Does a polarisationfilter do a measurement?

Click For Summary

Discussion Overview

The discussion revolves around the nature of measurements in quantum mechanics, particularly concerning polarization filters and their role in measuring the state of photons. Participants explore whether passing through a polarization filter constitutes a measurement and how it relates to the preparation of quantum states.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants propose that a polarization filter places a photon into a polarization eigenstate, suggesting that the filter is part of the measurement process.
  • Others argue that the concept of measurement may depend on interpretation, indicating that calling the filter a measurement is a matter of perspective.
  • A participant questions whether a particle prepared in a superposition of eigenvectors will always yield a specific measurement distribution, referencing the behavior of polarized photons.
  • Another participant clarifies that, according to quantum theory, there are no quantum jumps, and the evolution of the state is smooth, although polarization filters can be idealized as projection operators.
  • There is a query about whether the measurement principles discussed apply to other particles, such as electrons, and whether the same formulas for measurement distributions hold across different systems.
  • It is noted that the principles discussed are applicable to spin-1/2 particles and systems described by two-dimensional Hilbert spaces, including both electron spin and photon polarization.

Areas of Agreement / Disagreement

Participants express differing views on whether the action of a polarization filter constitutes a measurement, and while some agree on the technical aspects of state preparation, the discussion remains unresolved regarding the broader implications of measurement in quantum mechanics.

Contextual Notes

Participants acknowledge that the interpretation of measurements and the role of filters may depend on the specific context and definitions used, highlighting the complexity of quantum measurement theory.

entropy1
Messages
1,232
Reaction score
72
Consider a measurement of a photon after it has passed a polarisationfilter. Does the photon jump in a (polarisation-)eigenstate by passing the filter? Does the filter do a measurement? Is the filter part of the entire measurement?
 
Physics news on Phys.org
entropy1 said:
Consider a measurement of a photon after it has passed a polarisationfilter. Does the photon jump in a (polarisation-)eigenstate by passing the filter? Does the filter do a measurement? Is the filter part of the entire measurement?

Ordinarily, the filter places the photon into a polarization eigenstate as you say. You could say that it also takes the detector to complete the full measurement. Technically that would be accurate.

Certainly, a 2nd polarizer (or PBS) after the first would then allow you to see that the 1st did in fact place the photon into the eigenstate.

So to a certain extent, what you call it is a matter of taste in this case.
 
  • Like
Likes   Reactions: bhobba and entropy1
If I understand correctly, a measurement is made in a normalized basis. The measured particle eventually measures as a projection along one of eigenvectors of that basis. However, the particle can also be prepared in a superposition of eigenvectors of the measurement basis. (correct me if I'm wrong)

If I'm correct: consider a two-dimensional basis. Is a particle that is prepared in a superposition of eigenvectors of the basis in an angle α always measured with a distribution of cos2(α) resp. sin2(α) over several measurements? (such as is the case with polarized photons and polarizers?)
 
Exactly, and a polarization filter indeed does prepare photons of a given (linear) polarization state. Of course, nothing jumps since the time evolution in quantum theory is a smooth process governed by the equations of motion for state (statistical) operators and that of the operators representing observables. There are no quantum jumps according to quantum theory!

Of course, FAPP you can often just idealize a polarization filter as a projection operator and neglect the transition time of the state vector, because it's tiny compared to the time scales we resolve in such experiments.
 
  • Like
Likes   Reactions: entropy1
Does the formula I mentioned also hold for, say, measuring electron spin? In other words: does it hold for any particle? (not just for polarisation and photons?)
 
It's true for spin-1/2 particles and photons or for any system that's described by some two-dimensional Hilbert space like the spin component of spin-1/2 particles and the polarization of massless (relativistic) quanta, which both are described in a two-dimensional Hilbert space.
 
  • Like
Likes   Reactions: entropy1

Similar threads

Graduate Probability of entangled photons passing filter independent?
  • · Replies 4 ·
Replies
4
Views
2K
Undergrad How does a band pass filter increase the time of arrival of photons?
  • · Replies 2 ·
Replies
2
Views
1K
Undergrad Looking For Help Understanding Bell's Theorem, Hidden Variables & QM
  • · Replies 80 ·
3
Replies
80
Views
8K
Undergrad The mechanism of entangled particles
  • · Replies 38 ·
2
Replies
38
Views
4K
High School Bell inequality test without polarisation?
  • · Replies 8 ·
Replies
8
Views
2K
High School Why do photon polarization experiments show similar outcomes in different situations?
  • · Replies 11 ·
Replies
11
Views
3K
Graduate Perceived contradiction in non-locality principle
  • · Replies 61 ·
3
Replies
61
Views
6K
Graduate Are photons passing a polarization filter randomly?
  • · Replies 7 ·
Replies
7
Views
2K
High School Bell's Inequality && polarisation for the layman
  • · Replies 10 ·
Replies
10
Views
2K
Graduate How does a polarising filter work?
  • · Replies 13 ·
Replies
13
Views
2K