What do you think these Weak Measurement, Quantum Uncollapse Tests. Re-write QM?

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Discussion Overview

The discussion revolves around the implications of weak measurements and quantum uncollapse tests on the foundations of quantum mechanics (QM). Participants explore whether these concepts necessitate a revision of QM or if they simply represent interesting extensions of existing theories. The scope includes theoretical implications, experimental observations, and philosophical considerations regarding measurement in quantum mechanics.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants argue that weak measurements do not change the formalism of QM, suggesting that these experiments are intriguing but do not necessitate a rewrite of the theory.
  • Others propose that if weak measurements can be reversed, it raises questions about the nature of reality and whether it implies hidden variables, although some contend that it does not support the existence of hidden variables.
  • A participant highlights that the work of John Martinis and his group, while impressive, does not fundamentally alter the understanding of QM, as it builds on established experiments in quantum optics and other fields.
  • There is a discussion about the Hardy paradox and whether experiments confirming standard QM can resolve paradoxes inherent to the theory, with some participants questioning the nature of these paradoxes.
  • Some participants express confusion about reconciling Bohmian theory with the implications of weak measurements, suggesting that the relationship between particles and pilot waves may be contradicted by recent findings.

Areas of Agreement / Disagreement

Participants do not reach a consensus; there are multiple competing views regarding the implications of weak measurements on QM. Some believe these measurements reinforce existing QM principles, while others suggest they challenge or deepen the understanding of measurement and reality.

Contextual Notes

Participants express uncertainty regarding the definitions of terms like "rewrite" in the context of QM, and there are unresolved questions about the implications of weak measurements on the nature of reality and hidden variables. The discussion also touches on the limitations of current theories in addressing paradoxes.

Who May Find This Useful

This discussion may be of interest to those studying quantum mechanics, particularly in the areas of measurement theory, interpretations of quantum mechanics, and the philosophical implications of quantum experiments.

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Please comment on this! Thanks in advance!

http://www.newscientist.com/article...sity-doesnt-have-to-kill-the-quantum-cat.html

Curiosity doesn't have to kill the quantum cat
09 May 2007
NewScientist.com news service
Amanda Gefter
Enlarge
Back from the brink

It may not have the swirling cameras and intense music of a TV emergency room, but John Martinis's laboratory is about to provide just as much drama. Martinis, a physicist at the University of California, Santa Barbara, is preparing a landmark experiment. The objective? To bring an animal back from the brink of death. [...]
 
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Could persons with knowledge comment on the above post?? Are you shocked?

"This could be a very profound discovery. Since the birth of quantum theory we have become used to thinking of quantum measurements as creating reality: until things are measured, they don't have an absolute, independent existence. But if some forms of measurement, such as weak measurement, are reversible, then the fundamentals of quantum mechanics go even deeper than we realized. If you create reality with weak quantum measurements, does undoing them erase the reality you created?"
 


Wangf said:
Could persons with knowledge comment on the above post?? Are you shocked?

"This could be a very profound discovery. Since the birth of quantum theory we have become used to thinking of quantum measurements as creating reality: until things are measured, they don't have an absolute, independent existence. But if some forms of measurement, such as weak measurement, are reversible, then the fundamentals of quantum mechanics go even deeper than we realized. If you create reality with weak quantum measurements, does undoing them erase the reality you created?"

Absolutely none of this is any different than standard QM. Just another interesting aspect of what we have come to know and love for over 80 years. :smile:

It is very common for popular articles to try to stir up excitement by talking about being on the "verge" of exciting new science. If Martinis can come up with a modification of the HUP, that would be news! I seriously doubt he would mention that as a likely outcome, however.
 


If you can measure then undo the effects of a measurement, doesn't that imply that there is more state than given by the state vector => hidden variables?
 


denisv said:
If you can measure then undo the effects of a measurement, doesn't that imply that there is more state than given by the state vector => hidden variables?

To me it implies there are no hidden variables. But that is not an absolute conclusion, at least according to Bohmian theory.
 


DrChinese said:
If Martinis can come up with a modification of the HUP, that would be news! I seriously doubt he would mention that as a likely outcome, however.

