Interference seen in a member of an entangled pair

[JesseM]

Has anyone else here read with care the original Q.E. paper written by Scully and his colleagues in 1978? (Phys. Rep. 43, 13, p. 485) They analyze a modified S-G experiment, including a bi-level atom in one arm of the magnet as path detector. Under certain assumptions they derive a nearly diagonalized density matrix for the spinning molecule beyond the magnet, without any wavefunction perturbation at measurement. They infer from this that an erasure of information stored in the path detector should restore the interference pattern destroyed when which path was detected.

Because the S-G example is so important for understanding the quantum measurement process, I’d like to promote an experimental demonstration that the spinning particle actually emerges from the magnet in just one of the two paths, not in a superposition of both. It’s a simple, inexpensive experiment.

I still don't quite understand what this "experimental demonstration" would be--are you just talking about the experiment in the previous paragraph where Scully et al. "infer from this that an erasure of information stored in the path detector should restore the interference pattern", and are you proposing that they would be wrong, that it would be impossible to restore the interference pattern by erasing which-path information? What is the specific setup that they propose would erase this information, is it an interferometer similar to what's used to erase the which-path info of idler photons in the delayed choice quantum eraser?

 

[DrChinese]

Doing the experiment I’ve mentioned would require very little funding, almost inconsequential by standards of most such contemporary experiments. I’m sure it could be done in a small undergraduate college lab. If anyone is interested, please contact me via e-mail.

DocMike

You don't seem to want to address the fact that S-G type experiments are no different than what you can do with a photon and a polarizing beam splitter. These (photons + PBS) are very easy to do and a zillion versions have already been done. So what is so special about S-G? I mean, it's not like electrons display behavior that is fundamentally different than photons with regards to interference or spin. You say you want to understand what is "really" going on, but so do we all. You might be surprised to realize exactly how much new research is done in this area, both theoretically or experimentally. For example, check out any of these 100 articles:

Stern Gerlach

 

[Dbar_x]

I cannot find this paper, have you got a web reference?

Debra, I've searched for the title, "State Reduction in Quantum Mechanics: a Calculational Example", and not found a copy on the web. I got my copy the old way, from the journal, Physics Reports in 1978.

I still don't quite understand what this "experimental demonstration" would be--are you just talking about the experiment in the previous paragraph where Scully et al. "infer from this that an erasure of information stored in the path detector should restore the interference pattern", and are you proposing that they would be wrong, that it would be impossible to restore the interference pattern by erasing which-path information? What is the specific setup that they propose would erase this information, is it an interferometer similar to what's used to erase the which-path info of idler photons in the delayed choice quantum eraser?

There have been some relatively simple quantum eraser experiments published, like the one by Kwiat and Hilmer in Scientific American, "A Do-It-Yourself Quantum Eraser" in 2007, and Schneider and LaPuma in the American Journal of Physics, "A simple experiment for discussion of quantum interference..." in 2002, and Galvez et al. also in the Am. J. Phys. 2005, "Interference with correlated photons...". But the experiment I'm promoting is not a quantum eraser experiment. Rather, as I said, it's a simple experiment to demonstrate that our accepted understanding of the Stern-Gerlach experiment is mistaken. It seems to be taken for granted today, and was also assumed by Scully et al. in 1978, that the spinning silver atoms emerging from the magnet remain in a superposition of the two spin directions. I'm convinced that's incorrect. Each silver atom emerges from the magnet with a single definite spin direction, either up or down, because a measurement has occurred. Magnet deflection (macroscopic momentum transfer) has recorded spin direction, performed a measurement, or observation, and the result is an atom now with a definite eigenvalue of spin.

Essentially all quantum measurement analyses are based on our prototype, the S-G experiment. So it's cruicial, I believe that we actually understand that phenomenon. Thus the simple experiment I have in mind. I believe the results of this experiment must result in rethinking and reanalysis of essentially all our current measurement theories, including quantum erasure and Zurek's decoherence.

The simple experiment I'm proposing is a modification to the original one from the 1920's. So equipment required includes a small vacuum system, a source of polarized particles, like an oven to vaporize silver, and a powerful magnet. The results, I'm convinced, would be convincing. I don't own vacuum equipment and such, but anyone with access to that stuff who would like to discuss the experiment, may contact me via e-mail.

DocMike

 

[Dbar_x]

I still don't quite understand what this "experimental demonstration" would be--are you just talking about the experiment in the previous paragraph where Scully et al. "infer from this that an erasure of information stored in the path detector should restore the interference pattern", and are you proposing that they would be wrong, that it would be impossible to restore the interference pattern by erasing which-path information? What is the specific setup that they propose would erase this information, is it an interferometer similar to what's used to erase the which-path info of idler photons in the delayed choice quantum eraser?

Are you a skeptic of quantum erasure? Perhaps you know that Kwiat wrote about all the original Q.E. experiments, including his own experiments at Berkeley, and those of Mandel and coleagues at Rochester, that “none of these possesses all the attributes of a true quantum eraser.” Senitsky (Phys. Rev. Lett. 73, 3040, 1994) at the laser laboratory at U. of Southern California showed that the experiment Scully finally believed really demonstrated quantum erasure, the somewhat exotic crossed beams done by Zeilnger, et al. (“Frustrated Two-Photon Creation via Interference”, Phys. Rev. Lett. 72 629, 1994) was not an interference experiment at all, but rather an example of parametric amplification that can be completely explained by classical electromagnetic theory. I agree with him.

