Talk:Weak value

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This article was the subject of a Wiki Education Foundation-supported course assignment, between 10 March 2020 and 30 April 2020. Further details are available on the course page. Student editor(s): Keystonescience.

Above undated message substituted from Template:Dashboard.wikiedu.org assignment by PrimeBOT (talk) 12:46, 17 January 2022 (UTC)

A lot of the comments below relate to the old article (October 2015):

On october 20 (2015) I gave the article a major overhaul. I think we should see which talk pieces are still relevant and remove the rest. Yama jlac (talk) 17:06, 30 October 2015 (UTC)

Can this sentence be further explained/expanded upon:

The weak value of the observable becomes large when the post-selected state, $|\phi _{2}\rangle$, approaches being orthogonal to the pre-selected state, $|\phi _{1}\rangle$.

Although this sentence may make sense to those familiar with quantum measurement theory, it makes very little sense to someone without this background:

"After the measurement the measuring device is shifted by what is called the "weak value"."

How is the measurement device "shifted?" For instance, in the spin-1/2 experiment proposed by AAV, what is shifted by $A_{w}$? This should be explained, I think.

I added an example after this sentence to clarify. hope it's better this way. —Preceding unsigned comment added by Cap.fwiffo (talk • contribs) 08:42, 6 April 2009 (UTC)

Better link for [two state vector formalism] required

Lacking a better article to point the undefined internal link [two state vector formalism] with more precision, I defined it to point to [Quantum state|two state vector formalism] (ie: two state vector formalism).
Could someone understanding QM fix this, if necessary, by providing a better link or an article (perhaps a subsection) to point the original link [two state vector formalism]? . Thanks. Pmronchi (talk) 18:57, 27 April 2009 (UTC)

Paradoxes

The two state vector formalism creates a few paradoxes which can be solved by weak measurements. One such paradox is the famous Hardy's pradox, another is the three box paradox. Rather then having those as part of the main body, these should have a section devoted to paradoxes. Also the recent experimental results "solving" Hardy's paradox with photons are great examples of weak measurements but are a bit contervrersial, and in my opinion belong in the realm of scientific discussion rather then on wikipedia. Any comments on this?

Criticism of article

I think that the article on weak measurement existing on November 15, 2010, is highly misleading and should be entirely rewritten. My specific concerns are the following.

(1) The idea of weak measurement is controversial, but the article gives no indication of this.

(2) The article properly cites a seminal paper of Aharonov, Albert, and Vaidman as the origin of the weak measurement concept, but fails to note that many authors have questioned the mathematics of this paper [e.g., Peres, A., Phys. Rev. Letters 62 (1989), 2326, Duck, I.M., and Stevenson, P.M., Phys. Rev. D 40 (1989), 2112-2117, Parrott, S., arXiv:0909.0295.

(3) Important parts of the article are highly questionable or simply wrong. For example, it states that

"after the measurement the measuring device pointer is shifted by what is called the 'weak value'", and then goes on to say that "If |φ₁> and |φ₂> are the pre- and post-selected quantum mechanical states, the weak value of the observable ... is defined [emphasis mine] as ...",

where the defining expression "..." is generally a non-real complex number. Quite a few features of this quote are objectionable:

(a) To identify something as a physically measurable quantity (the shift of a pointer), and then define it as equal to a mathematical expression (which doesn't even have to be a real number!) is basically nonsense.

That is something like a physics text explaining that the acceleration of gravity in meters/sec/sec is the speed in m/sec which a freely falling object will gain in one second, and then defining the "acceleration of gravity" to be 2 m/sec/sec.!

(b) The notion of "pre- and post-selected quantum mechanical states" is not defined in the article and will probably not be obvious to someone who comes to Wikipedia hoping to learn about weak measurements.

(c) How can a physical pointer by shifted by a (nonreal) complex number? What is a naive reader to think of this?

(d) Most authors use the real part of the defining expression as the weak value. Other authors have obtained completely different (even arbitrary) weak values. [Jozsa, R., Phys. Rev. A 76 044103 (2007), Parrott, S., arXiv:0909.0295]. All this is part of the controversy about weak values.

(4) Not everybody agrees that "Hardy's paradox" is actually a paradox (Hardy did not present it as a paradox), nor that weak measurements contribute anything to understanding it. The article gives no indication that this, too, is controversial.

(5) The "Further Reading" section gives only one reference: "Discover Magazine article: "Back From the Future. A series of quantum experiments shows that measurements performed in the future can influence the present. http://discovermagazine.com/2010/apr/01-back-from-the-future."