Especially since Martinis is an experimentalist who -as far as I know- is not really working on the measurement problem. His group just happens to have some of the best solid state qubits around so they are using them for all sorts of experiments (usually with the help of collaborators in the relevant field).
The last time I attended one of his talk he was presenting work on number states in resonators, i.e. the solid-state analogue of cavity-QED and their ultimate goal is to build a useful quantum computer; i.e what they are talking about in this article was just a side project.

As far as I know neither his group nor anyone else has ever done anything "original" using a solid state qubit when it comes to the fundamentals of QM; all the experiments are essentially solid-state, on-chip versions of experiments that have already been done in quantum optics, NMR etc. It will take another few years before solid-state qubits can compete in this field.

My point is that the work they did was really interesting (and from an experimental point of view very impressive) but it did not really change our understanding of QM.
 
  • #10


denisv said:
If you can measure then undo the effects of a measurement, doesn't that imply that there is more state than given by the state vector => hidden variables?
DrChinese said:
To me it implies there are no hidden variables. But that is not an absolute conclusion, at least according to Bohmian theory.
I would like to draw your attention to papers
http://xxx.lanl.gov/abs/0706.2522 [New J. Phys. 9 165 (2007)]
http://xxx.lanl.gov/abs/0808.3324
In short, weak measurements can be used to make a simultaneous measurement of both position and velocity of the particle. The result turns out to coincide with Bohmian theory.
 
  • #11


Weak measurements aren't new, and certainly do not cause QM to be rewritten (whatever that means). I've highlighted at least one such experiments using such weak measurement technique to verify the Hardy Paradox.

https://www.physicsforums.com/showpost.php?p=2047556&postcount=76

If anything, they verify even stronger the QM rules!

Zz.
 
  • #12


ZapperZ, at the link you provide you say:
"This experiment appears to be the confirmation and resolution of the Hardy's paradox."
I want to ask you a question.
If the results of this experiment only confirm the predictions of standard QM, how the results of this experiment may resolve the Hardy "paradox", which is a "paradox" of standard QM?
I mean, an experiment may resolve a paradox only if the experiment gives some new information that is not provided by the theory. Otherwise, a paradox within a theory may only be resolved through better understanding of the - theory.
 
  • #13


Demystifier said:
ZapperZ, at the link you provide you say:
"This experiment appears to be the confirmation and resolution of the Hardy's paradox."
I want to ask you a question.
If the results of this experiment only confirm the predictions of standard QM, how the results of this experiment may resolve the Hardy "paradox", which is a "paradox" of standard QM?
I mean, an experiment may resolve a paradox only if the experiment gives some new information that is not provided by the theory. Otherwise, a paradox within a theory may only be resolved through better understanding of the - theory.

Paradox being something that appears to be impossible (a la Zeno), but obviously isn't. So the paradox must not actually reside in orthodox QM, agree?

Now you can't accept this as a paradox from the Bohmian side, lest it run afoul of this experiment. But I personally keep having trouble with reconciling the Bohmian theory and this type of experiment. I understand that the particle and the pilot wave are both "real" in this view, which is fine. But it seems to my simple brain as if that view is flatly contradicted here.
 
  • #14


DrChinese said:
Paradox being something that appears to be impossible (a la Zeno), but obviously isn't. So the paradox must not actually reside in orthodox QM, agree?
Agree!

DrChinese said:
Now you can't accept this as a paradox from the Bohmian side, lest it run afoul of this experiment. But I personally keep having trouble with reconciling the Bohmian theory and this type of experiment. I understand that the particle and the pilot wave are both "real" in this view, which is fine. But it seems to my simple brain as if that view is flatly contradicted here.
If you understood the GENERAL theorem explaining why Bohmian theory ALLWAYS gives the same predictions as orthodox QM, you would understand why Bohmian theory gives no contradictions. Nevertheless, it is instructive to discuss how Bohmian theory deals with particular cases such as this one, so let me do this.

First, as I am a theorist, I must say that I don't really understand the details of the actual experiment with two photons. Therefore, I will discuss the original thought experiment with an electron and a pozitron discussed by theorists (Hardy, Aharonov, and others).

