Paper detail

SL-invariant entanglement measures in higher dimensions: the case of spin $1$ and $3/2$

An SL-invariant extension of the concurrence to higher local Hilbert-space dimension is due to its relation with the determinant of the matrix of a $d\times d$ two qudits state, which is the only SL-invariant of polynomial degree $d$. This determinant is written in terms of antilinear expectation values of the local $SL(d)$ operators. We use the permutation invariance of the comb-condition for creating further local antilinear operators which are orthogonal to the original operator. It means that the symmetric group acts transitively on the space of combs of a given order. This extends the mechanism for writing $SL(2)$-invariants for qubits to qudits. I outline the method, that in principle works for arbitrary dimension $d$, explicitely for spin 1, and spin 3/2. There is an odd-even discrepancy: whereas for half odd integer spin a situation similar to that observed for qubits is found, for integer spin the outcome is an asymmetric invariant of polynomial degree $2d$.