摘要
‘I am a quantum engineer, but on Sundays I have principles.’ John Stewart Bell once started a colloquium with these words. The Bell inequality, one of the most important and fundamental results of quantum physics, probably originates from a flash of insight he had on a Sunday afternoon around 1964. The inequality proves Einstein wrong and provides a concrete method to tell apart quantum mechanics and classical theories. A variant of the Bell inequality, known as the CHSH inequality named after John Clauser, Michael Horne, Abner Shimony, and Richard Holt, states that |S| ≤ 2 in any classical theories where beginequation* S = łangle A_0 B_0 + A_0 B_1 + A_1 B_0 - A_1 B_1 ångle , endequation* and Ai, Bj are observables; however, quantum mechanically it is possible to violate the inequality and have $łeft|S g̊ht| = 2 sqrt2$.
