Entanglement Negativity of Spin-Orbit Correlations in a general Qubit-Qudit Setup

We present the complete eigenvalue spectrum of the partially transposed density matrix for a pure bipartite quantum state acting on a generic $2 \otimes n$ Hilbert space. The spectrum contains four non-zero eigenvalues, as, \begin{eqnarray} \lambda_{1,2}=\pm \sqrt{A}, ~~~ \lambda_{3,4}= \frac{1}{2}(1\pm\sqrt{1-4 A}), \nonumber \end{eqnarray} where $A$ is the determinant of the reduced density matrix (traced over the larger subspace). As $0 \leqslant A \leqslant1/4$, only one is negative among the four non-trivial eigenvalues. Within this qubit-qudit framework, we further studied the negativity as a measure of entanglement for the case of spin-orbit correlation of partons inside a proton. The entanglement negativity for spin-orbit correlations is found to be related to the gluon helicity PDF and the Hermitian angle of the associated Hilbert space for linearly polarized protons.