Bell Inequalities¶
The Bell inequalities describe a way to prove the validity of Quantum Mechanics and to rule out any kind of hidden variable theory that would preserve locality and realism, as described in EPR Experiments.
The are several forms of these inequalities. Here we will illustrate the CHSH inequality.
CHSH inequality¶
The Gedankenexperiment requires a large number of identical quantum systems, each made of two entangled particles.
One particle goes to Alice, who can measure the particle’s spin along two directions
The CHSH inequality is based on the evaluation of following quantity for each system that we we will call “samples” in the remainder of this section, and involves the outcomes
By reordering the terms, we observe that this quantity will only take the values
Thus the average
Contradiction¶
For each of the sample systems, Alice as well as Bob can only measure one of their spin components. Let us assume that the four terms are always well defined real quantities, even if they are not all measured. We also assume that the values of these quantities are defined locally, i.e. there are defined a priori, i.e. they do not depend neither on the choice of the measurements, nor on the measurement outcome of the other particle.
This means that, even if we have no access to the four measurement outcomes for each of the samples, we can perform an estimation of
We can also write these averages as expectation values given by the rules of Quantum Mechanics:
Let us go through one of the possible four outcomes:
We first measure
Then we measure
And finally we get:
The expectation values of the other terms follow the same procedure, and we get:
Now we introduce the particular experimental setup suggested for the CHSH inequality.
We define the directions of the components
These values lead to a value of
Interpretation¶
As presented in EPR Experiments, it has been experimentally confirmed that the world obeys the rules of Quantum Mechanics. This means that at least one of the assumptions of locality and realism, which are required by the hidden variable theory, does not hold. Such a theory cannot exist!
One of the key elements to understand how this contradiction occurs,
is to consider that we estimate the four terms contributing to