You don't seem to want to address the fact that S-G type experiments are no different than what you can do with a photon and a polarizing beam splitter. These (photons + PBS) are very easy to do and a zillion versions have already been done. So what is so special about S-G? I mean, it's not like electrons display behavior that is fundamentally different than photons with regards to interference or spin. You say you want to understand what is "really" going on, but so do we all. You might be surprised to realize exactly how much new research is done in this area, both theoretically or experimentally. For example, check out any of these 100 articles:

Happy to addrss it. The cruicial question, I think, is not whether photons and atoms both exhibit interference, but what exactly constitutes a quantum measurement, and what doesn’t. The question has definitely not been settled by physicists, resulting, I believe, in the continued confusion in quantum theory. Many measuement theorests today claim, for example, that what we do or do not know is an essential ingredient in the measurement process. Some, including Kwiat, have written that knowledge and the visability of interference are complementary variables in the Bohr theory (of complementarity). I think that the quantum theory, like all other physical theories, must be observer independent.

Obviously, I believe that the spin direction of silver atoms exiting a S-G magnet has been measured, so resulting in each atom going either one way or the other, not both. There is no experimental evidence of S-G interference, or anything else, yet, to refute or confirm my contention. That includes all the photon interference experiments.

If the atoms from a S-G magnet had not been measured, and actually remained in a spin superposition, there would not be the two separated traces left on the detector. If the atom continued in its spin supeposition it would be kicked back and forth, randomly, by each quantum in the macroscopic field, and would emerge still centered on the magnet, not at one of two separated spots. Instead, the measurement is amplified by each successive quantum exchange, certainly not something done in a photon beam splitter. A beam splitter cannot record (or register, or measure) which way the photon went. If it did measure which way, the photon would be reduced to just one path, with no interference.

 

[JesseM]

I still don't quite understand what this "experimental demonstration" would be--are you just talking about the experiment in the previous paragraph where Scully et al. "infer from this that an erasure of information stored in the path detector should restore the interference pattern", and are you proposing that they would be wrong, that it would be impossible to restore the interference pattern by erasing which-path information? What is the specific setup that they propose would erase this information, is it an interferometer similar to what's used to erase the which-path info of idler photons in the delayed choice quantum eraser?

Are you a skeptic of quantum erasure?

I see no reason to be, why would you imagine that? I was just asking whether you thought the way to test whether an electron (or atom) passing through a Stern-Gerlach device is in a superposition of states before its direction is measured would be to do some sort of erasure of the which-path information which according to orthodox QM should allow us to observe an interference pattern (which would presumably show that the wavefunction could not have been collapsed due to the SG device alone). If that's not what you were getting at with your comment "They infer from this that an erasure of information stored in the path detector should restore the interference pattern destroyed when which path was detected", then what would you consider a valid experimental demonstration that the electron is indeed in a superposition of states after leaving the SG device (but before its deflection angle is measured by us)? The only way I can think of to demonstrate superposition experimentally is to show that some type of interference effects can be measured...

Perhaps you know that Kwiat wrote about all the original Q.E. experiments, including his own experiments at Berkeley, and those of Mandel and coleagues at Rochester, that “none of these possesses all the attributes of a true quantum eraser.” Senitsky (Phys. Rev. Lett. 73, 3040, 1994) at the laser laboratory at U. of Southern California showed that the experiment Scully finally believed really demonstrated quantum erasure, the somewhat exotic crossed beams done by Zeilnger, et al. (“Frustrated Two-Photon Creation via Interference”, Phys. Rev. Lett. 72 629, 1994) was not an interference experiment at all, but rather an example of parametric amplification that can be completely explained by classical electromagnetic theory. I agree with him.

I don't know anything about the specifics of that particular experiment, but are you claiming that all quantum eraser experiments can be explained in a classical way? What about the delayed choice quantum eraser, for example?

 

[debra]

It seems to me that the crux of the matter is to decide if entangled electrons passing through a magnetic field removes the entanglement effect between the electrons.

 

[QuantumBend]

Particle no entangling after magnet field. We know this. Many wrong thing in theory - no good.

 

[DrChinese]

... The cruicial question, I think, is not whether photons and atoms both exhibit interference, but what exactly constitutes a quantum measurement, and what doesn’t. The question has definitely not been settled by physicists, resulting, I believe, in the continued confusion in quantum theory. Many measuement theorests today claim, for example, that what we do or do not know is an essential ingredient in the measurement process. Some, including Kwiat, have written that knowledge and the visability of interference are complementary variables in the Bohr theory (of complementarity). I think that the quantum theory, like all other physical theories, must be observer independent.

Obviously, I believe that the spin direction of silver atoms exiting a S-G magnet has been measured, so resulting in each atom going either one way or the other, not both. There is no experimental evidence of S-G interference, or anything else, yet, to refute or confirm my contention. That includes all the photon interference experiments.

If the atoms from a S-G magnet had not been measured, and actually remained in a spin superposition, there would not be the two separated traces left on the detector. If the atom continued in its spin supeposition it would be kicked back and forth, randomly, by each quantum in the macroscopic field, and would emerge still centered on the magnet, not at one of two separated spots. Instead, the measurement is amplified by each successive quantum exchange, certainly not something done in a photon beam splitter. A beam splitter cannot record (or register, or measure) which way the photon went. If it did measure which way, the photon would be reduced to just one path, with no interference.

OK, I guess this makes sense... I guess I didn't follow (and still don't) where you are going with this. I think you are asking WHEN is the observation to have occurred, and I can see your point that this is not specified precisely in current theory.

