do with consciousness) and it just looks like consciousness is causing the collapse to us because the way in which we observe things has the incidental effect of making this dynamical collapse extremely likely. For instance, the GRW interpretation posits that particles have a very small probability of initiating a collapse at any given time which propagates through any quantum system they are entangled with. This way, if you have a few particles entangled in an experiment the chances of a spontaneous collapse are vanishingly small. But, as soon as you perform a measurement and all of the particles that constitute the macroscopic measuring apparatus become entangled the odds of a collapse rapidly approach 1. -------------------------- What does the quantum eraser experiment tells us? up vote8down votefavorite 2 I am a beginner in this quantum-mechanics stuff. I understand the quantum eraser only from an experimental view. So I didn't understand the formalism that describes the quantum eraser. But what does the experiment tells us? Does the photon know that there is somebody watching it? And this is why it behaves in another way? Does the photon also see the future? quantum-mechanics shareimprove this question edited Feb 28 '11 at 10:33 asked Feb 27 '11 at 22:22 kame 280311 Obligatory: see "No Math, please". Any answer that is accurate without math is likely to be confusing; any answer that is simple without math is likely to be wrong. The formalism is required to answer this right. – kharybdis Feb 28 '11 at 6:07  add a comment 3 Answers activeoldestvotes up vote5down vote No, the photon doesn't see anyone watching it. And the photon doesn't see its future, either. In fact, the photon doesn't exist in any classical sense prior to its observation. All of its properties - e.g. which slits it could be taking; whether it behaves more as a particle or a wave etc. - are encoded in the wave function until the very moment of the measurement which is why they may always be "changed back" to the previous answers. For example, in quantum eraser, the photon is ordered to behave as a wave again, even though a premature argument could lead a sloppy person to think that the photon has already decided to behave as a particle forever. When you measure the photon, it is finally possible to think of its properties classically and the wave function allows one to calculate all probabilities that the outcome will be something or something else. In the case of the quantum eraser, we restore the interference pattern. But any attempt to "imagine" that the photon has obtained a classical property at any moment before it was measured would lead to wrong predictions. It is always essential to appreciate that the photon always behaves according to the laws of quantum mechanics and we're never allowed to approximate it by any classical intuition because the classical intuition fails. This strict requirement that classical mechanics is wrong may only be partly circumvented after the photon is actually detected (because then it interacts with a classical object that quickly decoheres) - but not earlier than that. In other words, quantum mechanics always holds: that's the main lesson of this experiment (and many others). Sb1 says that it was remarkable that the experiment behaved as Scully and Druhl predicted. I disagree with this wording. The prediction could have been made by any father of quantum mechanics - no new physics was used whatsoever and they could predict the behavior of any setup of this kind. It could have been remarkable in the 1920s but after the 1920s, all such experiments were mundane physics. shareimprove this answer answered Feb 28 '11 at 6:25 Luboš Motl 99.5k7128272 I don't agree. that an experiment confirms existing theory can also be remarkable, if the theory predicts something highly counter-intuitive. This gives strong emotional confidence on QM, and that is always remarkable, just like the EPR experiment was. –  lurscher Feb 28 '11 at 15:32 1 Perhaps I'm missing something, but can you edit this answer to include, specifically, what about the quantum eraser experiment is quantum? I can show, classically, that two orthogonal polarization states (of classical light) will not interfere, while changing to non-orthogonal polarization states will. Basically I have a problem with the tagging method which was used by Walborn et al. To me, it appears they are measuring the classical electromagnetic wave properties, and NOT probabilities and probability wave entanglement. – daaxix Jan 4 '13 at 22:55  @daaxix Same here. So far in all such experiments I've come across polarizers were used, and to me that is pretty controversial. –  jwalker Jan 27 at 19:41  @daaxix, the classical electromagnetic description may be OK for any kind of interference-like experiment including complex ones but the interference may be shown to persist even if the photons are being sent one-by-one so that they create individual dots, so the interference is a property of a single photon as well, and that requires quantum mechanics. –  Luboš Motl Jan 27 at 20:10 @LubošMotl, not really. I'm not saying that light is not quantized, but suppose for a moment that light packets were detected with solid state sensors (which are quantum mechanical), then those interference patterns would still be "individual dots" caused by the sensor, NOT necessarily the photons, with the polarizers still causing the interference. Has anyone disentangled this problem? –  daaxix Jan 28 at 5:54 show 5 more comments up vote2down vote "Quantum eraser" was first proposed by M. Scully and K. Druhl. In order to understand it, we must know the famous double slit experiment first which I suppose you already know. You must be aware that in the double slit experiment if photons are emitted one at a time an interference pattern forms at the detector screen. As soon as you try to observe which path the photon follows that is which slit it passes through, left or right slit, the interference disappears. That means a knowledge of "which path" information destroys the wave like character of a photon and hence no interference possible. But in 1982, Scully and Druhl suggested a stunning modification of the experiment. They proposed the following on the basis of their quantum mechanical calculation. Suppose a tagging device is attached by which we can know the "which path" information of the photon. Now if, just before the photon hits the detector screen, we eliminate the possibility of our knowledge of the "which path" information by erasing the mark registered by the tagging device, both possibilities that is the photon passed through the left slit and photon passed through the right slit should come back into play. Both histories should come back once again and interference pattern should reemarge. As if we are kind of shaping the past (warning: it is by no means that future is affecting the past). Experiment carried out by Raymond Chiao, Paul Kwiat and A. Steinberg. Remarkably it worked just as scully and Druhl predicted. Interference pattern indeed reemarged. shareimprove this answer answered Feb 28 '11 at 2:13 user1355 great explanation! can you elaborate a bit on the tagging mechanism used to know 'which path' the photon took? –  lurscher Feb 28 '11 at 15:27 1 @lurscher: Thanks. Briefly, it is a device which permits a photon to freely pass through a slit but forces its spin axis to point to a definite direction. Devices in front of the two slits make the photons spin in a different but specific manners. The detector screen registers a dot at the photon's impact position as well as keeps record of the photon's spin direction. –  user1355 Feb 28 '11 at 15:45 how does the "erasing the mark" procedure is made? –  HDE Mar 1 '11 at 11:39  @HDE: Details of the setup are not very important from theorist's point of view. Still if you are interested, see this wiki article en.wikipedia.org/wiki/Quantum_eraser_experiment for a good introduction. –  user1355 Mar 1 '11 at 11:51 add a comment up vote0down vote I don't think that Photon doesn't aware of observation. It reacts based on the observation. this is good one. http://www.youtube.com/watch?v=R-6St1rDbzo shareimprove this answer answered Dec 28 '12 at 14:33 Sakthi 1 Answers consisting primarily of a link are generally discouraged as Stack Exchange site strive to be repositoties of answers rather than mere link farms. Not the least because the links will decay in time. – dmckee♦ Dec 28 '12 at 23:53

