I would like to put in evidence the recent work "Quantum classification" which can be found at http://xxx.lanl.gov/PS_cache/arxiv/pdf/0809/0809.0444v2.pdf
An algebraic approach to the study of weakly excited
states for a condensate in a ring geometry, Buonsante P, Franco R, and Penna V, J. Phys. 38 8393-8408 (2005)
Discrete Wigner distribution for two qubits: a
characterization of entanglement properties, Franco R, and Penna V, J. Phys. A: Math. Gen. 39 (2006) 5907–5919