S-G is usually used to orient particle spin in a particular direction and/or show that the value of the spin is quantized. A PBS does the same basic thing with light, so I guess I am not seeing your point here either... I don't see how the physical impact of particle on the S-G apparatus is fundamentally any different than that of the PBS.

You have indicated your preference for the view that reality is observer independent, so I am guessing your proposed experiment is intended to show this one way or the other. After reading your posts again, it seems like you are well aware of key work in the field. So that leaves me just a bit confused, because there have been so many experiments that have demonstrated observer dependence - or at a bare minimum, failed to demonstrate observer independence where it might have been expected. I don't fault anyone for believing in observer independence, mind you, I just question how another experiment is likely to provide a result any different than all the others. But I acknowledge that there is always the better idea out there, hopefully yours is the one. :)

 

[Dbar_x]

It seems to me that the crux of the matter is to decide if entangled electrons passing through a magnetic field removes the entanglement effect between the electrons.

Particle no entangling after magnet field. We know this. Many wrong thing in theory - no good.

If I understand entanglement (I hope I do), Debra, the single electron, or silver atom, moving through a S-G magnet is not entangled, as QuantumBend said. It’s just a single particle. Entanglement always entails two or more particles, components of what von Neumann called a composite object, described by a single wavefunction. Think of the two photons in Aspect’s experiment, moving away in opposite directions. Because of the property of the composite system, total spin in that case, each photon must exhibit an opposed spin direction when measured.

I believe the crux of the matter, Debra, is whether the S-G magnet is the measuring apparatus which reduces the particle’s spin superposition to a single eigenvalue. I’m convinced that no detector screen is necessary.

DocMike

 

[JesseM]

believe the crux of the matter, Debra, is whether the S-G magnet is the measuring apparatus which reduces the particle’s spin superposition to a single eigenvalue. I’m convinced that no detector screen is necessary.

And again, what sort of experimental result would it take to convince you that you are wrong about this?

 

[Dbar_x]

I am skeptical of quantum erasure. I’m a committed believer in rational argument and directed, repeatable experimentation as the fundaments of science. Perhaps everyone else is also. I think we ought not believe anything presented as scientific fact unless it’s consistent, comprehensive, comprehensible, and has been subjected to experimental tests by refutation. (Not just confirmation, I would suggest, since a confirmatory test usually can be explained by another theory, as Senitzky did with the Zeilinger-Kwiat experiment.) I think it’s a profound mistake for us to accept something as scientific fact just because nearly everyone else is saying the same thing.

We must know how wrong the majority can be, even in science. Remember the difficulties Galilleo endured, or Boltzmann, whose rash belief in atoms was treated with disdain by nearly every scientist of his day. Plate techtonics is a more recent example.

So, I want to see the scientific evidence for quantum erasure, not just scores of similar publications. As I’ve said, the years of Q.E. experiments done at Berkeley and Rochester were dismissed afterwards by the experimenters themselves. If they don’t believe it...

some sort of erasure of the which-path information which according to orthodox QM should allow us to observe an interference pattern

I don’t think, JesseM, that information erasure at the detector, causing interference restoration, is orthodox quantum mechanics. What’s the basis in theory? There can be no reversible measurement via Schrodinger development; mixtures do not continuously evolve from superpositions. And Q.E. advocates do not accept von Neumann’s collapse theory. The closest I’ve seen to a plausible theory of Q.E. is from Fedorh Herbut this year. He suggests that the Everett-like relative state interpretation may be necessary. Or what he calls relative reality of unitarily evolving states.

I don't know anything about the specifics of that particular experiment, but are you claiming that all quantum eraser experiments can be explained in a classical way? What about the delayed choice quantum eraser, for example?

The recent delayed choice quantum eraser experiment, in which Scully participated, (Phys. Rev. Lett. 84, 1; “Delayed ‘Choice’ Quantum Eraser” ) is supposed to be the implementation of the original micromaser quantum eraser proposed by Scully and Druhl in 1982. If one reads the article critically (not simply accepting its conclusions) and carefully, it’s easy to see that not even their mathematical development is credible.

Look at Fig. 2 and their mathematical notation. They call L₀ the optical path length from the location on the BBO crystal where the bi-photon pair is created to the position on the interference pattern. (D₀, that is, where the signal photon is detected.) Crucially, the authors treat L₀ as a constant. It surely is not, but depends on which slit produces the photon pair and on location x. On the other hand, L_j (just below equation 3) is treated as a variable, dependent on x, but is, instead a constant optical distance. Contrary to what they say, t is the time when the photon pair is produced at the crystal.

Moreover, there’s no information erasure, at all, that occurs in this experiment, is there? The which way information is stored at either detector D₃ or D₄ (presumably on a computer disk). It is never erased, but is used, at a future time, to correlate with those coincident photons at D₀ which produce no interference.

I’ve done as much calculation as I can based on their limited published physical parameters, and I’m convinced that all their results, figures 3, 4, and 5, are accounted for by two pump photons at the crystal simultaneously creating four, not two, down-converted photons. If the detector thresholds are set for two simultaneous photons, (interpreting a single photon signal as jitter) one sees all the results they interpret as quantum erasure. If it were just one signal and one idler photon, as claimed, the coincidence rate in figure 4 ought to be about 30,000 per second, not about 150. That’s based on the singles rate of 300,000 they show (figure 4) and an overall detection efficiency of about 10 per cent. Because two simultaneous pump photons at the crystal are much less likely than just one, they actually get the low coincidence rate of about 150.