Let us assume that the full wave function (describing all possible paths of particles at once) they assume in their analysis is correct. In particular, and this is the crucial point, this wave function has a property that nothing special happens with the wave function at the points at which the electron wave function crosses with the positron wave function. This, according to the STANDARD quantum theory, means that electron and pozitron do NOT interact. In particular, such a wave function describes a situation in which there is NO ANNIHILATION between electron and pozitron. This is STANDARD QM, independent on the Bohmian interpretation. Nevertheless, the theorist above claim that electron and positron should annihilate if they come at the same position. But they are wrong! If the wave function is the one they assume it is, then they should not annihilate. Period!

Their paradox can be summarized as follows. First they (tacitly) assume that electron and pozitron do not interact. After that, they argue that they should interact and find a paradox. The paradox is a trivial artefact of the fact that they were not aware that they have tacitly assumed that electron and pozitron do not interact. With such an assumption (either tacit or explicit), it is not consistent to argue that they should interact.

Now the Bohmian interpretation. According to the Bohmian interpretation of THIS wave function, the trajectories of electron and pozitron may cross, but they will not annihilate. The motion of particles is completely described by the wave function (plus initial positions of the particles), and this is simply what this wave function predicts for their trajectories.

Of course, the wave function they use is not realistic because it does not use the fact that in reality electron and pozitron MAY annihilate. However, until someone calculates a more realistic wave function, it is difficult to say what theory (either standard or Bohmian) really predicts for such a case. For example, the true wave function may ruin the interference properties that naive wave functions have. Or it may not. I don't know in advance. Nobody has really calculated that yet. Nobody has performed the experiment as well. Therefore, I cannot say what is really predicted by either standard or Bohmian theory. But general theorems provide that both theories will have the same measurable predictions.

Now let me discuss the actual experiment with photons. I cannot explain what is going on there because I don't understand the details. In particular, I don't understand the physical mechanism that is supposed to destroy the photons when they both come at the same position at the same time. If someone can explain it to me in terms of QUANTUM MECHANICS (that is, by wave functions) then I will be able to explain how the results of this actual experiment are explained by Bohmian theory.
 
  • #15
Here's a recent experiment:
Direct observation of Hardy's paradox by joint weak measurement with an entangled photon pair
Kazuhiro Yokota et al 2009 New J. Phys. 11 033011 (9pp)
http://www.iop.org/EJ/abstract/1367-2630/11/3/033011/
 
  • #16
Demystifier said:
If someone can explain it to me in terms of QUANTUM MECHANICS (that is, by wave functions) then I will be able to explain how the results of this actual experiment are explained by Bohmian theory.

From the reference in post 15:

"Although it is natural to ask what the value of a physical quantity is in the middle of a time evolution, it is difficult to answer such a question in quantum mechanics, especially when post-selection is involved. Hardy's thought experiment [1] is a typical example in which we encounter such a difficulty. Figure 1(a) shows a photonic version of the experiment, which was recently demonstrated by Irvine et al [2]. The scheme consists of two Mach–Zehnder (MZ) interferometers MZ1 and MZ2 with their inner arms (O1, O2) overlapping each other at the 50 : 50 beam splitter BS3. If photons 1 and 2 simultaneously arrive at BS3, due to a two-photon interference effect, they always emerge at the same port. This corresponds to the positron–electron annihilation in the original thought experiment [1]. The path lengths of MZ1 are adjusted so that photon 1 should never appear at C1 by destructive interference, when photon 2 passes the outer arm NO2 and thus has no disturbance on MZ1. The path lengths of MZ2 are adjusted similarly. Then, a coincidence detection at C1 and C2 gives a paradoxical statement on which paths the detected photons have taken. The detection at C1 (C2) implies that MZ1 (MZ2) has been disturbed by photon 2 (1) traveling along O2 (O1)."

So when the path lengths have been adjusted properly, a photon is not destroyed - but the photon pairs exit one of the beamsplitters going the same way - thus there will be no simultaneous beeps on the 2 detectors (i.e. just 1 will fire).

See the above at: http://www.iop.org/EJ/article/1367-2630/11/3/033011/njp9_3_033011.html or via the PDF.

