The statement
Explanation
$S=E(a,b)+E(a,b')+E(a',b)-E(a',b')$.
Local realistic models cannot exceed 2.
Conditions
A compact formula is not a claim of unrestricted validity. Before applying this statement, check the assumptions stated in its wording (dimension, finiteness, equilibrium, model regime) and in the explanation above.
Deeper understanding
Noncommuting measurements, contextuality and Bell correlations constrain which classical descriptions can reproduce quantum predictions.
What this result tells us
A violation excludes the class of local hidden-variable models under the Bell-test assumptions; it does not permit superluminal messaging.
Mathematical reading guide
Read the equation from left to right: identify its premises, the operation being performed, and the resulting conclusion or bound. When a symbol represents an operator, verify its domain and whether Hermiticity, unitarity, positivity or finite dimensionality is assumed. When probabilities or entropies appear, check normalization and the logarithm convention.
Where it matters
Device-independent certification, quantum randomness, precision measurement and foundational experiments.
Worked example and interpretation
In a local deterministic model, $S=A_0(B_0+B_1)+A_1(B_0-B_1)$ with all outcomes $\pm1$. Exactly one parenthesis is $\pm2$ and the other is zero, so $|S|=2$; averaging yields $|\langle S\rangle|\le2$.
Specify the state, measurement settings, ensemble and experimental assumptions before interpreting the inequality or no-go result.
Check your understanding
Questions to work through
- What are the assumptions and the physical system for Classical CHSH bound?
- Which quantity is invariant, bounded, measurable or transformed in this statement?
- Can you demonstrate the claim on the simplest valid example, and identify a situation where its assumptions fail?
- How does this result connect to uncertainty and quantum foundations in an experiment or simulation?
Suggested approach to the answers
First identify the hypotheses in the original statement. Use the displayed equation and the worked example above; distinguish an exact theorem from an approximation, and state the operational meaning of each quantity. For the final question, relate the mathematical result to device-independent certification, quantum randomness, precision measurement and foundational experiments.