I think you are asking WHEN is the observation to have occurred, and I can see your point that this is not specified precisely in current theory.

I believe the key, DocChin, is not when the measurement occurs, but which object is the measuring apparatus. In this case, does the S-G magnet, itself, record spin direction due to the macroscopic energy and momentum transfer that occurs? Or is it necessary, as our textbooks say, to insert a detector screen to complete the measurement?

I don't fault anyone for believing in observer independence, mind you,

Let’s recall the history of physics. Galillean relativity is fundamental to all of classical physics. Einstein’s special and general theories are founded on the basic notion that all the laws of science are covariant. Form invariant, that is, in the reference frame of every observer. Does it make any sense to discard observer independence because some of the many, contradictory, proposed theories of quantum measurement require observer dependence?

what would you consider a valid experimental demonstration that the electron is indeed in a superposition of states after leaving the SG device (but before its deflection angle is measured by us)? The only way I can think of to demonstrate superposition experimentally is to show that some type of interference effects can be measured...

No interference pattern is required, JesseM. Contact me via e-mail if you’d like to discuss the experiment.

DocMike

 

[JesseM]

I don’t think, JesseM, that information erasure at the detector, causing interference restoration, is orthodox quantum mechanics. What’s the basis in theory? There can be no reversible measurement via Schrodinger development; mixtures do not continuously evolve from superpositions.

Can you elaborate on why you think quantum erasure would require "reversible measurement" or mixtures evolving from superpositions? The delayed choice quantum eraser involves the "signal" photon that goes through the slits being entangled with an "idler" which can tell you which slit the signal photon went through if it goes to one set of detectors (D3 or D4 in Scully's paper) but this information will be lost if it's detected at a different set of detectors (D1 or D2). If the signal photons are detected first, then you could use the 2-particle wavefunction for the signal/idler pair to calculate the probability distribution for just the signal photon to end up at different points on the screen; if the idlers are detected first, then according to the standard procedure for calculating probabilities this would cause a discontinuous collapse in the 2-particle wavefunction which would alter the probabilities of the signal photon landing at different points on the screen. However, I presume that if you calculate P(signal photon detected at position X|idler detected at detector D1)*P(idler detected at detector D1) + P(signal photon detected at position X|idler detected at detector D2)*P(idler detected at detector D2) + P(signal photon detected at position X|idler detected at detector D3)*P(idler detected at detector D3) + P(signal photon detected at position X|idler detected at detector D4)*P(idler detected at detector D4), then you should get almost exactly the same answer as you'd get for P(signal photon detected at position X) in the case where the signal photon is detected first so there was no prior state reduction in the 2-particle wavefunction caused by the detection of the idler (I say almost exactly because there may be cases in which the idler just misses all 4 detectors, but you should be able to make it exact by adding additional detectors so that there was no possible direction the idler could go without hitting a detector).

No interference pattern is required, JesseM. Contact me via e-mail if you’d like to discuss the experiment.

Required for what, exactly? I'm specifically asking for an experiment that could falsify your claim that the atoms are not in a superposition after traveling through the SG device, not one that you think would "prove" that claim (if you're thinking of the latter type of experiment, it may be that your idea of proof is based on a misconception, and that in fact orthodox QM would predict exactly the same result that you predict even if we assume the particle does remain in a superposition after passing through the device, with the collapse not occurring until we actually detect its position). So, are you in fact thinking of an experiment that would falsify your claim, and that would provide evidence that the atom was in a superposition after it passed through the SG device?

 

[QuantumBend]

I believe the crux of the matter, Debra, is whether the S-G magnet is the measuring apparatus which reduces the particle’s spin superposition to a single eigenvalue. I’m convinced that no detector screen is necessary.

DocMike

OK, so particle not interfere with itself after go out from magnet field. Screen show no interference. You want proove this - yes / no?

 

[msumm21]

Back to an earlier discussion within this thread regarding the Dopfer experiment. I wanted to clarify why I would be surprised if the pattern seen by Bob after performing the experiment a million times were the same, independent of the location of Alice's detector D1. Here are some definitions similar to the ones given before:

D1 at focal plane:
A1 - Probability distribution function A1 is revealed behind the double slit when D1 fails to register (failure to register occurs with probability P1).
B - Probability distribution function B is revealed behind the double slit when D1 registers a photon (as colorspace pointed out this may be a superposition of numerous interference patterns).

D1 at imaging plane:
A2 - Probability distribution function A2 is revealed behind the double slit when D1 fails to register (failure to register occurs with probability P2).
C - Probability distribution function C is revealed behind the double slit when D1 registers a photon.

Note: initially I said A=A1=A2, but it was pointed out by colorpsace that A1 and A2 are indeed different.

Now, given that A1 and A2 are different, and evidentially B and C are different, why must it be true that P1*A1+(1-P1)*B (the distribution when D1 is at the focal plane) is the same as P2*A2+(1-P2)*C (the distribution when D1 is at the imaging plane)? For B, C, P1, and P2 to take on values in just the right way to make the equality hold seems too remarkable.

Also, I believe someone said these results (overall net probability distribution functions) were not given in Dopfer's thesis. Is that true (I can't read it unless there's an English version)? I would think that this would be THE most interesting result of the experiment, unless there is already some theory explaining why P1*A1+(1-P1)*B must be exactly equal to P2*A2+(1-P2)*C.