Demystifier: I am not doubting that if the equivalence principle holds, then you get the same results from BI as would be expected from oQM. The question I have is whether BI really has a realistic footing, when all is said and done. In my mind, if BI is contextual then it is not realistic. If it is realistic, then it should not be possible to re-assemble probability waves back into a superposition after a measurement is performed. So to a certain degree my issues are semantic more than technical.
 
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  • #17


Why almost everyone, here talk about Bohmian theory?? There are many other interpretations.

i found it is odd that from general media and general lectures, i barely heard of Bohmian interpretation. Did he got Nobel Prize? To me, there are real giants in QM that far far more influential than Bohmian.
 
  • #18


DrChinese said:
In my mind, if BI is contextual then it is not realistic.
Why not? Why reality could not change when the context is changed? After all, classical physics is also contextual, i.e., even classical properties may change by the act of measurement. The only essential difference between classical and quantum contextuality is that the latter is nonlocal.

DrChinese said:
If it is realistic, then it should not be possible to re-assemble probability waves back into a superposition after a measurement is performed.
Why not? If the performed measurement is a WEAK measurement, then the measurement has not induced a decoherence. If decoherence has not happened, then it is not a problem to re-assemble probability waves back into a superposition after a measurement is performed. Are you familiar with the theory of decoherence (which is an experimentally verified interpretation-independent phenomenon)?
 
  • #19


Wangf said:
Why almost everyone, here talk about Bohmian theory?? There are many other interpretations.

i found it is odd that from general media and general lectures, i barely heard of Bohmian interpretation. Did he got Nobel Prize? To me, there are real giants in QM that far far more influential than Bohmian.
Maybe Bohm was not so influential in the past, but it seems that it changes now.
 
  • #20


DrChinese said:
The question I have is whether BI really has a realistic footing, when all is said and done.
Perhaps you would agree with Sculy et al that Bohmian trajectories are not realistic but surrealistic. Namely, they have shown that Bohmian trajectories differ from trajectories obtained by weak measurements, from which they have concluded that Bohmian trajectories are surreal. However, Bohmians (as well as some theoretical experts for weak measurements) disagree with such a conclusion. If you think more carefully what a weak measurement really is, you should realize that a weak measurement is not a true measurement, but an indirect (and somewhat naive) conclusion about some not-really-measured variable from a true (strong) measurement of some other variable. In fact, Bohmians could argue that weak values are the ones which are not realistic but "surrealistic".

And again, as I have already stressed, if weak values are to be taken seriously (which I think they shouldn't), then SOME weak measurements may be used as an experimental verification of the Bohmian interpretation.
 
  • #21
DrChinese said:
See the above at: http://www.iop.org/EJ/article/1367-2630/11/3/033011/njp9_3_033011.html or via the PDF.
DrChinese, this paper itself is enough to explain why weak measurements should not be taken very seriously. One only needs to read carefully (and critically) the explanation around Eq. (3). In particular:

1) They say
"... this gives an operational way of defining what the value of A is ..."
The crucial word is DEFINING. This definition is somewhat arbitrary. True, it may look intuitively appealing to someone, but the Bohmian definition of velocity may also look appealing to someone. Anyway, this definition is based on measuring AVERAGE values of something, and average values, in general, do not need to be a good estimate of the actual values (unless the standard deviation is small, which is often not the case when beam splitters are present.)

2) The weak value expressed by Eq. (3) does not even need to be a real number. How would you interpret that?

3) If the operator A in Eq. (3) is the velocity operator (i.e., momentum operator divided by mass) and if |phi> is the position eigenstate |x>, then real part of Eq. (3) is nothing but the Bohmian velocity. Would you be ready to take such a weak measurement of particle velocity at a given position as an experimental verification of the Bohmian theory?
 
  • #22


Demystifier said:
DrChinese, this paper itself is enough to explain why weak measurements should not be taken very seriously. One only needs to read carefully (and critically) the explanation around Eq. (3). In particular:

1) They say
"... this gives an operational way of defining what the value of A is ..."
The crucial word is DEFINING. This definition is somewhat arbitrary. True, it may look intuitively appealing to someone, but the Bohmian definition of velocity may also look appealing to someone. Anyway, this definition is based on measuring AVERAGE values of something, and average values, in general, do not need to be a good estimate of the actual values (unless the standard deviation is small, which is often not the case when beam splitters are present.)

