Someone reads about a loophole-free Bell test and argues: "The experiment measured…
PHYSICS · Multi-part · Fermi University Physics · Quantum Information & Technology · Quantum states, measurement and entanglement · Quantum states, measurement and entanglement
Quantum Information and Technology, Ch Nonlocality: EPR, Bell inequalities and the CHSH game
Problem
Someone reads about a loophole-free Bell test and argues: "The experiment measured S>2 with the locality and detection loopholes closed. The CHSH inequality is derived assuming locality, so the violation proves nature is nonlocal. Realism (the idea that a measured quantity has a value whether or not it is measured) and measurement independence (the idea that the hidden variable is uncorrelated with the choice of settings) are untouched by this: it is locality specifically that has to go."
((a)) State exactly which assumptions enter the derivation of the CHSH inequality ∣S∣≤2.
((b)) Explain why the observed violation, by itself, cannot single out locality as the assumption that fails, and give one example of an alternative account (naming the assumption it drops instead) that is equally consistent with the data.
((c)) Say, in one sentence, what a Bell-test violation does establish about the three assumptions taken together.