Quantum Mechanics: Does the delayed-choice quantum eraser experiment imply that conscious awareness collapses the quantum wave function?

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"Scully and Drühl found that there is no interference pattern when which-path information is obtained, even if this information was obtained without directly observing the original photon, but that if you somehow "erase" the which-path information, an interference pattern is again observed."

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Scott Aaronson

Scott Aaronson

55 upvotes by Frank Heile, Manan Shah, Stephan Hoyer, (more)

No.

In general, if someone asks, "Does experiment X imply that conscious awareness collapses the quantum wave function?", then the answer is no.

With any quantum experiment, a crucial point is that the interference pattern is destroyed whenever the "which-path" information leaks out into the external environment.  It doesn't matter, for this purpose, whether the information leaks into the brain of a human observer, or the brain of a frog, or just into stray air molecules and radiation -- the interference gets destroyed all the same.

  

Written 3 Apr, 2013. Asked to answer by Lyric Duveyoung.

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Charles H Bennett

Charles H Bennett

3 upvotes by Joshua Engel, Christine Johal, and Niraj Kumar.

Expanding on Scott's terse No, the question itself is both ambiguous and too anthropocentric.  If you take it to mean "Is conscious awareness the only or usual cause of collapse?" then the answer is No--the air molecules would do as well.  If you take the question to mean "Does conscious awareness always cause collapse?", then the answer is still no, at least in principle.  Whatever physical brain processes are involved in conscious awareness can in principle be undone, just as a measurement is undone in a quantum eraser experiment. After this had happened, the human observer would have forgotten their awareness of which path the particle followed, and the interference pattern would be restored.  In other words, the collapse, whether caused by an air molecule or a human observer, can in principle be undone, though of course it is exceedingly difficult in practice.  This is not so different from the in-principle reversibility of classical irreversible phenomena in statistical mechanics.

  

Written 9 Apr, 2013.

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Richard Malcolm Smythe

Richard Malcolm Smythe

No. There is a fault on our part when we give too much credence to the objective reality of a wave function collapse. All that can be said is that all experiments to date are consistent with this. The interference pattern simply means the experiment was done without asking which slit the particle went through. It doesn't matter if this was determined 'in front' of the slits or 'behind it' or before 'going through' the slits or after 'going through'. Merely that in any way; the decision to see which slit it went through wasn't made. Anything else we concern ourselves with is just baggage we carry trying to impose on nature our 'classical' world view and experiences. 

Delayed choice experiment is a real thing ?

Top page (correct Bohr model including the two-electron atoms)
Strange "spin" is NOT a real thing.
Bell inequality violation can be explaied by waves.

• Wheeler's delayed choice experiment can be explained by classical waves.
• Delayed choice quantum eraser can be explained by classical waves.

Wheeler's delayed choice experiment can be explained by classical waves.

Can we really change ( choose ) the past events ??

Wheeler's delayed choice experiment is a thought experiment proposed by John Wheeler in 1978, and is confirmed experimentally later.
Surprisingly, in this experiment, we can choose (change) the photon's past state from the future !
This means we can control the past events from the future.
But in this real world, is it really possible ??
Here we try to explain the "tricks" of these mysterious phenomena.

Mach-Zehnder interferometer shows wave-particle of a photon ?

(Fig.1) "Particle" nature of a photon is observed ?

Fig.1 is a Mach-Zehnder interferometer.
In Fig.1, a single photon is split at the half beam splitter 1 ( BS 1 ).
(In the half beam splitter, almost half of the light is reflected, and the remaing half is transmitted. )
Of course, according to the quantum mechanical interpretation, a single photon can NOT be divided by the splitter.

A single photon is through ONLY one of two paths ?

So the single photon exists in only one of the two paths after the beam splitter according to the quantum mechanics.
When we detect a photon at the detectors D1 and D2, we can detect the single photon at only one of these two detectors at the same time. ( D1 or D2 )
(We can NOT detect the photon at D1 and D2 at the same time.)
This experimental result shows that the photon is a particle.
In Fig.1 when we detect the single photon at D1, it shows the photon has passed the A route.
( When we detect at D2, it has passed B route. )
So we can get the photon which-path information in Fig.1, and in this case the photon shows "particle" nature.

A single photon, but interference from two paths is observed !

(Fig.2) "Wave" nature of a photon is observed ?

In Fig.2, we insert the half beam splitter 2 (BS 2), too.
We suppose the beam splitter 2 ( BS 2 ) reverses the wave phase of only one reflection (downward), as shown in Fig.2.
( The transmitted light doesn't change. )
As a result, we always detect the single photon only at D2 detector by the interference from the A and B paths.
And at D1 detector, the electromagnetic waves from the A and B paths cancel each other. So we can not detect the photon at D1.
This means the photon shows "wave" nature by inserting the beam splitter 2 !

Delayed choice = we can change past events !?

This is a very mysterious thing.
According to Fig.1, after the photon has passed the beam splitter 1 ( BS 1 ), the photon becomes "particle".
But if we insert the beam splitter 2 after the photon has passed BS 1, the photon particle changes to the waves !
This means that we can change the past events ( particle or wave ) from the future ! (= delayed choice )
But in this real world, is it really possible ?

