It's difficult to follow. IIUC they are using the density matrix D, and calling the coefficients "probabilities". Only the diagonal has an interpretation as probabilities, the other parts are more difficult to interpret and can be negative (and are not probabilities).<p>Anyway, I'd like to see an example of <a href="https://en.wikipedia.org/wiki/Stern%E2%80%93Gerlach_experiment#Sequential_experiments" rel="nofollow noreferrer">https://en.wikipedia.org/wiki/Stern%E2%80%93Gerlach_experime...</a> to see how it works.