Spin squeezing of mixed systems
  The notion of spin squeezing has been discussed in this paper using the density matrix formalism. Extending the definition of squeezing for pure states given by Kitagawa and Ueda in an appropriate manner and employing the spherical tensor representation, we show that mixed spin states which are non-oriented and possess vector polarization indeed exhibit squeezing. We construct a mixed state of a spin 1 system using two spin 1/2 states and study its squeezing behaviour as a function of the individual polarizations of the two spinors. 
K.S. Mallesh, Swarnamala Sirsi, Mahmoud A.A. Sbaih, P.N. Deepak and G.   Ramachandran