Wave or particle can be determined from the future !?

(Fig.3) Can we change (choose) the past event ( particle or wave ) ?

Delayed choice experiment = classical electromagnetic waves

Actually, we can explain these mysterious phenomena of the delayed choice experiment ONLY by the classical electromagntic ( E-M ) waves .
Here we show this mechanism.
As shown on this page, We suppose the photon detector can detect it when the light intensity is 6 as a single photon.
( So when its light intensity is less than 6, it can not be recognized at the detector as a photon, even if the electromagnetic wave exists. )

Classical wave + "detection threshold" is the trick of delayed choice.

For example, the first light intensity is 10, then it is split by the beam splitter ( BS1 ) into 6 + 4 waves.
( 10 is detected as one photon, not 2 photons (=12). )
The 4 side is not detected, only 6 side ( D1 or D2 ) is detected as a photon due to the detection threshold.
( If the E-M waves is split into 5 + 5, no photons can be detected. This case is not recognized at all, so it can be ignored. )
So we can explain the "particle" nature of the photon of Fig.1 by the classical electromagnetic waves.
When we insert another half beam splitter 2 ( BS 2 ) as shown in Fig.2, "6" is supposed to be split into "3+3", and "4" is supposed to be split into "2+2".
The light amplitude is a square root of the light intensity.
As shown in Fig.2, the D2 detector side can detect a single photon due to the increased amplitude by the interference.

Inteference + detection threshold at a single photodetector.

(Eq.1) Light intensity at D2. 

And the D1 side can not detect the photon by canceling the waves.
(Eq.2) Light intensity at D1. 

As a result, the wave nature of Fig.2 can be explained using the classical electromagnetic waves naturally !
This means the photon is originally NOT a particle but an electromagnetic wave. (= "Photon particle" is a illusion. )
This is the trick of the delayed choice experiment, and unfortunately we can not change (choose) the past event from the future.

Delayed choice quantum eraser can be explained by classical waves.

Which-path information changes photon into a particle.

Using the concept of Wheeler's delayed choice experiment, the "delayed choice quantum eraser" experiment was performed (Phys. Rev. Lett. 2000 84 1-5).
In this delayed choice quantum eraser experiment, depending on the measument devices, the photon changes to particle or wave.
When we know which route the photon has passed, the photon changes to a particle !
But when we erase the information of which route the photon has passed, the superposition state of a single photon into the two paths is restored, and the photon changes to wave !
(= Delayed choice quantum eraser. )
But is this strange phenomenon really occurring ??

Photon is wave or a particle ? ← we can control ?

(Fig.4) Delayed choice quantum eraser experiment.

In this experiment, an single photon is generated by Argon ion pump laser beam.
This laser (= single photon) is divided by the double slit into A and B paths (= what we call, "superposition" ).
( Or this single photon passes only one of slits, upper (= B ) or lower (= A ).)
After passing the slits, the beta barium crystal ( BBO ) causes spontaneous parametric down conversion (SPDC), converting the photon into two identical entangled photons with 1/2 the frequency of the original photon.
So the total energy is conserved.
( Surprisingly, a single photon particle can be divided into two particles by BBO ! )
In Fig.4 case, the light (= single photon ) with the intensity 10 is divided into 6+4 at the double slit apparatus.
And at each route ( A and B ), a pair of two identical photons with 1/2 frequency is generated ( 6 → 6+6, 4 → 4+4 ).
One of the two engtangled photons is sent to the upper route, and detected at D0 detector (= signal photon ).
( The position of the D0 detector can be moved in the x direction, as shown in Fig.4. )
And another photon (= idler photon ) is sent to the lower route, divided at the half beam splitters such as BSA, BSB, and BS.
( Of course, single photon particle can not be divided by the beam splitter according to the quantum mechanics.)

When photon is detected at D3 or D4 → which-path information (= A or B ) is gotten ?

As shown in Fig.4, when we detect the idler photon at D4, we can know the information that the idler photon (= single photon ) has passed the "B" slit.
(= which-path information ).
When we detect the idler photon at D3, we can know that the idler photon has passed the "A" slit.
Suprisingly, in these cases ( D3 or D4 detection ), the signal photon at D0 shows the "particle" pattern of the double slit experiment !
At the last beam splitter (= "BS" of Fig.4), only one reflection is supposed to reverse the light phase, as shown in Fig.5.
( The transmitted light doesn't change. )
(Fig.5) 

So if the idler photon has the "wave" property, the light can not be detected at D1 detector, because their waves always cancel each other.
And when we detect the idler photon at D2 (of course, in this case D3 and D4 can't detect the idler photon ), we can not know the idler photon has passed A or B.
Because the light from both A and B routes can enter the D2 detector (and interfere with each other).

When a photon is detected at D2, interference of a single photon happened !?

Suprisingly, in this case ( D2 detection ), the signal photon at D0 shows the "wave" interference pattern of the double slit experiment !
This means when we choose the detector ( D2, D3, or D4 ), the signal photon changes to particle or wave depending on it !
If we use D2 detector, the which-path information is erased, which means the superposition state of single photon is restored.
So the interference of the signal photon with itself occurs according to the quantum mechanical interpretation.
But does this strange phenomenon really occur ??

" Trick " of delayed choice quantum eraser.

Here we explain the reason why D0 detector shows "particle" pattern of double slit experiment when we detect the idler photon at D3 or D4detectors.
( Here we use only the classical electromagnetic wave theory to explain the delayed choice eraser like the upper section. )
(Fig.6) Which-path information ( A or B ) changes the photon into a "particle" ??

As shown in Fig.6, when we detect the idler photon at D4 detector, it means the idler photon passes the "B" route ( NOT "A" ).
The first single photon intensity is 10, and almost all of the light needs to pass the upper slit (= "B" ) for us to detect it at D4.
Because at the first half beam splitter (= BSB ), the light is split into almost half ( in the classical waves ).
Like the upper section, the light intensity needs to reach at least "6" to detect it as a single particle.

