I have read many explanations of Bell’s proof that mention in passing something like “According to QM, the correlation between measurements of spin at different angles should be given by the cosine of the angle between them.” Sometimes they talk about 1-cos(x)/2. Sometimes they talk about cos^2(x). I suppose that the cos is in some wavefunction, whose amplitude squared is the probability. What is that wavefunction and where does it come from? Some explanations involve Pauli matrices, which I understand somewhat. But I don’t understand the rules for using them in this calculation. Can you explain them in terms of some Hamiltonian and Schrodinger’s equation? Or is it just a matter of an expectation-value calculation from state vectors?