Tensorial characterization and quantum estimation of weakly entangled   qubits
  In the case of two qubits, standard entanglement monotones like the linear entropy fail to provide an efficient quantum estimation in the regime of weak entanglement. In this paper, a more efficient entanglement estimation, by means of a novel class of entanglement monotones, is proposed. Following an approach based on the geometric formulation of quantum mechanics, these entanglement monotones are defined by inner products on invariant tensor fields on bipartite qubit orbits of the group SU(2)xSU(2). 
P. Aniello, J. Clemente-Gallardo, G. Marmo, G. F. Volkert
quant-ph math-ph math.MP