How Does Detector Rotation Affect Spin Correlation in Bell's Test Experiments?

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Discussion Overview

The discussion revolves around the effects of detector rotation on spin correlation in Bell's test experiments, particularly focusing on the measurement of spin states of particles such as electrons and positrons produced from decaying pi mesons. Participants explore the implications of changing detector angles and the resulting correlations in spin measurements, as well as the underlying quantum mechanics principles involved.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions how rotating a detector affects spin measurements, specifically asking about the correlation when one detector is at 0 degrees and the other at 45 degrees.
  • Another participant suggests that measuring on non-commuting bases does not reveal anything unusual, but asserts that Bell's theorem implies a statistical mixture must be considered.
  • A participant provides a mathematical perspective, stating that the probability of measuring opposite spins at different angles is given by cos²(A-B), leading to a 50% chance when one detector is at 0 and the other at 45 degrees.
  • Some participants discuss the implications of pre-existing spin states and whether experimental results can rule out predetermined outcomes, referencing the need for specific conditions to validate such claims.
  • There is a mention of the detector efficiency loophole and its relevance to Bell test experiments, with references to previous experiments that have addressed this issue.
  • One participant raises a concern about the timing of measurements and the potential influence between entangled particles, questioning how simultaneous measurements could complicate the interpretation of results.

Areas of Agreement / Disagreement

Participants express differing views on the implications of detector rotation and the interpretation of spin correlations. While some agree on the mathematical probabilities involved, others challenge the assumptions about pre-existing states and the implications of measurement timing. Overall, the discussion remains unresolved with multiple competing perspectives.

Contextual Notes

Participants highlight limitations such as the need for perfect detector efficiency and the assumptions required for certain interpretations of Bell's theorem. The discussion also touches on the complexities of entangled particle measurements and the conditions necessary to draw conclusions from experimental data.

  • #91
SeventhSigma said:
You can't control what spin a particle assumes afaik (edit: I have no idea if you actually can do this). It comes out either spin up or spin down with equal probability and you have no way of controlling this. You can control what kinds of spins are allowed to exit a detector, but this doesn't change the fact that the particle assumed that particular spin in the first place.

Even if you could change the spin by forcing it, you'd be breaking the entanglement anyway.

These types of experiments are often conducted with the spin controlled and the measuring devices rotated to see the effects, especially the effects when 2 matching particles are detected. See http://arxiv.org/PS_cache/quant-ph/pdf/0205/0205171v1.pdf" as an example of preparing entangled spin up particles (photons in this case).
 
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  • #92
edguy99 said:
These types of experiments are often conducted with the spin controlled and the measuring devices rotated to see the effects, especially the effects when 2 matching particles are detected. See http://arxiv.org/PS_cache/quant-ph/pdf/0205/0205171v1.pdf" as an example of preparing entangled spin up particles (photons in this case).

Maybe you can quote something. Because it doesn't say that.
 
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  • #93
DrChinese said:
Maybe you can quote something. Because it doesn't say that.

"To create the state | EPRi or something close to it, we adjust the parameters which determine the laser polarization. First we adjust l to equalize the coincidence counts N(0◦, 0◦) and N(90◦, 90◦). Next we set l by rotating the quartz plate about a vertical axis to maximize N(45◦, 45◦). When performing these optimizations, we typically collect a few hundred photons per point which requires an acquisition window of a few seconds."
 
  • #94
edguy99 said:
"To create the state | EPRi or something close to it, we adjust the parameters which determine the laser polarization. First we adjust l to equalize the coincidence counts N(0◦, 0◦) and N(90◦, 90◦). Next we set l by rotating the quartz plate about a vertical axis to maximize N(45◦, 45◦). When performing these optimizations, we typically collect a few hundred photons per point which requires an acquisition window of a few seconds."

Yes, an EPR state is entangled. Notice the discussion of coincidences?
 
  • #95
DrChinese said:
Yes, an EPR state is entangled. Notice the discussion of coincidences?

Yes, they have "fixed" the orientation of the photon stream, and are rotating the two measuring devices to see the effect on the number of coincident (entangled) hits. An entangled hit is essentially when they see both detectors go off at "almost exactly" the same time.
 
  • #96
That paragraph just talks about how they calibrated the equipment as far as I can tell -- it's not saying that they're somehow "fixing" the streams to produce certain types of spins that break the entanglements. They adjust the first parameter (the theta-l) so that N(0,0) and N(90,90) produce roughly the same output counts, and then they determine where the proper 45 degree mark is by going between those two thresholds (hence the "maximize N(45,45)" part).

Someone else can correct me if I'm wrong, here.

Nowhere in here is it implying that it's breaking entanglement by "fixing" the spins in any way. The conclusion of that paper is also consistent with what's been put forth in this thread (local realistic variables contradicted, Bell's Inequality violated, etc).
 
  • #97
edguy99 said:
Yes, they have "fixed" the orientation of the photon stream, and are rotating the two measuring devices to see the effect on the number of coincident (entangled) hits. An entangled hit is essentially when they see both detectors go off at "almost exactly" the same time.

Yes, and we are looking at the subset where Alice's side is all up, which is 50% of the total stream from the PDC crystal.

The point is that the stream going to Bob consists of ones in which Alice is up, and where Alice is down. You must coincidence match to find the desired subset of Alice=up.
 
  • #98
edguy99 said:
The sequence of post is maybe a little unclear. We start with our standard electron/positron setup and measure 100% up at one end (the electron) and 100% down at the other end (the positron).

We then turn the positron end by 120 degrees and restart the measurement.

Without touching the electron end, we still see 100% up at the electron end, but we now see 75% up at the positron end.

We then start the experiment at the electron end: The first set of measurements with the 2 detectors matched will result in 75% up at the positron end and 75% down at the electron end.

OK, I think I see what you are wanting to do. Let's just refer to this as the subset so it is clear. Then everything you are saying is fine. We will simply ignore the other subgroup for our discussion purposes, knowing that the "true" universe does not have this attribute.
 
  • #99
SeventhSigma said:
That paragraph just talks about how they calibrated the equipment as far as I can tell -- it's not saying that they're somehow "fixing" the streams to produce certain types of spins that break the entanglements. They adjust the first parameter (the theta-l) so that N(0,0) and N(90,90) produce roughly the same output counts, and then they determine where the proper 45 degree mark is by going between those two thresholds (hence the "maximize N(45,45)" part).

Someone else can correct me if I'm wrong, here.

Nowhere in here is it implying that it's breaking entanglement by "fixing" the spins in any way. The conclusion of that paper is also consistent with what's been put forth in this thread (local realistic variables contradicted, Bell's Inequality violated, etc).

If the photon is modeled as http://en.wikipedia.org/wiki/Polarization_(waves)" , he is setting the properties of the photons to keep the photon properties consistent throughout the experiment. This allows him to continue the experiment where the only change being made is the rotation of the measuring device. My use of the word "fix" means the properties of the particle stream are known and unchanging.
 
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  • #100
DrChinese said:
OK, I think I see what you are wanting to do. Let's just refer to this as the subset so it is clear. Then everything you are saying is fine. We will simply ignore the other subgroup for our discussion purposes, knowing that the "true" universe does not have this attribute.

I am not sure I understand if you are agreeing or not (is there a 50% chance of us agreeing?). What probabilities would you assign to the OP question?
 

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