Detected at D4 → particle nature of photon appears ?

If the first light is divided at the double slit into A and B routes (for example, 10 → 4 (A) + 6 (B) ) like Fig.4, we can detect the idler photon atneither D3 nor D4.
Because at the first half beam splitter ( BSA or BSB ), the light is divided again ( for example, 6 → 3+3 ), which does NOT reach the threshold detection light intensity "6".
So, when we detect the idler photon at D4 (or D3 ), it means the first light passes ONLY "B" path (or only "A" path ).
As a result, the interference between A and B lights doesn't occur in the upward direction (= "D0" ), which shows "particle" pattern of Young double slit experiment.

Detected at D2 → wave nature (= interference ) of photon ?

(Fig.7) "Erasing" which-path information ( A or B ) changes the photon into a "wave" ??

Fig.7 shows the case in which we detect the idler photon at D2 detector.
Surplisingly, in these cases ( = D2 or D1 detection ), D0 detector shows the interference pattern between A and B lights.
Again, here we explain this trick using the usual classical electromagnetic waves.
If the first light passes only one ( A or B ) of the double slit as shown in Fig.6, can we detect the idler photon at D2 or D1 ?
Answer is NO ( we can detect the idler photon at neither D2 nor D1 when the first light is not divided at the double slit. ).
Because the light needs to pass two half beam splitters ( for example, BSB (BSA) and BS ) to reach D2 (or D1 ).
Then the first light intensity "10" becomes almost a quarter of the original value ( 10 → BSB (or BSA) → 5+5 → BS → 2.5 + 2.5 ) at D2 or D1, which is very difficult to detect as a photon ( we need "6" ).
To detect a single photon at D2 or D1, we have to take the power of the interference, as shown in Fig.7.
In Fig.7, the first light is divided into almost half "5+5" at the double slit.
After BBO, the idler photon ( = of course "classical" electromagnetic wave ) passes both the red and blue thick lines, and is divided again at BSB and BSA beam splitters ( for example, 5 → 3+2 ).
And furthermore, they are divided at the last beam splitter ( BS ) ( 3 → 1.5+1.5 ).
The light amplitude is a square root of the light intensity, and they are increased by the inteference from the red and blue lines.

Classical lights + detection threshold is the trick !

(Eq.3) Light amplitude by the interference after BS. 

The light intensity is a square of the light amplitude.
(Eq.4)

As a result, we can detect the idler photon at D2 ONLY when the first light is divided into almost half at the double slit !
In this case ( Fig.7 ), the interference from A and B routes occur also at D0 detector, which shows "wave pattern".
This is a trick of the delayed choice quantum eraser.
As you notice, if we admit the particle nature of the photon, we have to accept the strange concepts such as entanglement, delayed-choice experiment, and many-world like superposition.
But if we treat the photon as usual classical electromagnetic wave, we need not accept those strange ideas.
You can easily judge which case ( photon is a particle or wave ) is more natural.

2011/11/17 updated. Feel free to link to this site.

