CORRELATED SPIN STATES IN THE PHASE-SPACE REPRESENTATION

Calculations of the state evolution in a Stern-Gerlach type device by means of the spin extended Weyl-Wigner-Moyal phase space representatio n manifest a new feature of the Wigner functions: the spin part at eac h phase space point must be interpreted as a mixed state with negative probabilities. We analyse here, at which phase space regions and unde r which conditions this is necessary and give a sound interpretation.