 

[Dbar_x]

Can you elaborate on why you think quantum erasure would require "reversible measurement" or mixtures evolving from superpositions?

Marlan Scully is the father of quantum erasure. His mathematical analysis from 1978 with Shea and McCullen is the genesis of all the QE experiments that have followed. So, let me quote from an article by Scully and Walter, “An Operational Analysis of Quantum Eraser and Delayed Choice,” Found. of Phys. 28, 3, 1998. On p. 400 they say, “...if we put a Welcher Weg detector in place (so we lose interference even if we don’t look at the detector) and then erase the which way information after the particles have passed...such a ‘quantum eraser’ process (would) restore the interference fringes.” (Notice that in this instance QE is described as observer independent, since the detector is responsible for destroying interference whether or not any person is looking.)

The detector is a measurement apparatus and the measurement destroys the interference. Measurement implies a single real eigenvalue for the property observed; which path in this case. So, the density matrix is diagonalized. Von Neumann showed that diagonalization of the density matrix is a thermodynamically irreversible process. But, by erasing the information now stored in the apparatus, QE is supposed to restore interference. That’s a reversal of the measurement.

this information will be lost if it’s detected at a different set of detectors (D1 and D2).

As we know, the wavefunction predicts the probability that the idler photon might be absorbed at either D3 or D4 (or D1 or D2). If the idler is not absorbed at D3 or D4 (but at D1 or D2 instead) no information about which way the signal photon went is recorded at a detector. The which way information isn’t erased or lost, it never was.

If the signal photons are detected first

In the Kim, ... Scully experiment the signal photons are always detected first.

if the idlers are detected first

The idler photons are never detected first in this experiment. The authors say, on the third page, that “...photon 2 must be at least 7.7 ns later than the registration of photon 1.”

I'm specifically asking for an experiment that could falsify your claim that the atoms are not in a superposition after traveling through the SG device

You’re asking the wrong person for such an experiment. I know of no experiment that will do it. There is, however, a simple experiment that will refute or falsify the widely-held belief (hypothesis) that the particle remains in its spin-direction superposition through the magnetic field.

So, are you in fact thinking of an experiment that would falsify your claim, and that would provide evidence that the atom was in a superposition after it passed through the SG device?

No. For what I think are obvious reasons, I’m not willing to discuss details of the experiment in this public forum.

OK, so particle not interfere with itself after go out from magnet field. Screen show no interference. You want proove this - yes / no?

Essentially, yes.

 

[JesseM]

Marlan Scully is the father of quantum erasure. His mathematical analysis from 1978 with Shea and McCullen is the genesis of all the QE experiments that have followed. So, let me quote from an article by Scully and Walter, “An Operational Analysis of Quantum Eraser and Delayed Choice,” Found. of Phys. 28, 3, 1998. On p. 400 they say, “...if we put a Welcher Weg detector in place (so we lose interference even if we don’t look at the detector) and then erase the which way information after the particles have passed...such a ‘quantum eraser’ process (would) restore the interference fringes.” (Notice that in this instance QE is described as observer independent, since the detector is responsible for destroying interference whether or not any person is looking.)

But presumably any calculation that shows that erasing the information from the detector would "restore the interference fringes" depends on modeling the detector's interaction with the particle as creating an entangled detector/particle system rather than the detector collapsing the wavefunction.

The detector is a measurement apparatus and the measurement destroys the interference. Measurement implies a single real eigenvalue for the property observed; which path in this case. So, the density matrix is diagonalized. Von Neumann showed that diagonalization of the density matrix is a thermodynamically irreversible process. But, by erasing the information now stored in the apparatus, QE is supposed to restore interference. That’s a reversal of the measurement.

Have you studied at the actual mathematical analysis of how erasing the information in the apparatus is supposed to restore interference? Again, I would assume that such an analysis would not involve the assumption that the detector caused a collapse of the wavefunction when it made the original measurement, but would instead model the detector as becoming entangled with the particle it measured, would then show that if you don't assume the detector's state is measured (and its wavefunction collapsed) until after the which-path information has been erased, in this case you can show there will be interference effects in the system. Think about how the situation would be analyzed if you treat the idler as a miniature "detector" which carries information about which path the signal photon went through, but where the idler's information can later be erased if it is measured in a certain way--I'm pretty sure you don't assume that when the signal photon passes through the slits, the which-path information potentially available in the idler causes an immediate collapse of the 2-particle wavefunction!

As we know, the wavefunction predicts the probability that the idler photon might be absorbed at either D3 or D4 (or D1 or D2). If the idler is not absorbed at D3 or D4 (but at D1 or D2 instead) no information about which way the signal photon went is recorded at a detector. The which way information isn’t erased or lost, it never was.

But that's just the sort of situation the words "quantum eraser" are used for! What's "erased" if the idler ends up and D1 or D2 is the potential that existed at earlier times to gain the which-way information by measuring the idler in the right way. After all, if you really want to know the which-way information immediately after the signal-idler pair are created, you can always guarantee that the idler will end up at a detector that gives you the which-way information by altering the experimental setup at that moment--in the diagram of the experiment in Scully's paper, just imagine removing the two beam-splitters BSA and BSB, in which case no photons will be deflected towards D1 or D2, they'll all go to D3 or D4. By making the path length of the idler long enough in comparison to the path length of the signal photon, you could even set it up so you didn't have to decide whether you wanted to have a guarantee of getting which-path information (by putting nothing in the spot of BSA and BSB so all the idlers go to D3 or D4) or if you wanted to have a guarantee of no which-path information (by putting mirrors in the spots of BSA and BSB so all the idlers go to D1 or D2) until after you'd already detected the signal photon at D0. If you make the latter choice, I'd assume Scully would deem this a choice to "erase" the which-path information.