Wheeler's Classic Delayed Choice Experiment by Ross Rhodes N.B. Familiarity with The Reality Program Chapter 2 is assumed.
John Archibald Wheeler is one of those thinkers who takes the ideas of quantum mechanics seriously. After studying the Copenhagen explanation of the double slit experiment – with its emphasis on what the observer knows and when it is known – Wheeler realized that the observer's choice might control those variables in a test.      "If what you say is true," he said (in effect), "then I may choose to know a property after the event should already have taken place." [1]  Wheeler realized that in such a situation, the observer's choice would determine the outcome of the experiment – regardless of whether the outcome should logically have been determined long ago.     "Nonsense," said the reductionists. "Rubbish," said the materialists. "Completely absurd," said the naïve realists.  "Yup," said the mathematicians.     And so Wheeler's thought experiment and the predictions of quantum mechanics were brought to the laboratory for testing. [2]  This is what happens. Basic delayed choice
1. Photon (or other quantum unit) is sent toward double slit. 2. Photon passes through double slit unobserved, logically either through one, through the other, or through both. To obtain an interference pattern, we surmise that something must pass through both slits; to obtain a particle distribution, we surmise that the photon must pass through one or the other. Whatever the photon does, it presumably does it now when it passes through the slits. 3. After passing through the slits, the photon is in transit towards the back wall. At the "back wall," we have available two separate methods of detecting the photon. 4. First, we have a screen (or other detection system that can measure the horizontal placement of a photon hit, but is not able to distinguish where the photon came from). The screen can be removed, as indicated by the dotted line. It can be removed quickly, very quickly, after the photon has passed the double slits but beforethe photon reaches the plane of the screen. That is, the screen can be removed while the photon is in transit in region 3. Or the screen can be left in place. This is the experimenter's choice, which is delayed until after the photon has passed the slits (2) in whatever manner it happens to do so. 5. If the screen is removed, we reveal two telescopes. The telescopes are tightly focused on, watching, observing, just the narrow space of one slit only. The left telescope watches the left slit; the right telescope watches the right slit. (The mechanism/metaphor of the telescope assures that if you are looking through the telescope, you will see a flash of light if the photon went either wholly or in part through the slit on which it is focused; otherwise not. Therefore, you will obtain "which-path" information about the photon hit.)
Now suppose we have a photon in transit in region 3. The photon has already passed the slits.     We can yet choose to leave the screen in place, in which case we do not know which slit the photon went through.     Or we can choose to remove the screen. If we remove the screen, we will expect to see a flash at one telescope or the other (or both, except that never happens) for every photon sent through. Why? Because the photon has to go through either one slit, the other slit, or both slits. Those are all the possibilities. By watching both slits, we must see one of the following: a flash at the left telescope and no flash at the right telescope, indicating that the photon went through the left slit; or a flash at the right telescope and no flash at the left telescope, indicating that the photon went through the right slit; or a weak half-flash at both telescopes indicating that the photon went through both slits. Those are all the possibilities.     Upon observation at the screen QM tells us what we will get: Pattern 4r, which is exactly reminiscent of wave interference caused by two symmetrical waves, one emanating from each of the slits.     Upon observation at the telescopes QM tells us what we will get: Pattern 5r, which is exactly reminiscent of particle-like behavior traveling from the source, through one slit or the other, and being detected at the telescopes. Consider the difference in the experimental set up depending on our choice of detection. If we choose to leave the screen in place, we get a particle distribution consistent with the interference pattern that would be produced by two hypothetical symmetrical waves, each emanating from one of the slits. We might say (although we are extremely reluctant to say this) that the photon traveled as a wave from the point of origin, through both slits, and on to the screen.     On the other hand, if we choose to remove the screen, we get a particle distribution consistent with the clumping pattern that would be produced by particle motion from the point of origin through one slit or the other and to the left telescope or to the right telescope. After all, the particle "appeared" (we saw a flash) at one telescope or the other, rather than "appearing" at some other point along the length of the screen.     In summary, we have chosen whether to know which slit the particle went through, by choosing to use the telescopes or not, which are the instruments that would give us the information about which slit the particle went through. We have delayed this choice until a time after the particles "have gone through one slit or the other slit or both slits," so to speak. Yet, it seems paradoxically that our later choice of whether to obtain this information determines whether the particle passed through one slit or the other slit or both slits, so to speak. If you want to think of it this way (I don't recommend it), the particle exhibited after-the-fact wave-like behavior at the slits if you chose the screen; and it exhibited after-the-fact particle-like behavior at the slits if you chose the telescopes. Therefore, our delayed choice of how to measure the particle determines how the particle actually behaved at an earlier time. Does our choice "change the past"?     How long can we delay the choice? In Wheeler's original thought experiment, he imagined the phenomenon on a cosmic scale, as follows: 1. A distant star emits a photon many billions of years ago. 2. The photon must pass a dense galaxy (or black hole) directly in its path toward earth. "Gravitational lensing" predicted by general relativity (and well verified) will make the light bend around the galaxy or black hole. The same photon can, therefore, take either of two paths around the galaxy and still reach earth – it can take the left path and bend back toward earth; or it can take the right path and bend back toward earth. Bending around the left side is the experimental equivalent of going through the left slit of a barrier; bending around the right side is the equivalent of going through the right slit. 3. The photon continues for a very long time (perhaps a few more billion years) on its way toward earth. 4. On earth (many billions of years later), an astronomer chooses to use a screen type of light projector, encompassing both sides of the intervening and the surrounding space without focusing or distinguishing among regions. The photon will land somewhere along the field of focus without our astronomer being able to tell which side of the galaxy/black hole the photon passed, left or right. So the distribution pattern of the photon (even of a single photon, but easily recognizable after a lot of photons are collected) will be an interference pattern.5.  Alternatively, based on what she had for breakfast, our astronomer might choose to use a binocular apparatus, with one side of the binoculars (one telescope) focused exclusively on the left side of the intervening galaxy, and the other side focussed exclusively on the right side of the intervening galaxy. In that case the "pattern" will be a clump of photons at one side, and a clump of photons at the other side. Now, for many billions of years the photon is in transit in region 3. Yet we can choose (many billions of years later) which experimental set up to employ – the single wide-focus, or the two narrowly focused instruments. We have chosen whether to know which side of the galaxy the photon passed by (by choosing whether to use the two-telescope set up or not, which are the instruments that would give us the information about which side of the galaxy the photon passed). We have delayed this choice until a time long after the particles "have passed by one side of the galaxy, or the other side of the galaxy, or both sides of the galaxy," so to speak. Yet, it seems paradoxically that our later choice of whether to obtain this information determines which side of the galaxy the light passed, so to speak, billions of years ago.     So it seems that time has nothing to do with effects of quantum mechanics. And, indeed, the original thought experiment was not based on any analysis of how particles evolve and behave over time – it was based on the mathematics. This is what the mathematics predicted for a result, and this is exactly the result obtained in the laboratory. If you are serious about the idea that the result is determined only upon observation ... Can we delay the choice even longer? Can we delay the choice until after the photons have "hit" the telescopes or the screen? It turns out we can. We do so with a handy quantum eraser. But that's a whole 'nuther experiment.  See the Delayed Choice Quantum Eraser Commentary. This is great stuff, isn't it? Bonus picture!
Canton, Ohio March 23, 2003 References: [1] See Wheeler's "delayed choice", in Quantum Theory and Measurement, edited by J.A. Wheeler and W.H. Zurek, Princeton Univ. Press (1983). [2] E.g., V. Jacques, et al., "Experimental realization of Wheeler's gedankenexperiment," Science 315 966 (2007), e-print athttp://www.arxiv.org/abs/quant-ph/0610241 ; A.G. Zajonc et al., Nature, 353, 507 (1991); P.G. Kwiat et al., Phys. Rev. A 49, 61 (1994); T.J. Herzog et al., Phys. Rev. Lett., 75, 3034 (1995); T.B. Pittman et al., Phys. Rev. Lett., 77, 1917 (1996).  The Jacques experiment is described in Physics World, "Photons denied a glimpse at their observer" (Feb. 15, 2007), http://physicsworld.com/cws/article/news/27106 .

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http://www.bottomlayer.com/bottom/kim-scully/kim-scully-web.htm

Excerpts from  "A Delayed Choice Quantum Eraser" by Yoon-Ho Kim [1], R. Yu, S.P. Kulik, Y.H. Shih, and Marlon O. Scully http://xxx.lanl.gov/pdf/quant-ph/9903047 (citations omitted) Phys.Rev.Lett. 84 1-5 (2000). With commentary by Ross Rhodes RhodesR@BottomLayer.comwww.bottomlayer.comSome references to QM concepts introduced at length in The Reality Program, chaps. 1-2.