2) The weak value expressed by Eq. (3) does not even need to be a real number. How would you interpret that?

3) If the operator A in Eq. (3) is the velocity operator (i.e., momentum operator divided by mass) and if |phi> is the position eigenstate |x>, then real part of Eq. (3) is nothing but the Bohmian velocity. Would you be ready to take such a weak measurement of particle velocity at a given position as an experimental verification of the Bohmian theory?

2. They specified that only the real part was to be considered.

3. Sure I would. There better be a lot of experimental support for Bohmian theory or else we're all wasting a bunch of time. :smile: Of course that would not be expected to support BI over oQM due to quantum equivalence hypothesis.
 
  • #23


DrChinese said:
3. Sure I would. There better be a lot of experimental support for Bohmian theory or else we're all wasting a bunch of time. :smile: Of course that would not be expected to support BI over oQM due to quantum equivalence hypothesis.
But if oQM is correct (and almost nobody doubts that it is), then the experiment will CERTAINLY "confirm" the Bohmian interpretation.
You may be surprised that, for me, it would not be an experimental support for Bohmian theory at all.
Nevertheless, I would be happy if someone would make such an experiment (I have no doubts on the result) and publish it in Nature, Science, or Physical Review Letters. I would be happy because it would certainly increase the interest of physicists for the Bohmian interpretation, even if that interest would be initially caused by a wrong reason. (Just as it is equally wrong to interpret some different (already existing) weak measurements as experimental support against the Bohmian theory.)

By the way, what do you mean by the quantum equivalence hypothesis? The equivalence between oQM and BM? I cannot overemphasize that it is not a hypothesis but a theorem (valid for "ideal" strong measurements).
 
  • #24


DrChinese said:
2. They specified that only the real part was to be considered.
Fine, but don't you find it slightly artificial to simply discard a part of a complex number simply because you have no idea how to interpret it?
I'm just trying to convince you how illusive and unnatural weak measurements are.
 
  • #25


Further arguments against weak measurements:

1. By a suitable choice of the post-selected state, the weak value can be made ARBITRARILY large. In particular, the title of the first (PRL) paper on weak measurements reads:
"How the result of a measurement of a component of the spin of a spin-1/2 particle can turn out to be 100"
I don't think that such arbitrariness (made by experimentalists!) should be viewed as a measurement of a true value existing out there in nature itself.

If this is not convincing enough, then the following should:

2. The resolution of the original Hardy paradox with electrons and positrons is based on a NEGATIVE number of particles obtained by a weak measurement. What, for the God sake, a negative number of particles means? It is nothing but a number obtained when some POSITIVE numbers directly obtained by measurements are put into a weird MATHEMATICAL FORMULA supposed to represent a "physical" quantity called "weak value".

Perhaps weak values can be best understood through an every-day analogy in the classical world. The physical (i.e., strong) amount of money cannot be negative. Yet, a weak value of the amount of money can be negative. Namely, there is a reasonable MATHEMATICAL formula that in some cases attributes a negative amount of money in certain situations in which the amount of money is not strongly measured. Indeed, we have a standard word for such a negative amount of money - DEBT.

Let me push the analogy further. There is a guy called Bohm who proposed that money exists even when nobody observes it. It is called the Bohmian interpretation of economics. According to this interpretation, the amount of money is allways non-negative. However, weak measurements of money show that the amount of money can be negative. Some economists take this as an indication that the Bohmian theory of money is wrong, or at least that it is highly unnatural.

Nevertheless, the Bohmian theory is very simple and natural. It proposes a double ontology, according to which both money (particles) and human rules of behavior (wave function) separately exist. A "negative amount of money", that is debt, is not really an amount of money, but a part of human rules which say that some humans should give money to other humans whenever certain circumstances take place. We see that money (which is a hidden variable which exists even when nobody observes it) is guided by the pilot human rules. We also see that this hidden variable is highly contextual (although local, or course).
 
