On the separability of Bell-diagonal states with absolutely positive partial transposition
The study investigates if absolutely PPT (APPT) states are separable within two Bell-diagonal families: two-qutrit and two-ququart lattice bases. For the two-qutrit case, every APPT state is shown to be separable. For the two-ququart lattice case, it is proven that entanglement becomes non-positive partial transposition (NPT) after a suitable permutation of Bell coefficients, implying separability.