Abstract This paper reports a "delayed choice quantum eraser" experiment proposed by Scully and Drühl in 1982. The experimental results demonstrated the possibility of simultaneously observing both particle-like and wave-like behavior of a quantum via quantum entanglement. The which-path or both-path information of a quantum can be erased or marked by its entangled twin even after the registration of the quantum.

In the two-slit experiment, the common "wisdom" is that the position-momentum uncertainty relation . . . makes it impossible to determine which slit the photon (or electron) passes through without at the same time disturbing the photon (or electron) enough to destroy the interference pattern. However, it has been proven that under certain circumstances this common interpretation may not be true.     In 1982, Scully and Drühl [2] found a way around this position-momentum uncertainty obstacle and proposed a quantum eraser to obtain which-path or particle-like information without scattering or otherwise introducing large uncontrolled phase factors to disturb the interference. To be sure the interference pattern disappears when which-path information is obtained. But it reappears when we erase (quantum erasure) the which-path information.      Since 1982, quantum eraser behavior has been reported in several experiments; however, the original scheme has not been fully demonstrated. . . .      We wish to report a realization of the above quantum eraser experiment. The schematic diagram of the experimental setup is shown in Fig. 2.      [S]pontaneous parametric down conversion (SPDC) is used to prepare the entangled two-photon state. SPDC is a spontaneous nonlinear optical process from which a pair of signal-idler photons is generated when a pump laser beam is incident onto a nonlinear optical crystal.
	Comment: In this experiment, a single photon is aimed at the double-slit. If it passes through the left slit, it will hit a crystal placed behind the slit on the left side of the crystal; if through the right slit, it will hit the crystal on the right side. We will not actually be measuring the interference vel non of the incoming photon, but of the subsequently generated photons. When the incoming photon hits the crystal, it is destroyed and a pair of entangledphotons is generated by the crystal at the spot where it hit.  Because of entanglement, the properties of the two entangled photons will forever be correlated. Therefore, if we can later identify one of the entangled photons as having come from, say, the left side of the crystal, we will thereby know, retroactively, that its twin also came from the left side of the crystal, so that we will then know where bothentangled photons came from.  On the other hand, if we cannot later identify where either of the entangled photons came from, then we will have no which-path information for either.  The remainder of the experiment consists of manipulating the pair of entangled photons. We are able to preserve which-path information for as long as we are able to identify either one of the entangled photons with respect to which side of the crystal generated it. If at any time in the course of the experiment we lose the ability to determine which side of the crystal generated the entangled pair, we thereby lose the which-path information relating to both entangled photons.
In this experiment, the 351.1nm Argon ion pump laser beam is divided by a double-slit and incident onto a type-II phase matching nonlinear optical crystal BBO (b – BaB2O4) at two regions A and B.
	Comment: The laser is able to spit out photons in a highly controlled manner. Photons from the laser are aimed at the double-slit where they are "divided," in the QM sense, between the left slit and the right slit.  (This double slit is a bit of a red herring. It is only a method of randomizing emissions from region A or region B of the crystal, which are themselves the equivalent of the two slits of Young's experiment.)  The optical crystal is placed immediately behind the slits. An incoming photon shines on the crystal ("is incident upon the crystal") at two well-defined areas of the crystal (region A and region B) because the crystal is right up next to the slits. Note that, because of the nature of QM with its superpositions, we can say that a single incoming photon is "incident onto a … crystal … at two regions," despite the fact that it will "hit" the crystal at only one or the other region. Dividing the photon via the double slit makes its "impact" on the crystal subject to quantum randomness. That is, it is quantumly random whether the incoming photon will actually appear at region A or region B.
A pair of 702.2nm orthogonally polarized signal-idler photon is generated either from A or B region. The width of the SPDC region is about 0.3mm and the distance between the center of A and B is about 0.7mm. A Glen-Thompson prism is used to split the orthogonally polarized signal and idler.
	Comment:When the incoming photon hits the crystal, a pair of entangled photons is generated at the point of incidence – either region A of the crystal or region B of the crystal. The two regions of the crystal, A and B, are some distance apart. (This will be approximately the distance between the two slits.) In this experiment the separation is approximately 0.7 millimeters.  The entangled pair of photons generated by the crystal (at either region A or region B) are "orthogonally polarized." That is to say, if one of the pair is polarized horizontally, the other will be polarized at right angles ("orthogonally"), i.e., vertically; and vice versa. This correlation in the respective polarizations of the pair of photons is part and parcel of their "entanglement."  The Glen-Thompson prism at the crystal is used to separate the two photons generated at the crystal. One of the pair is sent in one direction, the other in a different direction. (Here bear in mind that because of the entanglement, QM dictates that the correlation in their respective polarizations will remain regardless of spatial separation.)  Although the pair of photons generated by the crystal are identical (and correlated), the experiment will use them in different ways. Thus, one of the pair (sent in one direction) is called the "signal" photon; the other of the pair (sent in another direction) is called the "idler" photon. The designation of a particular photon as "signal" or "idler" is a matter of convention based on which direction they are sent (and how they will be used in the experiment), not any inherent difference between the two entangled photons generated at the crystal.
The signal photon (photon 1, either from A or B) passes a lens LS to meet detector D0, which is placed on the Fourier transform plane (focal plane for collimated light beam) of the lens. The use of lens LS is to achieve the "far field" condition, but still keep a short distance between the slit and the detector D0.
		Comment:"Collimated" means "made parallel." The lens LS focuses photons from either of these paths onto a single detector D0.  The lens LS also allows us to control the distance (f) from the place of origin (region A or B) to the detector D0. In this way, we can fix the time it will take for the signal photon to be detected at D0.  The "far field" condition essentially makes the photons arrive at D0as though they had traveled a long distance, even though the path is foreshortened. Like binoculars.