Similarly, if you could have a macroscopic detector which was completely isolated from interactions with the external environment (decoherence) that might carry away information about its state (a practical impossibility similar to Schroedinger's cat), and the detector worked in such a way that if you waited long enough the information would be unavailable (imagine Schroedinger's cat makes a measurement and writes it down, then you have the option to either open the box immediately or else wait a few trillion years until the cat and everything else in the box have gone to a state of maximum entropy and become some sort of featurless liquid/gas combination), then if you do in fact wait, one could say as you did above that "the which way information isn't erased or lost, it never was". It's really just a question of what you mean by the words "erased", but whatever you say about this situation, you should say the same about the idler which also contains the potential to give you the which-way information at earlier times but where it may not give you any which-way information if you wait until later times.

I'm specifically asking for an experiment that could falsify your claim that the atoms are not in a superposition after traveling through the SG device

You’re asking the wrong person for such an experiment. I know of no experiment that will do it. There is, however, a simple experiment that will refute or falsify the widely-held belief (hypothesis) that the particle remains in its spin-direction superposition through the magnetic field.

That doesn't really make sense to me. If there are two alternative possibilities (in this case, either the atoms are in superposition or aren't after passing through the SG device), then the only way to "falsify" either possibility is to show that they yield different predictions about a given experiment, and then demonstrate one prediction is confirmed while the other isn't. If you don't even know what an advocate of the "atoms do remain in superposition after passing through the SG device" view would predict about your own experiment, how can you possibly say that if the experiment has the results you predict it'd falsify this view? Maybe you're just thinking about it wrong and an advocate of the superposition view would actually predict the same thing you're predicting. And if you do know what they'd predict and it's different from what you predict, then if the actual results matched their prediction but not yours, wouldn't that be a falsification of your own view that they don't remain in superposition?

No. For what I think are obvious reasons, I’m not willing to discuss details of the experiment in this public forum.

The reasons are not obvious to me, you could probably get a lot more useful feedback than you will with your current cryptic approach. If you're afraid you'd run afoul of the rules against challenging mainstream theories, you can just present the experiment as a way of distinguishing the two views on superpositions without making any confident predictions about what the results would be, or asking for feedback about what orthodox QM would predict about your experiment. If you're afraid someone will steal your idea and take credit, then I don't really understand why it would be any better to share it with anonymous users via PM as you have offered to do, and in any case a public post outlining your idea would show that you were the first to publicly propose this experiment.

 

[debra]

No. For what I think are obvious reasons, I’m not willing to discuss details of the experiment in this public forum.

Essentially, yes.

But presumably any calculation that shows that erasing the information from the detector would "restore the interference fringes" depends on modeling the detector's interaction with the particle as creating an entangled detector/particle system rather than the detector collapsing the wavefunction.

Well I kind of agree with DBar_x but some of the details get lost on me.
I am re-reading them.

In this well explained but sensationally written offering:
http://grad.physics.sunysb.edu/~amarch/

I am question the conclusions. See the well known diagram:

http://grad.physics.sunysb.edu/~amarch/PHY5657.gif

Isn't it true that a proportion of the photons passing through
both quarter wave plates will have the same polarization - by
probability? - and will interfere normally? Then the filter
labelled Polarizer examines those only. Not exotic at all? I
may be wrong here and its all wrapped up and proved.


Has this been done with electrons to check the erasure as polarizers can
throw confusing results? Can we believe what they claim in this
reference above? (photons sending secret messages about filters etc).
I am to be convinced - but maybe its all accepted and wrapped up?

The SG discussion seems similar to me, but maybe I am off topic.

 

[zonde]

Can we believe what they claim in this
reference above? (photons sending secret messages about filters etc).

If you consider that optically active devices can send electromagnetic "signatures" along the same path that photons go (only in opposite direction) down to BBO then you will have more sensible picture than these secret messages.

Of course it is not what QM says.

 

[debra]

If you consider that optically active devices can send electromagnetic "signatures" along the same path that photons go (only in opposite direction) down to BBO then you will have more sensible picture than these secret messages.

Of course it is not what QM says.

Quantum erasure experiments often speak about backwards in time messages and I was being sarcastic about 'secret messages' even though they actually claim this in the above reference. I am questioning the experimental set up and analysis of results and am hoping the SG discussion in this thread will throw some clarity on these entanglement and beam splitting questions without resorting to FTL and similar exotics.

 

[JesseM]

Isn't it true that a proportion of the photons passing through
both quarter wave plates will have the same polarization - by
probability? - and will interfere normally? Then the filter
labelled Polarizer examines those only. Not exotic at all? I
may be wrong here and its all wrapped up and proved.

According to the chart on the page underneath the diagram you posted, it's true that some photons going through slit 1 have polarization R, and some photons going through slit 2 also have polarization R...but the ones that have polarization R after going through slit 1 are all matched to entangled p photons that had polarization x, while the ones that have polarization R after going through slit 2 are all matched to entangled p photons that had polarization y. So by matching up each s photon at detector Ds with its entangled p photon at detector Dp, you should always be able to tell which of the slits the p photon went through, so there should be no interference. And you can see on the graph underneath the chart that when they actually performed this experiment and looked at the data, they didn't see an interference pattern:

http://grad.physics.sunysb.edu/~amarch/PHY5658.gif

 

[zonde]

Quantum erasure experiments often speak about backwards in time messages and I was being sarcastic about 'secret messages' even though they actually claim this in the above reference. I am questioning the experimental set up and analysis of results and am hoping the SG discussion in this thread will throw some clarity on these entanglement and beam splitting questions without resorting to FTL and similar exotics.

