Quasiprobability Based Criterion for Classicality and Separability of   States of Spin-1/2 Particles
  A sufficient condition for a quantum state of a system of spin-1/2 particles (spin-1/2s) to admit a local hidden variable (LHV) description i.e. to be classical is the separability of the density matrix characterizing its state, but not all classical states are separable. This leads one to infer that separability and classicality are two different concepts. These concepts are examined here in the framework of a criterion for identifying classicality of a system of spin-1/2s based on the concept of joint quasiprobability (JQP) for the eigevalues of spin components. The said criterion identifies a state as classical if a suitably defined JQP of the eigenvalues of spin components in suitably chosen three or two orthogonal directions is non-negative. In agreement with other approaches, the JQP based criterion leads to the result that all non-factorizable pure states of two spin-1/2s are non-classical. Furthermore, the validity of the criterion is confirmed by comparing its predictions with those arrived at by other methods when applied to several mixed states of two spin-1/2s and the Werner like state of three spin-1/2s (G.Toth and A.Acin,Phys.Rev. A74, 030306(R) (2006)). The JQP based approach, formulated as it is along the lines of the P-function approach for identifying classical states of the electromagnetic field, offers a unified approach for systems of arbitrary number of spin-1/2s and the possibility of linking classicality with the nature of the measurement process. 
R.R. Puri
quant-ph