Detector D0 can be scanned along its x-axis by a step motor.
		Comment:Scanning the detector along one axis provides the position information – where the signal photon actually landed on the detector D0.  At this point, we do not have which-path information. (The straight lines of Fig. 2 are diagrammatic only.) Detector D0 by itself will not be able to distinguish between a photon originating at region A, and a photon originating at region B, because nothing about this part of the experiment provides which-path information.  Without more, we would expect the pattern developing at detector D0 to be an interference pattern. QM predicts that without which-path information, photons arriving from either A or B should interfere and distribute themselves one-by-one according to the statistical distribution of interfering waves.  Mind you, if we did have which-path information, the results should be quite different. In that case, QM would predict the "clumping" pattern typical of particle motion.
The idler photon (photon 2) is sent to an interferometer with equal-path optical arms. The interferometer includes a prism PS, two 50-50 beamsplitters BSA, BSB, two reflecting mirrors MA, MB, and a 50-50 beamsplitter BS.
	Comment: The idler photon first encounters the prism PS, where it's path is bent to ensure it heads off where it is supposed to – one path for photons from region A, a different path for photons from region B. The idler photon next encounters a 50-50 beamsplitter BSA or BSB. The beamsplitter will either reflect the idler photon off course and into the detector D3 or D4; or it will allow the photon to pass through and continue (toward the reflecting mirror MA or MB). QM dictates that it will go one way or the other a random 50% of the time, i.e., a 50-50 chance either way.  If the idler photon is reflected at BSA or BSB into the detector D3 or D4, it will be detected with which-path information intact. Therefore, for every photon detected at D3 or D4, we know which region of the crystal generated both it and its twin signal photon: if detected at D3, we know that both twins came from region A; if detected at D4, then both twins came from region B.  As stated earlier in the paper, "The registration of D3 or D4 provides which-path information (path A or path B) of [idler] photon 2 and in turn provides which-path information of [signal] photon 1 because of the entanglement nature of the two-photon state . . .."  If the idler photon is not reflected at BSA or BSB, it will continue toward its next encounter, the reflecting mirror MA or MB. There it will be reflected toward the single 50-50 beamsplitter BS, which is the quantum eraser.  Here it is most interesting to note, as do the authors in the published version of the paper, that the "choice" of which direction the photon will take at BSA or BSB is made by QM itself. That is, the path at this juncture is 50-50 random. As we will see, this "choice" will determine the information available at the conclusion of the experiment. (The authors note that in other quantum eraser experiments, the choice is made by the experimentalist. [3])  The beamsplitter BS randomizes the path information, and thereby makes the which-path information unavailable, in the following manner.  For an idler photon coming from reflecting mirror MA, the beamsplitter BS will eitherreflect the idler photon into the detector D1; or it will allow the photon to pass through and continue toward the detector D2. There is a 50-50 chance either way. Similarly, for an idler photon coming from the other reflecting mirror MB, the beamsplitter BS willeither reflect the idler photon into the detector D2; or it will allow the photon to pass through and continue toward the detector D1.  You can see that, with this 50-50 beam splitter, any photon arriving at detector D1 or D2 has an equal 50-50 chance of having been reflected from reflecting mirror MA or MB. It is therefore impossible to tell whether the photon was generated at region A of the crystal, or at region B. The which-path information encoded into the photons arriving at detectors D1 and D2 has been "erased."
Detectors D1 and D2 are placed at the two output ports of the BS, respectively, for erasing the which-path information.      The triggering of detectors D3 and D4 provide which-path information of the idler (photon 2) and in turn provide which-path information of the signal (photon 1).
	Comment:This is key. There is no which-path information for the signal photons when they initially arrive at D0. Which-path information for those signal photons is obtained onlylater, when the twin idler photon is later detected at D3 or D4 (and not obtained if the twin idler photon is detected at D1 or D2).  As discussed below, the experimental setup ensures that this which-path information for the signal photons is obtained or erased only after the signal photon has been detected and the information is winging its way toward the Coincidence Circuit.
The electronic output pulses of detectors D1, D2, D3, and D4 are sent to coincidence circuits with the output pulse of detector D0, respectively, for the counting of "joint detection" rates R01, R02, R03, and R04.
	Comment:Detector D0 fires once for every photon generated by the pump laser, because regardless of whether the photon hits region A or region B, a signal photon is sent to D0 for detection. By comparing the detections of the signal photons at D0 with the other detectors, we can determine where the corresponding idler photon hit. There will be only one joint detection for each incoming photon from the pump laser. R01 records a joint detection at detector D0 and detector D1 (which-path info not available) R02 records a joint detection at detector D0 and detector D2 (which-path info not available)  R03 records a joint detection at detector D0 and detector D3 (which-path info available)  R04 records a joint detection at detector D0 and detector D4 (which-path info available)  The joint detection event at detectors D3 or D4 (given by R03, R04) gives us new information about a photon that was previously registered at detector D0. We have already registered that photon, and the scanning has registered where it hit D0 – its position along the D0 x-axis. But when we correlate the later detection of the idler at D3 or D4, with that previous detection at D0, we now know that the photon registered at D0 came from either region A or region B of the crystal.  QM predicts that if which-path information is not available at the time of measurement, the pattern will be an interference pattern, as though wave-like photons passed through both slits and "interfered with themselves" to produce the distinctive interference pattern of hits. This is the case at detector D0 at all times.  Because which-path information is not available for photons registered at D0 even after a joint detection at the post-erasure detectors D1 and D2, we learn nothing new about the detections that have occurred at D0 and so QM predicts that R01 and R02 will exhibit this interference pattern in counting photon hits at D0.  QM also predicts that if which-path information is available at the time of measurement, the pattern will be a "clumping," as though particle-like photons passed through a slit and on to a detector in a more-or-less straight line. Because which-path information is available for photons registered at D0 once a joint detection has been indicated at the pre-erasure detectors D3 and D4, QM predicts that R03 and R04 will exhibit this "clumping" pattern. In this experiment, "the time of measurement" is after the correlation of the joint detections, which takes place at the Coincidence Circuit. However, the count of photon hits that will be displayed represents hits at D0, registered earlier.
