Quantum measurement: first kind vs non-demolition

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Discussion Overview

The discussion centers on the differences between quantum measurement of the first kind and quantum non-demolition (QND) measurement. Participants explore theoretical aspects, practical implications, and the conceptual nuances of these measurement types within quantum mechanics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants describe quantum non-demolition measurements as involving hermitian operators that commute with the Hamiltonian, allowing for simultaneous solutions and coexistence of states.
  • One participant mentions a recent experiment involving neutrino and light measurements, suggesting that weak-field interactions may challenge traditional uncertainty principles, although practical limitations were noted.
  • Another participant discusses weak measurements potentially leaving the system in its original eigenbasis and avoiding wave function collapse.
  • Some participants express confusion about the clear distinction between the two types of measurements, with one noting that they found explanations of both but not a comparative analysis.
  • There is a claim that quantum measurement of the first kind results in wave function collapse, while QND measurements may allow for reversing changes to maintain superposition states.
  • One participant argues that the two types of measurements should not be defined differently, suggesting they are effectively the same in practice.
  • References to external resources are provided to clarify the differences, with one participant stating that the difference became clear after reviewing a specific paper.

Areas of Agreement / Disagreement

Participants express varying levels of understanding and agreement regarding the differences between the two measurement types. Some assert clear distinctions, while others see them as fundamentally similar or are uncertain about the definitions.

Contextual Notes

Participants highlight the complexity of the concepts and the lack of comprehensive resources that compare both measurement types directly. There are unresolved questions about the implications of wave function collapse and the conditions under which measurements can be classified.

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

Thanks!


you ?
asking that ?

...very suspect
 
audioloop said:
you ?
asking that ?

...very suspect
Why? Do you think this question is too easy? If so, can YOU answer it?
 
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.
 
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:
Demystifier said:
Why? Do you think this is too easy? If so, can YOU answer it?

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.
Demystifier said:
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.
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:
audioloop said:
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,...
Thanks, the difference is now clear to me.
 

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