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

  1. Jul 30, 2009 #1
  2. jcsd
  3. Jul 30, 2009 #2

    DrChinese

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  4. Jul 31, 2009 #3
    Re: What do you think these Weak Measurement, Quantum Uncollapse Tests. Re-write QM??

    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. [...]
     
    Last edited by a moderator: Aug 4, 2009
  5. Aug 4, 2009 #4
    Re: What do you think these Weak Measurement, Quantum Uncollapse Tests. Re-write QM??

    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 realised. If you create reality with weak quantum measurements, does undoing them erase the reality you created?"
     
  6. Aug 4, 2009 #5

    Demystifier

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

    For the most recent criticism of weak measurements and weak values, see
    http://lanl.arxiv.org/abs/0908.0035
     
  7. Aug 4, 2009 #6

    DrChinese

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

    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.
     
  8. Aug 4, 2009 #7
    Re: What do you think these Weak Measurement, Quantum Uncollapse Tests. Re-write QM??

    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?
     
  9. Aug 4, 2009 #8

    DrChinese

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

    To me it implies there are no hidden variables. But that is not an absolute conclusion, at least according to Bohmian theory.
     
  10. Aug 4, 2009 #9

    f95toli

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

    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.
     
  11. Aug 6, 2009 #10

    Demystifier

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

    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.
     
  12. Aug 6, 2009 #11

    ZapperZ

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

    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.
     
  13. Aug 6, 2009 #12

    Demystifier

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

    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.
     
  14. Aug 6, 2009 #13

    DrChinese

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

    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.
     
  15. Aug 7, 2009 #14

    Demystifier

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

    Agree!

    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.
     
  16. Aug 7, 2009 #15
    Re: What do you think these Weak Measurement, Quantum Uncollapse Tests. Re-write QM??

    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/
     
  17. Aug 7, 2009 #16

    DrChinese

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

    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.
     
    Last edited: Aug 7, 2009
  18. Aug 8, 2009 #17
    Re: What do you think these Weak Measurement, Quantum Uncollapse Tests. Re-write QM??

    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.
     
  19. Aug 10, 2009 #18

    Demystifier

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

    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.

    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)?
     
  20. Aug 10, 2009 #19

    Demystifier

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

    Maybe Bohm was not so influential in the past, but it seems that it changes now.
     
  21. Aug 10, 2009 #20

    Demystifier

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

    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.
     
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