  • #26


Wangf said:
Why almost everyone, here talk about Bohmian theory?? There are many other interpretations.


Because Demystifier, who posts frequently here, is a Bohm fan who has written quite a few academic papers on this subject. So he can explain this topic well to others here and that leads to frequent discussions on this topic. :smile:
 
  • #27


Count Iblis said:
Because Demystifier, who posts frequently here, is a Bohm fan who has written quite a few academic papers on this subject. So he can explain this topic well to others here and that leads to frequent discussions on this topic. :smile:
It may be a part of the reason, but note that it was DrChinese who first mentioned Bohm in this topic in post #8. In my understanding, he is not a fan of Bohm, but admits that it has some merits.

Another, more important reason is that several proponents of weak measurements argue that weak measurements (unlike ordinary strong measurements) are in contradiction with the Bohmian interpretation. Such arguments need to be discussed properly.
 
  • #28


Demystifier said:
It may be a part of the reason, but note that it was DrChinese who first mentioned Bohm in this topic in post #8. In my understanding, he is not a fan of Bohm, but admits that it has some merits.

You are right about my feelings about the Bohmian interpretation, but I consider myself still a "student" trying to understand more about it. Fortumately for me, you and others here have stepped up to the task of assisting in my (slow) education.

Actually I am quite a fan of Bohm the person, although I don't agree with all of his ideas. (You are probably familiar with the Holographic paradigm.) I consider him instrumental in the development of a lot of important work.
 
  • #29


DrChinese said:
Actually I am quite a fan of Bohm the person, although I don't agree with all of his ideas. (You are probably familiar with the Holographic paradigm.)
I don't care much about Bohm as a philosopher, including his holographic paradigm.
On the other hand, I care much about the Bohmian interpretation of QM because this interpretation is naturally suggested by the EQUATIONS.
 
  • #30


Let me continue with my criticism and demystification of weak measurements.

In most cases, the essence of weak measurements is hidden behind relatively complicated cases considered in practice. So let us consider the simplest (and the most honnest) case in which no final experimental outcomes are discarded. In other words, let us consider the case in which the post-selected state is equal to the pre-selected state. In this case, the weak value in nothing but the well-understood average value
<psi|A|psi>
Is that a good representation of an actual value? As an example, consider a simple experiment with one 50:50 beam splitter and two standard particle detectors. Take A to be the position operator. One expects that the position of the photon should be either in the left arm or in the right arm of the experimental setting. Nevertheless, the weak position of the particle is in the middle between these two arms, where the particle will NEVER be found by a strong measurement.

If it is still not obvious to you that such a weak value should not be taken as an actual value, then the following should convince you. The weak value of the photon position above is completely analogous to the fact (that every American knows) that the average American family has 2.6 children. How can any family have 2.6 children? Of course it can't. This is just the average, that is the "weak value" of the number of children.

You can also make it "more complicated" by postselecting only those families that live in Manhattan, for example. In this case you will not get 2.6 but a smaller number. Nevertheless, it is still clear and trivial: the number you will get (say 1.7) is only an average and does not describe any real family.

An orthodox experimentalist may say: "But that is the number that I've measured, so I am obliged to take it seriously." But he is wrong, he has NOT measured this number. Instead, he has CALCULATED it. He has measured the total number of children Nc. He has also measured the the total number of families Nf. However, the number of children per family (nf) is a result of CALCULATION through a mysterious formula
nf=Nc/Nf

Mysterious? No, trivial! Silly? If you interpret the weak value as an actual value of an individual system, then it is more than silly.

To conclude, in a Ballentine style: A strong measurement reveals a property of an individual system, but a weak measurement only reveals a property of a large STATISTICAL ENSEMBLE of equally prepared systems. A weak measurement says nothing about properties of an individual system. All weirdness of weak values results from attempts to interpret properties of an ensemble (2.6 children) as properties of an individual system (a family).

Therefore, no weak measurement can be taken as an indication against the Bohmian interpretation or any other hidden-variable theory. Essentially, Bohmian interpretation says that children exist even when nobody watches them, and that the number of children in a family is allways an integer. The fact that the average American family has 2.6 children does not contradict the Bohmian interpretation.
 
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