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Quantum measurement: first kind vs non-demolition

  1. Aug 9, 2012 #1

    Demystifier

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    Can someone explain to me the difference (if any) between measurement of the first kind and quantum non-demolition measurement?

    Thanks!
     
  2. jcsd
  3. Aug 9, 2012 #2
    Quantum non-demolition measurements corresponde to the case of acting with an operator which is hermitian and commutes with the Hamiltonian (or another operator of the system). In this case both operators can have simultaneous solutions. the system can therefore coexist in both states at the same time.
    Last year in Canada a group of scientists took neutrino measurements as well as light measurements to determine the position and momentum of a particle at the same time, an impossibility in the regular courses that people take (the first kind of measurement). however the weak-field interactions take place via the W and Z bosons, meaning they may contain a different abstract space from the electromagnetic, thereby giving hopes in going beyond the uncertainty principle limitations in experiments. However they had to redistribute the neutrino data at a later time due to a lack of data points, and they had to somewhat guess, where and when the neutrino's had come from. So although in theory may work, it is far far from practically determining the positions and momenta of the incident particles exactly.
     
  4. Aug 9, 2012 #3
    I may have deviated a bit from the main theme here. The idea is that weak measurements may leave the system in the original eigenbasis, since they operate via a different channel, and have much different lifetimes and linewidths. Essentially avoiding the collapse of the wave function.
     
  5. Aug 9, 2012 #4

    you ?
    asking that ?

    ....very suspect
     
  6. Aug 10, 2012 #5

    Demystifier

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    Why? Do you think this question is too easy? If so, can YOU answer it?
     
  7. Aug 10, 2012 #6

    Demystifier

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    Ardie, thanks for the explanation of QND, but I ask specifically for the DIFFERENCE between the two. I found the explanations of either of the two concepts in many places, but I could not find any single book or article that explains BOTH concepts at one place. And I must admit, from various explanations of these concepts at various places, I still cannot see clearly what exactly is the DIFFERENCE between them.
     
  8. Aug 10, 2012 #7
    I was under the impression, that quantum measurement of the first kind would leave the wave function collapse. clearly that's not the case.
    in a quantum non-demolishing experiment you may reverse the change you had done to the system to leave it back in a superposition state. Information taken from the measurement will help you achieve this, by classical transmission. In a measurement of the first kind, the wave function effectively does not collapse as wave functions exist on different spaces.
    I find it unnecessary that they should be defined differently, after all they are the same act in practice.
     
    Last edited: Aug 10, 2012
  9. Aug 10, 2012 #8
    nothing to do with the easiness or the difficulty in itself.
    i refer to your credentials, you are a theoretical physicist, a lot of publications etc




    but now i understand.

    First-kind measurements, non-demolition measurements, and
    conservation laws
    http://arxiv.org/pdf/quant-ph/9804026v3.pdf

    ...Any QND measurement is of the FK, but the inverse becomes true only when Qˆ is a constant-of-motion of S,...
     
    Last edited: Aug 10, 2012
  10. Aug 13, 2012 #9

    Demystifier

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    Thanks, the difference is now clear to me.
     
  11. Nov 17, 2013 #10
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