Measurement in QM

  • #1

Main Question or Discussion Point

When talking about measurement in QM it's completely normal to think right away about someone doing "something" to get or "observe" a definite quantum state....

......but how about the screen in a standard DS? ....there we get clumps if we measure a quantum state at the slits or fringes if not.....

...... can't we consider the screen a measuring device that can tell Alice if Bob measured a quantum state or not?....the no communication theorem applies to measurement of quantum states only, so.... does Alice really needs to see what Bob was doing at the slits to tell if Bob was measuring a quantum state or not?....
 

Answers and Replies

  • #2
atyy
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Yes, the screen can be considered a measurement device.

Measurement is a subjective notion in quantum mechanics, and depends on whether you consider that a definite (often also called "irreversible" or "macroscopic" or "classical" or "real") measurement outcome (eg. a mark on the screen) was obtained or not. A measurement has occurred if there is a mark on the screen, and you retain the information about the mark on the screen. If you throw the information away, you can consider it as a measurement having occurred and the information has been thrown away, or you can consider it as if a measurement has not occurred (by treating the particle and the screen as a joint quantum system undergoing unitary time evolution).
 
  • #3
DrChinese
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Velocity is a component of momentum. Momentum does not commute with position. As a practical matter, you may be able to deduce velocity - although you may be losing accuracy about position.

So as with everything... it depends. :smile:
 
  • #4
Measurement is a subjective notion
Agree... hope it don't bother if i share something with you about measurement, but this time about the detectors....

.....have read about DS experiments with detectors at both sides of the slits, by only one slit, or no detectors at all.....but do you know of any experiment with only one detector placed above the slits in a way that it will beep for any photon passing thru any slit or even if it goes thru both slits? .....no position being measured, still will tell a photon just went thru the slits...wonder what the outcome will be at the screen......
 
  • #5
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You’ll get a double slit pattern. The detector behind the slits does what you want already.
 
  • #6
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.....have read about DS experiments with detectors at both sides of the slits, by only one slit, or no detectors at all.....but do you know of any experiment with only one detector placed above the slits in a way that it will beep for any photon passing thru any slit or even if it goes thru both slits? .....no position being measured, still will tell a photon just went thru the slits...wonder what the outcome will be at the screen......
A photon does not split into two to go through the two slits.... you can't get a click at both and expect an interference pattern to still appear.
 
  • #7
vanhees71
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A photon is not a classical particle in any way. Forget about this utterly wrong picture, which unfortunately is used even in otherwise good textbooks. The reason seems to be that textbook writers just copy the introductory sections about the qualitative and heuristic meaning of QT, although in fact this might be the most important section for the student in the sense of setting him or her on the right track of "quantum thinking" from the very beginning. The challenge of non-relativistic QM is not so much the math, which is not as complicated as, e.g., vector calculus needed for classical electromagnetics, but to learn to think "quantum mechanically" and get rid of thinking in terms of "classical physics". This re-learning process is hindered by bad heuristics in the beginning. The two most serious sins commit still by textbook writers in the introductory section are (a) to claim that photons are particle like (they are not, they don't even have a position in the usual quantum sense of massive particles) and (b) describe the old quantum mechanics in great detail (including wave-particle duality, which is not part of modern quantum mechanics since 1926 anymore and the Bohr-Sommerfeld model of atoms since electron trajectories do not make any sense in atomic bound states nor is the ground state of the hydrogen a flat-disk configuration but spherically symmetric).
 
  • #8
53
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Yes, the screen can be considered a measurement device.

Measurement is a subjective notion in quantum mechanics, and depends on whether you consider that a definite (often also called "irreversible" or "macroscopic" or "classical" or "real") measurement outcome (eg. a mark on the screen) was obtained or not. A measurement has occurred if there is a mark on the screen, and you retain the information about the mark on the screen. If you throw the information away, you can consider it as a measurement having occurred and the information has been thrown away, or you can consider it as if a measurement has not occurred (by treating the particle and the screen as a joint quantum system undergoing unitary time evolution).
cannot we say that to throw the information away we have to erase all what could help to know which peculiar output was obtained? we have so to interact with the screen, the environment etc to say that no measurement occured
 
  • #9
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the only way i see to consider that unitarity is not broken is to adopt the MWI.
is there another way to throw away the information?
 
  • #10
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de-Broglie-Bohm has a unitary evolution as well.
 
  • #11
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i know the argument of Wigner's friend. he considers the cat the poison and Wigner as a quantic system that has to be measured. suppose that wigners has an answer today, wigner's friend is free to measure it tomorrow because the system will continue to evolve unitarily.
this is only possible if he ignores everything about the real cat.
consider now that the excited atom is an atomic bomb and the measuring device a japonese town. it is not possible to ignore what occured and that things evolved unitarily until now.
unless you are a MWist (and this is respectable)
 
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