Apparatus used to test violations of Bell's inequalities

Question

I believe most people are familiar with Bell's Theorem. If I understand correctly, the violations of Bell's inequalities are possible due to the existance of entangled quantum states. Although popular discussion often circles around nonlocality questions and doubts about what the experiments say with regards to local realism, we rarely see lengthy discussions about the implications of creating the apparatus itself.

I wonder if it is possible to reasonably view the apparatus as a sort of quantum state detector? In asking this, I am not thinking about a detector as a device that detects a particle, but as a device that has detected the existance of a possible state of particles.

If one thinks of the spin correlation diagram in the article, one might interpret the possible outcomes as two possible system response curves (or possibly zero or one). It seems that if the apparatus is properly tuned to a particular type of particle system, one should always see the quantum result.

Is there some clarity that can be brought to this issue?

Answer 1

When one discusses with experimentalists building an apparatus creating entangled state, one quickly understands that violating Bell's inequalities is a key benchmark for them in order to be sure to have created entanglement. So the Bell's theorem is actually used as "quantum state detector", or more technically, as an entanglement witness.

There exists other quantum witnesses, and they are also used, but a Bell's inequality violation is mainly used by experimentalist because:

• it is historically the first entanglement witnesses : it is well known and has been used for almost 3 decades.
• it is device-independent. If your apparatus is not well tuned (i.e. if you don't measure what you think you are measuring), many entanglement witness may report entanglement when the is none. For example, if you forget to rotate your polarizer between measurement, it is easy to be in a situation where you think to have perfect quantum correlations while these correlation are classical. However, due to the minimal assumptions needed to derive Bell's theorem, the violation of Bell's inequality is a proof that there is entanglement somewhere in the apparatus, even if is misaligned.

I don't know if I answer your question, but I hope I brought some clarity.