Towards a complete, continuous, Wigner function for an ensemble of spins   or qubits
  We present a new quasi-probability distribution function for ensembles of spin-half particles or qubits that has many properties in common with Wigner's original function for systems of continuous variables. We show that this function provides clear and intuitive graphical representation of a wide variety of states, including Fock states, spin-coherent states, squeezed states, superpositions and statistical mixtures. Unlike previous attempts to represent ensembles of spins/qubits, this distribution is capable of simultaneously representing several angular momentum shells. 
Derek Harland, M. J. Everitt, Kae Nemoto, T. Tilma, T. P. Spiller
quant-ph math-ph math.MP