In this experiment the optical delay (Li-L0) is chosen to be @ [approximately equal to] 2.5m, where L0 is the optical distance between the output surface of BBO and detector D0, and Li is the optical distance between the output surface of theBBO and detectors D1, D2, D3, and D4, respectively. This means that any information one can learn from photon 2 must be at least 8ns later than what one has learned from the registration of photon 1. Compared to the 1ns response time of the detectors, 2.5m delay is good enough for a "delayed erasure".
	Comment: The delay between the detection of one paired photon at D0 and the detection of its twin at D1, D2, D3 or D4 is the feature that drives home the point. At the moment when D0 registers a photon, all of the following still apply: There is no which-path information about the signal photon (photon 1) detected at D0.  The which-path information about its twin idler photon (photon 2) is "available" because the twin is still in flight along a well-defined path coming from either region A or region B. The twin has not yet reached the first 50-50 beamsplitterBSA or BSB.  The which-path information about the signal photon itself is "available" in principle at D0. This is because we can correlate every photon at D0 with its twin, so that learning the which-path information about the twin (at D3 or D4) will necessarily reveal the which-path information of the signal photon at D0, albeit after-the-fact.  But the signal photon has already been detected, and the information about its position at D0 is already winging its way to the Coincidence Circuit, while the information about its twin idler photon is still inchoate.  The detecting mechanism that has tagged the which-path information (i.e., the generation of an entangled pair at either region A or region B) has already been accomplished, but it has not yet yielded up its which-path information to any observer.  However, at the subsequent moment when the twin is detected at D1, D2, D3 or D4, the which-path information may or may not be available, depending on whether the detection occurs at D1 or D2 (which-path information not available), or at D3 or D4(which-path information available).  As stated earlier in the paper, "After the registration of photon 1 [at D0], we look at these 'delayed' detection events of D1, D2, D3, and D4 which have constant time delays . . . relative to the triggering time of D0. It is easy to see these 'joint detection' events must have resulted from the same photon pair."
Figs. 3, 4, and 5 report the experimental results, which are all consistent with prediction.       Figs. 3 and 4 show the "joint detection" rates R01 and R02 against the x coordinates of detector D0. It is clear we have observed the standard Young's double-slit interference pattern. . . .        Fig. 5 reports a typical R03 (R04), "joint detection" counting rate between D0 and "which-path" D3 (D4), against the xcoordinates of detector D0. An absence of interference is clearly demonstrated. There is no significant difference between the curves of R03 and R04 except the small shift of the center.
	Comment: To the physicist, the results "are all consistent with prediction." To the layperson, the results should be shocking. Let us review the course of the experiment as it unfolds, beginning when the incoming photon from the laser generates an entangled pair at the crystal.  Time 1. The entangled pair leaves either region A or region B of the crystal. The signal photon heads off to detector D0, and the idler photon heads off to the interferometer.  Time 2. The signal photon is registered and scanned at detector D0according to its position. This information (the position of the signal photon upon "impact" at D0) is sent on its way to the Coincidence Circuit.  Time 3. The idler photon reaches the first pair of beamsplitters, BSA,BSB. There, QM makes a choice which direction the idler photon will go – either to detectors D3, D4; or to the quantum eraser BS and on to detectors D1, D2.  Time 4a. If the idler photon is shunted to detectors D3, D4, it is detected with which-path information intact. Then and only then do we know which-path information for its twin signal photon that already has been detected, scanned, registered and recorded at D0.  Time 4b. If the idler photon passes through to detectors D1, D2, it is detected with no which-path information (the which-path information having been "erased" at BS).  Time 5. The Coincidence Circuit correlates the arrival of a signal photon at detector D0 with the arrival of its twin at D1, D2, D3, or D4. If the correlation is with an idler arriving at D3 or D4, then we know (after-the-fact) the which-path information of the signal photon that arrived earlier at D0. If the correlation is with an idler arriving at D1 or D2, then we have no which-path information for the signal photon that arrived earlier at D0.  Time 6. Upon accessing the information gathered by the Coincidence Circuit, we the observer are shocked to learn that the pattern shown by the positions registered at D0 at Time 2 depends entirely on the information gathered later at Time 4 and available to us at the conclusion of the experiment.  The position of a photon at detector D0 has been registered and scanned. Yet the actual position of the photon arriving at D0 will be at one place if we later learn more information; and the actual position will be at another place if we do not.  Ho-hum. Another experimental proof of QM. This is the way it works, folks.  Canton, Ohio September 4, 2002
Fig. 2 from Yoon-Ho Kim, et al., "A Delayed Choice Quantum Eraser" http://xxx.lanl.gov/pdf/quant-ph/9903047  Phys.Rev.Lett. 84 (2000) 1-5   [additions by Ross Rhodes]
Endnotes1. Your commentator wishes to thank Dr. Kim for reviewing this commentary before posting. 2. M. O. Scully and K. Drühl, Phys. Rev. A 25, 2208. "Quantum eraser: A proposed photon correlation experiment concerning observation and 'delayed choice' in quantum mechanics." 3. T. Helmuth, et al., Phys. Rev. A 35, 2532 (1987); J. Baldzuhn, E. Mohler, and W. Martienssen, Z. Phys. B 77, 347 (1989); B.J. Lawson-Daku et al., Phys. Rev. A. 54, 5042 (1996).  More recently, see V. Jacques, et al., "Experimental realization of Wheeler's gedankenexperiment," Science 315 966 (2007), e-print athttp://www.arxiv.org/abs/quant-ph/0610241 .

Thought experiments in quantum mechanics

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Pages in category "Thought experiments in quantum mechanics"

The following 12 pages are in this category, out of 12 total. This list may not reflect recent changes (learn more).

E Elitzur–Vaidman bomb tester EPR paradox H Hardy's paradox Heisenberg's microscope R Renninger negative-result experiment S Schrödinger's cat W Wheeler's delayed choice experiment Wigner's friend

I Interaction-free measurement Q Quantum pseudo-telepathy Quantum suicide and immortality Quantum tic-tac-toe