Nonlocal Parameters for Multiparticle Density Matrices
Noah Linden, Sandu Popescu, Anthony Sudbery · Physical Review Letters · 1999
As far as entanglement is concerned, two density matrices of $n$ particles are completely equivalent if one can be transformed into the other by local unitary transformations. We present two methods to find whether or not two generic density matrices of arbitrary numbers of spin-1/2 particles are equivalent. Both methods describe density matrices in terms of a finite number of invariant parameters.