# Proving Entanglement - Do we need Bells Theorem?

1. Apr 13, 2012

### San K

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): [Broken]

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?

Last edited by a moderator: May 5, 2017
2. Apr 14, 2012

### Demystifier

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: Apr 14, 2012
3. Apr 14, 2012

### San K

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

4. Apr 15, 2012

### zonde

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