Proving Entanglement - Do we need Bells Theorem?

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Discussion Overview

The discussion centers around the necessity of Bell's theorem in the context of proving quantum entanglement, exploring whether simpler proofs suffice and addressing the implications of local hidden variable (LHV) theories.

Discussion Character

  • Debate/contested
  • Exploratory
  • Technical explanation

Main Points Raised

  • Some participants argue that numerous simpler proofs of quantum entanglement exist, such as those involving SPDC, fiber couplers, quantum dots, and atomic cascades.
  • One participant questions the need to focus on Bell's theorem, suggesting it may serve as secondary proof rather than a necessity for establishing entanglement.
  • Another participant contends that while the experiments demonstrate nonlocality, they do not rigorously prove it, as LHV theories could potentially explain the results in an artificial manner.
  • A later reply clarifies that the effects observed in experiments can only be validated through coincidence measurements of both entangled photons, not instantaneously as initially suggested.
  • One participant emphasizes the importance of Bell's theorem to differentiate between correlations arising from common causes versus those where one event causes another.

Areas of Agreement / Disagreement

Participants express differing views on the necessity of Bell's theorem, with some advocating for its importance while others suggest that simpler proofs may be adequate. The discussion remains unresolved regarding the role of Bell's theorem in proving entanglement.

Contextual Notes

There are unresolved questions about the implications of LHV theories and the interpretations of experimental results, particularly concerning the nature of correlations observed in entangled photon experiments.

San K
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Question: Do we really need to spend too much time on Bell's theorem/test when there are numerous/easier proofs of quantum entanglement?

The numerous/easier proofs are: - Almost all experiments in which two photons are generated via

a) SPDC (Delayed Choice Quantum Eraser, Mach–Zehnder interferometer et el.)
b) Fiber coupler
c) Quantum dots
d) Atomic cascades (used in the original Bell's test/theorem)

For example in DCQE (http://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser):

one of the twin/entangled photon's path can be manipulated to get (or erase) which-way information and the effect can be instantaneously seen on its remote twin in term of the patterns the twin would make on the screen.

Is there a way/logic that LHV (local hidden variable) theory can explain this? Are there any loopholes?

Spending time on Bell's theorem might be useful as it serves as additional/secondary proof and it proves/confirms the cosine relationship (from QM theory)

however do we need to argue/doubt the existence of Quantum Entanglement?
 
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San K said:
one of the twin/entangled photon's path can be manipulated to get (or erase) which-way information and the effect can be instantaneously seen on its remote twin in term of the patterns the twin would make on the screen.
No, that is not true. (If it was, it would mean that entanglement can be used to send a controlled signal superluminally, which cannot be done.) The effect cannot be seen instantaneously on the screen of the remote twin. The effect can only be seen through coincidences in measurements of BOTH members of entangled pairs.

The experiments you mention nicely DEMONSTRATE nonlocality, but do NOT PROVE it rigorously. (In principle, these experiments could be explained in terms of LHV's, but such an explanation would probably look quite artificial.)
 
Last edited:
San K said:
one of the twin/entangled photon's path can be manipulated to get (or erase) which-way information and the effect can be instantaneously seen on its remote twin in term of the patterns the twin would make on the screen.

EDIT: the effect can be seen/validated after comparison of the two entangled photon via coincidence counter

EDITORS note: I forgot to mention that. Thanks for reminding, Demystifier
 
San K said:
Question: Do we really need to spend too much time on Bell's theorem/test when there are numerous/easier proofs of quantum entanglement?
Yes we really need Bell's theorem/tests to distinguish between two cases:
- correlations between events that have common cause
- correlations between events where one event is cause of the other
 

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