06 5 v 1 1 0 O ct 2 00 1 A quantum measurement of the spin direction

We give a first physical model for the quantum measurement of the spin direction. It is an Arthurs-Kelly model that involves a kind of magnetic-dipole interaction of the spin with three modes of radiation. We show that in a limit of infinite squeezing of radiation the optimal POVM for the measurement of the spin direction is achieved for spin 1 2 .