The quote about the future influencing the present is from the "teaser" for the article, but could easily be interpreted as a summary and endorsement by the Wikipedia author. I could find no evidence in the article for its claim that actual experiments show that the future can influence the present (in which case the present can influence the past, which influences the present, and so on ad infinitum). There is no indication in the Wikipedia article that this, too, is highly controversial (to say the least!). Maybe the controversial nature of the statement should be obvious, but it may not be obvious to those without scientific training. My personal opinion is that this reference has no place in any serious discussion of weak measurements, and to link to it without caveats does a disservice to naive readers.

In case the reader is wondering why I don't rewrite the article, there are several reasons. First, I anticipate that it could involve me in a time-consuming controversy. Second, I am not familiar with the mechanics of editing Wikipedia pages and am hesitant to invest the time to learn for this sole purpose. Third, I would probably have to cite my own work, and I am not sure if that would be proper, nor that if proper, I would want to do it. As a substitute, I am posting this on the "discussion" page.

A great weakness of the Wikipedia model is that there is no control over authorship, nor any way for readers to judge the competence and lack of bias of authors. Perhaps in theory, questionable articles will eventually be rewritten, but there is no guarantee. For example, the "weak measurement" article has been questionable for a long time. Stephen Parrott (Parrott_SK) —Preceding unsigned comment added by Parrott SK (talk • contribs) 03:33, 16 November 2010 (UTC) Parrott SK (talk) 23:05, 3 January 2011 (UTC)

POV-check tag added; link to example of more neutral writeup given

I am going to try to add a "POV-check" box to the weak measurement article to alert readers that many parts of the existing article are controversial, and the article gives no indication of this. (I say "try" because I have never done this before and cannot be sure that I have correctly interpreted the procedure given in the "NPOV_dispute" page.) Specific objections are given in the preceding section, "Criticism of the article"

An example of the kind of writeup which I would like to see along with further discussion can be found in the November 28, 2010 entry on the "papers" page of my web site http://www.math.umb.edu/~sp . Parrott SK (talk) 23:06, 3 January 2011 (UTC)

Disclaimer from Stephen Parrott added June 6, 2011

Disclaimer: I want to make clear that I did not add the reference: "Stephen Parrott questions of the meaning and usefulness of weak measurements, as described above", though I am flattered that someone thought my views worth referencing. Stephen Parrott. Parrott SK (talk) 02:54, 6 June 2011 (UTC)

A more detailed summary of Lundeen et al.

Chris Howard (talk · contribs) summarized a recent paper by Lundeen et al. as follows:

In 2011, a direct measurement of the wavefunction was reported, which was performed by means of a weak measurement of a first variable and a subsequent measurement of a second variable, complementary to the first.

If there are no objections, I'd like to replace this summary with a slightly more detailed one:

In 2011, weak measurements of many photons prepared in the same pure state, followed by strong measurements of a complementary variable, were used to reconstruct the photons' common wavefunction.

Vectornaut (talk) 20:43, 21 November 2011 (UTC)

Yes, that is much better. Suggestion: I would prefer "overall wavefunction" rather than "common wavefunction", because it lets the reader think of (simple) superposition rather than problems of entanglement etc. But that may be a mere matter of taste. --Chris Howard (talk) 12:36, 22 November 2011 (UTC)

When you say "overall wavefunction," I think of the tensor-product wavefunction of the composite system consisting of all the photons. Maybe that's the same problem you have with "common wavefunction"? In any case, I've gone ahead and edited the article; take a look at the wording I've used, and see if that's clear. Vectornaut (talk) 19:12, 22 November 2011 (UTC)

Yup, seems to be the same kind of problem. A question of mental association.

I have looked at the text and actually liked your previous version better than the sentence you have put in the article now. Given that in Lundeen's article (abstract) is written "The result is that the real and imaginary components of the wavefunction appear directly on our measurement apparatus." I think the (shape of the) wavefunction as such should be mentioned.

(Maybe just use your text suggestion on this page but drop the adjective - "common" or "overall" or "total" or whatever - and leave it at "the photons' wavefunction"?) --Chris Howard (talk) 21:12, 22 November 2011 (UTC)

Related to Quantum Nondemolition Measurement?

--Rainald62 (talk) 01:14, 30 December 2013 (UTC)

Quantum Weak Measurement is deemed Pseudoscience -- Read the following discussion

http://motls.blogspot.de/2012/09/pseudoscience-hiding-behind-weak.html — Preceding unsigned comment added by 2001:4CA0:0:F254:221:6AFF:FE2B:4C08 (talk) 16:53, 24 July 2014 (UTC)