I understand your sarcasm but I am taking a bit different approach.
Say I believe that experimental set up and analysis of results are free from serious errors and I am looking for realistic explanation from that point.
And the only realistic explanation that comes to my mind is that optically active devices interact with photon at the moment of emission with that interaction happening at light speed and not FTL. This interaction at light speed is what I meant with that electromagnetic "signature".

 

[debra]

According to the chart on the page underneath the diagram you posted, it's true that some photons going through slit 1 have polarization R, and some photons going through slit 2 also have polarization R...but the ones that have polarization R after going through slit 1 are all matched to entangled p photons that had polarization x, while the ones that have polarization R after going through slit 2 are all matched to entangled p photons that had polarization y. So by matching up each s photon at detector Ds with its entangled p photon at detector Dp, you should always be able to tell which of the slits the p photon went through, so there should be no interference. And you can see on the graph underneath the chart that when they actually performed this experiment and looked at the data, they didn't see an interference pattern:

http://grad.physics.sunysb.edu/~amarch/PHY5658.gif

[/URL]

**********************************************************


No, not that experiment, I was questioning the Quantum
Erasure experiment below that one in:
http://grad.physics.sunysb.edu/~amarch/

Here is the set up:
http://grad.physics.sunysb.edu/~amarch/PHY5657.gif

This results is an interference pattern:
http://grad.physics.sunysb.edu/~amarch/PHY5653.gif

I am suggesting (maybe wrongly) that the Polariser filter (see diagram)
is acting to select specific photons resulting in a normal interference
pattern.

Its not (IMO) that the Polarisation Filter is 'acting upon' the pattern
(FTL, or backward time) it is simply selecting photons that we see,
because only photons getting through Polariser are counted.

 

[debra]

I understand your sarcasm but I am taking a bit different approach.
Say I believe that experimental set up and analysis of results are free from serious errors and I am looking for realistic explanation from that point.
And the only realistic explanation that comes to my mind is that optically active devices interact with photon at the moment of emission with that interaction happening at light speed and not FTL. This interaction at light speed is what I meant with that electromagnetic "signature".

OK, so I do not understand your 'signature' as you put it, - sounds suspiciously like a local variable to me and Bells Inequality shows that is probably not possible.

My view is that the entangled photons act as if their states are correlated. I view it as though they are synchronously clocked until entanglement is ended - although nobody has proved a Universe clock exists, it just how I think of it. So nothing travels from one particle to the other its just that their states are somehow synchronised (or correlated) by some unknown means. To retain my sanity I pretend they are externally clocked. It seems to explain things for me, until I see something that changes that view (maybe in this thread).

 

[DrChinese]

And the only realistic explanation that comes to my mind is that optically active devices interact with photon at the moment of emission with that interaction happening at light speed and not FTL. This interaction at light speed is what I meant with that electromagnetic "signature".

Experiments have already been done where Alice and Bob's polarizers are changed mid-flight, at a time at which it would be too late for them to have the kind of light speed influence that you imagine. So your "realistic" explanation will not work.

 

[Dbar_x]

would instead model the detector as becoming entangled with the particle it measured, would then show that if you don't assume the detector's state is measured (and its wavefunction collapsed) until after the which-path information has been erased, in this case you can show there will be interference effects in the system.

Credible, useful physical theories must be clear and unabmigiuous, consistent, and testable by falsification (or refutation) via experimental observation. As, for example, special relativity or quantum electrodynamics. It’s incumbant on those proposing or supporting a theory to meet such criteria, not on the rest of us to try to figure out what in the world they mean.

If quantum erasure means that entanglement between the apparatus and observed system causes an interference pattern to be destroyed, then restored when information is erased, then it must be clear precisely when the two objects are entangled. What unambiguous definition does QE give for entanglement? Is it distinct from the von Neumann measurement correlation between object and apparatus?

that's just the sort of situation the words "quantum eraser" are used for! What's "erased" if the idler ends up and D1 or D2 is the potential that existed at earlier times to gain the which-way information by measuring the idler in the right way.

How would an experimenter measure the potential to gain which-way information? What’s the equation for such a potential?

 

[JesseM]

Credible, useful physical theories must be clear and unabmigiuous, consistent, and testable by falsification (or refutation) via experimental observation. As, for example, special relativity or quantum electrodynamics. It’s incumbant on those proposing or supporting a theory to meet such criteria, not on the rest of us to try to figure out what in the world they mean.

If quantum erasure means that entanglement between the apparatus and observed system causes an interference pattern to be destroyed, then restored when information is erased, then it must be clear precisely when the two objects are entangled. What unambiguous definition does QE give for entanglement? Is it distinct from the von Neumann measurement correlation between object and apparatus?

