Quantum Malu's law

Abstract

For measurements of spin orientation passed trough a linear polarizer we derive from quantum mechanics a quantum version of Malus' law. We show that Malus' law can be applied to quantum mechanical amplitudes leading to a random average over the initial spin orientation. This derivation shows how close quantum mechanics is in form to a hidden-variable theory based on random average of the “unknown” angle of the spin polarization. This calculation shows also the obvious differences between quantum mechanical averaging and some semiclassical averaging based on the purely classical Malus law analogy.