Since it is impossible in practice to maintain a macroscopic measuring-apparatus in isolation from its environment and only observe it after enough time has passed that no which-path information would be gained from the observation, we are not dealing with predictions about actual experiments here (in future we might be able to create large isolated multiparticle systems using quantum computers, so this sort of thing is not impossible in principle, just impossible with present technology). But it's routine in theoretical physics to look at what the formalism of a given theory would tell us about an experiment which is impossible in practice for us to do today, like analyzing what would be seen by an observer diving into a black hole in general relativity. In QM, if a system composed of all the particles making up a macroscopic object could be maintained in isolation for some time and then observed, the formalism would say you should set up a giant multiparticle wavefunction for the system and evolve it forward until the moment of observation using the standard rules for wavefunction evolution, and then at the moment of observation you'd use the usual Born rule to figure out the probabilities it will be in different configurations. This is what physicists imagine doing in the Schroedinger's cat thought-experiment for example. It has nothing to do with "quantum erasure" specifically, it's just the universal quantum rules for dealing with any isolated quantum system composed of multiple interacting particles (and this is exactly what's done for smaller isolated multiparticle systems consisting of just a few particles).

But if Scully didn't include any detailed mathematical analysis to go with the statement you quoted, namely:

...if we put a Welcher Weg detector in place (so we lose interference even if we don’t look at the detector) and then erase the which way information after the particles have passed...such a ‘quantum eraser’ process (would) restore the interference fringes.

...then it seems a little pointless to waste too much time worrying about an offhand remark about an impossible-in-practice thought experiment similar to Schroedinger's cat (aside from pointing out that your notion that his statement assumes 'mixtures evolving from superpositions' is almost certainly a misreading of what he had in mind). Let's focus instead on the actual quantum eraser experiment, which doesn't involve a giant multiparticle wavefunction, but just a 2-particle wavefunction for the signal/idler pair. In that vein, you ask:

How would an experimenter measure the potential to gain which-way information? What’s the equation for such a potential?

"Potential" just means on any given trial, if you choose you can always set up the experiment so that you do gain which-way information on that trial (and if the idler path length is long enough you can make this choice after the signal photon has already been detected). It's similar to saying that on any trial involving a single particle going through the double-slit, prior to the time when the particle reaches the slits there is the potential to gain that particle's which way-information by placing a detector near the slit. You could demonstrate this potential by putting a detector near the slits on 100% of the trials, and in that case for near 100% of particles (allowing for some small amount of experimental error) you would gain the which-way information. Similarly, if you removed the beam-splitters BSA and BSB in Scully's experiment on 100% of trials, for near 100% of signal/idler pairs you could gain the signal photon's which-way information. If you don't like this use of the word "potential", fine, as long as you agree about the basic physical idea that you can set up the experiment so that you gain which-path information on nearly 100% of trials, then this is just a semantic dispute with no real physical content.

I would appreciate it if you'd address my more specific questions about how you are proposing to demonstrate that the particle does not remain in superposition after passing through the SG device:

That doesn't really make sense to me. If there are two alternative possibilities (in this case, either the atoms are in superposition or aren't after passing through the SG device), then the only way to "falsify" either possibility is to show that they yield different predictions about a given experiment, and then demonstrate one prediction is confirmed while the other isn't. If you don't even know what an advocate of the "atoms do remain in superposition after passing through the SG device" view would predict about your own experiment, how can you possibly say that if the experiment has the results you predict it'd falsify this view? Maybe you're just thinking about it wrong and an advocate of the superposition view would actually predict the same thing you're predicting. And if you do know what they'd predict and it's different from what you predict, then if the actual results matched their prediction but not yours, wouldn't that be a falsification of your own view that they don't remain in superposition?

 

[zonde]

Experiments have already been done where Alice and Bob's polarizers are changed mid-flight, at a time at which it would be too late for them to have the kind of light speed influence that you imagine. So your "realistic" explanation will not work.

Thank you for your comment!
Didn't knew about that Austrian experiment with ultra-fast random analyzer settings (http://arxiv.org/abs/quant-ph/9810080). It was very enlightening to find out about it. I suppose that was the one of the experiments that you were referring to.
Hmm, it would be interesting to know if there are plans to prepare quantum eraser experiments like that with ultra-fast random changing of settings.

 

[zonde]

Still ... in that Austrian experiment there is one small hole. Detection rates with static polarizer settings are not known, at least in paper there is said nothing about something like that.
If for example detection rate with static polarizer settings would rise twice then it's easy to see that sampling can be adjusted so that actual polarization much expected polarization.
I do not say that this is the case but it would be more comforting to discard such possibility.

 

[DrChinese]

Thank you for your comment!
Didn't knew about that Austrian experiment with ultra-fast random analyzer settings (http://arxiv.org/abs/quant-ph/9810080). It was very enlightening to find out about it. I suppose that was the one of the experiments that you were referring to.
Hmm, it would be interesting to know if there are plans to prepare quantum eraser experiments like that with ultra-fast random changing of settings.

That is exactly the experiment I was thinking of.

A little thought should indicate why this is not so critical anymore. Obviously, it has been ruled out as a factor with entangled photons. On the other hand, the neat trick with eraser experiments is that you can have one side's results already in the can BEFORE the other side decides whether to erase or not. And yet, the eraser still works. Pretty cool, eh! Defies "logic".

 

[DrChinese]

Still ... in that Austrian experiment there is one small hole. Detection rates with static polarizer settings are not known, at least in paper there is said nothing about something like that.
If for example detection rate with static polarizer settings would rise twice then it's easy to see that sampling can be adjusted so that actual polarization much expected polarization.
I do not say that this is the case but it would be more comforting to discard such possibility.

They calibrate the apparatus with static settings. So not much to consider there. Any way you run it, the results match QM predictions closely.